1.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation
2.
Logarithmic scale
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A logarithmic scale is a nonlinear scale used when there is a large range of quantities. Common uses include the strength, sound loudness, light intensity. It is based on orders of magnitude, rather than a linear scale. In particular our sense of hearing perceives equal ratios of frequencies as equal differences in pitch, the top left graph is linear in the X and Y axis, and the Y-axis ranges from 0 to 10. A base-10 log scale is used for the Y axis of the left graph. The top right graph uses a scale for just the X axis. A slide rule has logarithmic scales, and nomograms often employ logarithmic scales, the geometric mean of two numbers is midway between the numbers. Before the advent of graphics, logarithmic graph paper was a commonly used scientific tool. If both the vertical and horizontal axes of a plot are scaled logarithmically, the plot is referred to as a log–log plot, if only the ordinate or abscissa is scaled logarithmically, the plot is referred to as a semi-logarithmic plot. Bit Byte Decade John Napier Level Logarithm Logarithmic mean Preferred number Dehaene, Stanislas, Izard, Véronique, Spelke, Elizabeth, Pica, distinct intuitions of the number scale in Western and Amazonian indigene cultures. American Association for the Advancement of Science, why using logarithmic scale to display share prices
3.
Voyager 1
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Voyager 1 is a space probe launched by NASA on September 5,1977. Part of the Voyager program to study the outer Solar System, Voyager 1 launched 16 days after its twin, having operated for 39 years,7 months and 1 day, the spacecraft still communicates with the Deep Space Network to receive routine commands and return data. At a distance of 138 AU from the Sun as of March 2017, the probes primary mission objectives included flybys of Jupiter, Saturn, and Saturns large moon, Titan. It studied the weather, magnetic fields, and rings of the two planets and was the first probe to provide detailed images of their moons. After completing its mission with the flyby of Saturn on November 20,1980, Voyager 1 began an extended mission to explore the regions. On August 25,2012, Voyager 1 crossed the heliopause to become the first spacecraft to enter interstellar space, in the 1960s, a Grand Tour to study the outer planets was proposed which prompted NASA to begin work on a mission in the early 1970s. Information gathered by the Pioneer 10 spacecraft helped Voyagers engineers design Voyager to cope effectively with the intense radiation environment around Jupiter. Initially, Voyager 1 was planned as Mariner 11 of the Mariner program, due to budget cuts, the mission was scaled back to be a flyby of Jupiter and Saturn and renamed the Mariner Jupiter-Saturn probes. As the program progressed, the name was changed to Voyager. Voyager 1 was constructed by the Jet Propulsion Laboratory and it has 16 hydrazine thrusters, three-axis stabilization gyroscopes, and referencing instruments to keep the probes radio antenna pointed toward Earth. Collectively, these instruments are part of the Attitude and Articulation Control Subsystem, the spacecraft also included 11 scientific instruments to study celestial objects such as planets as it travels through space. The radio communication system of Voyager 1 was designed to be used up to, the communication system includes a 3. 7-meter diameter parabolic dish high-gain antenna to send and receive radio waves via the three Deep Space Network stations on the Earth. The craft normally transmits data to Earth over Deep Space Network Channel 18, using a frequency of either 2.3 GHz or 8.4 GHz, while signals from Earth to Voyager are broadcast at 2.1 GHz. When Voyager 1 is unable to communicate directly with the Earth, signals from Voyager 1 take over 19 hours to reach Earth. Voyager 1 has three radioisotope thermoelectric generators mounted on a boom, each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The RTGs generated about 470 W of electric power at the time of launch, the power output of the RTGs declines over time, but the crafts RTGs will continue to support some of its operations until 2025. As of 2017-04-06, Voyager 1 has 73. 14% of the plutonium-238 that it had at launch, by 2050, it will have 56. 5% left. Since the 1990s, space probes usually have completely autonomous cameras, the computer command subsystem controls the cameras
4.
Space probe
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A space probe is a robotic spacecraft that does not orbit the Earth, but, instead, explores further into outer space. A space probe may approach the Moon, travel through space, flyby, orbit, or land on other planetary bodies. Approximately fifteen missions are currently operational, once a probe has left the vicinity of Earth, its trajectory will likely take it along an orbit around the Sun similar to the Earths orbit. To reach another planet, the simplest practical method is a Hohmann transfer orbit, more complex techniques, such as gravitational slingshots, can be more fuel-efficient, though they may require the probe to spend more time in transit. Some high Delta-V missions can only be performed, within the limits of modern propulsion, a technique using very little propulsion, but requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network. First man-made object to land on the Moon, or any other extra terrestrial surface. First mission to photograph the far side of the Moon, launched in 1959, first robotic sample return probe from the Moon. It was sent to the Moon on November 10,1970, first successful in-place analysis of another planet. It may have also been the first space probe to impact the surface of another planet, the Venera 7 probe was the first spacecraft to successfully soft land on another planet and to transmit data from there back to Earth. Upon its arrival at Mars on November 13,1971, Mariner 9 became the first space probe to orbit around another planet. Although, the spacecraft failed shortly after landing, the Mars Exploration Rovers, Spirit and Opportunity surface and geology, and searched for clues to past water activity on Mars. They were each launched in 2003 and landed in 2004, communication with Spirit stopped on sol 2210. JPL continued to attempt to regain contact until May 24,2011, Opportunity arrived at Endeavour crater on 9 August 2011, at a landmark called Spirit Point named after its rover twin, after traversing 13 miles from Victoria crater, over a three-year period. As of January 26,2016, Opportunity has lasted for more than twelve years on Mars — although the rovers were intended to last only three months. The first dedicated missions to a comet, in this case and they dropped landers and balloons at Venus before their rendezvous with Halleys Comet. This Japanese probe was the first non-US, non-Soviet interplanetary probe, a second Japanese probe, it made ultraviolet wavelength observations of the comet. The first space probe to penetrate a comets coma and take images of its nucleus. First solar wind sample return probe from sun-earth L1, first sample return probe from a comet tail
5.
Kuiper belt
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It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets, Pluto, Haumea, and Makemake, some of the Solar Systems moons, such as Neptunes Triton and Saturns Phoebe, are also thought to have originated in the region. The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not actually predict its existence, in 1992,1992 QB1 was discovered, the first Kuiper belt object since Pluto. Since its discovery, the number of known KBOs has increased to over a thousand, the Kuiper belt should not be confused with the theorized Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc, Pluto is the largest and most-massive member of the Kuiper belt and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Plutos status as part of the Kuiper belt caused it to be reclassified as a planet in 2006. It is compositionally similar to other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as plutinos. After the discovery of Pluto in 1930, many speculated that it not be alone. The region now called the Kuiper belt was hypothesized in various forms for decades and it was only in 1992 that the first direct evidence for its existence was found. The number and variety of speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it. The first astronomer to suggest the existence of a population was Frederick C. That same year, astronomer Armin O. Leuschner suggested that Pluto may be one of many long-period planetary objects yet to be discovered. Kuiper was operating on the common in his time that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuipers hypothesis correct, there would not be a Kuiper belt today, the hypothesis took many other forms in the following decades. Cameron postulated the existence of a mass of small material on the outskirts of the solar system. Observation, however, ruled out this hypothesis, in 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a blink comparator, the device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before
6.
Centaur (minor planet)
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Centaurs are minor planets with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets, Centaurs typically behave with characteristics of both asteroids and comets. They are named after the centaurs that were a mixture of horse. It has been estimated there are around 44,000 centaurs in the Solar System with diameters larger than 1 km. The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory, however, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo, which at 260 km in diameter is as big as a mid-sized main-belt asteroid, however, the lost centaur 1995 SN55 may be somewhat larger. No centaur has been photographed up close, although there is evidence that Saturns moon Phoebe, imaged by the Cassini probe in 2004, in addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus. As of 2008, three centaurs have been found to display comet-like comas, Chiron,60558 Echeclus, and 166P/NEAT, Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs, such as 52872 Okyrhoe and 2012 CG, are suspected of having shown comas, any centaur that is perturbed close enough to the Sun is expected to become a comet. The generic definition of a centaur is a body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Though nowadays the MPC often lists centaurs and scattered disc objects together as a single group, the Jet Propulsion Laboratory similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune. In contrast, the Deep Ecliptic Survey defines centaurs using a classification scheme. These classifications are based on the change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation and this definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit. The collection The Solar System Beyond Neptune defines objects with an axis between those of Jupiter and Neptune and a Jupiter – Tisserands parameter above 3. The JPL Small-Body Database lists 324 centaurs, there are an additional 65 trans-Neptunian objects with a perihelion closer than the orbit of Uranus. The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on either side of the debate, thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy. Other objects caught between these differences in classification methods include 944 Hidalgo which was discovered in 1920 and is listed as a centaur in the JPL Small-Body Database
7.
Neptune trojan
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Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same period as Neptune and follow roughly the same orbital path. Seventeen Neptune trojans are known, of which thirteen orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune. The Neptune trojans are termed trojans by analogy with the Jupiter trojans and it is suspected that large Neptune trojans could outnumber Jupiter trojans by an order of magnitude. In 2010, the discovery of the first known L5 Neptune trojan,2008 LC18, was announced, Neptunes trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars. However, New Horizons may not have had sufficient downlink bandwidth, in 2001, the first Neptune trojan was discovered,2001 QR322, near Neptunes L4 region, and with it the fifth known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, on August 12,2010, the first L5 trojan,2008 LC18, was announced. It was discovered by a survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds. This suggests that large L5 trojans are as common as large L4 trojans, to within uncertainty and it would have been possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto. Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizonss flight path, allowing detection of objects that the spacecraft could image. 2011 HM102, the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU. However, New Horizons may not have had sufficient downlink bandwidth, the orbits of Neptune trojans are highly stable, Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System. Neptunes L5 can host stable trojans equally well as its L4, Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10, 000-year period. Neptune trojans that escape enter orbits similar to centaurs, although Neptune cannot currently capture stable trojans, roughly 2. 8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, the unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole. The existence of high-inclination Neptune trojans points to a capture during planetary migration instead of in situ or collisional formation. The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture, the capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the period at which the locations where Uranus passes Neptune circulate can resonate with the periods of Neptune trojans
8.
Trans-Neptunian object
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A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune,30 astronomical units. Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, the first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly,1992 QB1, as of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Centers List of Transneptunian Objects. Of these TNOs,2,000 have a perihelion farther out than Neptune, as of November 2016,242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation. The largest known object is Pluto, followed by Eris,2007 OR10, Makemake. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division. The orbit of each of the planets is slightly affected by the influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune, the search for these led to the discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptunes mass from the Voyager 2 flyby in 1989 showed that the problem was spurious, Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has an inclination to the ecliptic than most other large TNOs. After Plutos discovery, American astronomer Clyde Tombaugh continued searching for years for similar objects. For a long time, no one searched for other TNOs as it was believed that Pluto. Only after the 1992 discovery of a second TNO,1992 QB1, a broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers, Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union. Kuiper belt objects are classified into the following two groups, Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1,2 resonance are called twotinos, and objects with a 2,3 resonance are called plutinos, after their most prominent member, classical Kuiper belt objects have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1,50000 Quaoar and Makemake, the scattered disc contains objects farther from the Sun, usually with very irregular orbits. A typical example is the most massive known TNO, Eris, scattered-extended —Scattered-extended objects have a Tisserand parameter greater than 3 and have a time-averaged eccentricity greater than 0
9.
Classical Kuiper belt object
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A classical Kuiper belt object, also called a cubewano, is a low-eccentricity Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto and that is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets. The name cubewano derives from the first trans-Neptunian object found after Pluto, similar objects found later were often called QB1-os, or cubewanos, after this object, though the term classical is much more frequently used in the scientific literature. Most cubewanos are found between the 2,3 orbital resonance with Neptune and the 1,2 resonance,50000 Quaoar, for example, has a near-circular orbit close to the ecliptic. Plutinos, on the hand, have more eccentric orbits bringing some of them closer to the Sun than Neptune. The majority of objects, have low inclinations and near-circular orbits, a smaller population is characterised by highly inclined, more eccentric orbits. The Deep Ecliptic Survey reports the distributions of the two populations, one with the inclination centered at 4. 6° and another with inclinations extending beyond 30°, the vast majority of KBOs have inclinations of less than 5° and eccentricities of less than 0.1. The hot and cold populations are different, more than 30% of all cubewanos are in low inclination. The parameters of the orbits are more evenly distributed, with a local maximum in moderate eccentricities in 0. 15–0.2 range. See also the comparison with scattered disk objects, when orbital inclinations are compared, hot cubewanos can be easily distinguished by their higher inclinations, as the plutinos typically keep orbits below 20°. In addition to the orbital characteristics, the two populations display different physical characteristics. The difference in colour between the red cold population and more heterogeneous hot population was observed as early as in 2002, another difference between the low-inclination and high-inclination classical objects is the observed number of binary objects. Binaries are quite common on low-inclination orbits and are typically similar-brightness systems, binaries are less common on high-inclination orbits and their components typically differ in brightness. There is no definition of cubewano or classical KBO. However, the terms are used to refer to objects free from significant perturbation from Neptune. The Minor Planet Center and the Deep Ecliptic Survey do not list cubewanos using the same criteria, many TNOs classified as cubewanos by the MPC are classified as ScatNear by the DES. Dwarf planet Makemake is such a borderline classical cubewano/scatnear object,2002 KX14 may be an inner cubewano near the plutinos. Furthermore, there is evidence that the Kuiper belt has an edge, in that an apparent lack of objects beyond 47–49 AU was suspected as early as 1998
10.
Resonant trans-Neptunian object
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In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean-motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e. g.1,2,2,3 etc, resonant TNOs can be either part of the main Kuiper belt population, or the more distant scattered disc population. The diagram illustrates the distribution of the known trans-Neptunian objects, resonant objects are plotted in red. The designation 2,3 or 3,2 both refer to the resonance for TNOs. There is no ambiguity, because TNOs have, by definition, the usage depends on the author and the field of research. Detailed analytical and numerical studies of Neptune’s resonances have shown that the objects must have a precise range of energies. If the objects semi-major axis is outside these ranges, the orbit becomes chaotic. As TNOs were discovered, more than 10% were found to be in 2,3 resonances and it is now believed that the objects have been collected from wider distances by sweeping resonances during the migration of Neptune. During this relatively short period of time, Neptunes resonances would be sweeping the space, the 2,3 resonance at 39.4 AU is by far the dominant category among the resonant objects, with 92 confirmed and 104 possible member bodies. The objects following orbits in this resonance are named plutinos after Pluto, the objects are rather small and most of them follow orbits close to the ecliptic. Twotinos have inclinations less than 15 degrees and generally moderate eccentricities, there are far fewer objects in this resonance than plutinos. Consequently, it might be that twotinos were originally as numerous as plutinos and these Neptune trojans, termed by analogy to the Trojan asteroids, are in 1,1 resonance with Neptune. One of the concerns is that weak resonances may exist and would be difficult to due to the current lack of accuracy in the orbits of these distant objects. Many objects have orbital periods of more than 300 years and most have only observed over a short observation arc of a couple years. A true resonance will smoothly oscillate while a coincidental near resonance will circulate, simulations by Emel’yanenko and Kiseleva in 2007 show that 2001 XT254 is librating in a 3,7 resonance with Neptune. This libration can be stable for less than 100 million to billions of years, Emel’yanenko and Kiseleva also show that 1995 TL8 appears to have less than a 1% probability of being in a 3,7 resonance with Neptune, but it does execute circulations near this resonance. The classes of TNO have no universally agreed definitions, the boundaries are often unclear. The Deep Ecliptic Survey introduced formally defined dynamical classes based on long-term forward integration of orbits under the combined perturbations from all four giant planets
11.
Plutino
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In astronomy, a plutino is a trans-Neptunian object in 2,3 mean-motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times, the term plutino derived from the dwarf planet Pluto, the largest and the first plutino discovered. The term does not imply common physical characteristics, Plutinos are named after mythological creatures associated with the underworld. Plutinos form the part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects. Plutinos are the largest class of the resonant trans-Neptunian objects, aside from Pluto itself, the first plutino,1993 RO, was discovered on September 16,1993. It is thought that objects that are currently in mean orbital resonances with Neptune initially followed independent heliocentric paths. As Neptune migrated outward early in the Solar Systems history, the bodies it approached would have been scattered, during this process, the 3,2 resonance is the strongest and most stable among all resonances. This is the reason it contains the largest number of bodies. The orbital periods of plutinos cluster around 247.3 years, the gravitational influence of Pluto is usually neglected given its small mass. However, the width is very narrow and only a few times larger than Pluto’s Hill sphere. Consequently, depending on the eccentricity, some plutinos will be driven out of the resonance by interactions with Pluto. Numerical simulations suggest that the orbits of plutinos with an eccentricity 10%–30% smaller or larger than that of Pluto are not stable over Ga timescales, the plutinos brighter than HV=6 include, David Jewitt on Plutinos Minor Planet Center, List of TNOs MPC List of Distant Minor Planets
12.
Scattered disc
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The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40° and these extreme orbits are thought to be the result of gravitational scattering by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. Although the closest scattered-disc objects approach the Sun at about 30–35 AU and this makes scattered objects among the most distant and coldest objects in the Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, many Oort cloud objects are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, during the 1980s, the use of CCD-based cameras in telescopes made it possible to directly produce electronic images that could then be readily digitized and transferred to digital images. Because the CCD captured more light than film and the blinking could now be done at a computer screen. A flood of new discoveries was the result, over a thousand objects were detected between 1992 and 2006. The first scattered-disc object to be recognised as such was 1996 TL66, three more were identified by the same survey in 1999,1999 CV118,1999 CY118, and 1999 CF119. The first object presently classified as an SDO to be discovered was 1995 TL8, as of 2011, over 200 SDOs have been identified, including 2007 UK126,2002 TC302, Eris, Sedna and 2004 VN112. Known trans-Neptunian objects are divided into two subpopulations, the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the Oort cloud, has been hypothesized, some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with detached objects. Those in 3,2 resonances are known as plutinos, because Pluto is the largest member of their group, in contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune. Scattered-disc objects come within range of Neptune at their closest approaches. Some objects, like 1999 TD10, blur the distinction and the Minor Planet Center, the MPC also makes a clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits from those in scattered orbits. Another term used is scattered Kuiper-belt object for bodies of the scattered disc and this delineation is inadequate over the age of the Solar System, since bodies trapped in resonances could pass from a scattering phase to a non-scattering phase numerous times. That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the disc over time. In the a >30 AU region, the region of the Solar System populated by objects with semi-major axes greater than 30 AU, the Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Under this definition, an object with a greater than 40 AU could be classified as outside the scattered disc
13.
Detached object
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Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects. This reflects the dynamical gradation that can exist between the parameters of the scattered disk and the detached population. At least nine such bodies have been identified, of which the largest, most distant. Those with perihelia greater than 75 AU are termed sednoids, as of 2016, there are two known sednoids, Sedna and 2012 VP113. Detached objects have much larger than Neptunes aphelion. They often have highly elliptical, very large orbits with semi-major axes of up to a few hundred astronomical units, such orbits cannot have been created by gravitational scattering by the giant planets. Instead, a number of explanations have been put forward, including an encounter with a star or a distant planet-sized object. The classification suggested by the Deep Ecliptic Survey team introduces a distinction between scattered-near objects and scattered-extended objects using a Tisserands parameter value of 3. Detached objects are one of five distinct classes of TNO, the other four classes are classical Kuiper-belt objects, resonant objects, scattered-disc objects. Detached objects generally have a distance greater than 40 AU, deterring strong interactions with Neptune. However, there are no boundaries between the scattered and detached regions, since both can coexist as TNOs in an intermediate region with perihelion distance between 37 and 40 AU. One such intermediate body with a well determined orbit is 2003 FY128, although Sedna is officially considered a scattered-disc object by the MPC, its discoverer Michael E. This classification of Sedna as an object is accepted in recent publications. They have orbital periods of more than 300 years and most have only observed over a short observation arc of a couple years. Further improvement in the orbit and potential resonance of objects will help to understand the migration of the giant planets. For example, simulations by Emel’yanenko and Kiseleva in 2007 show that many distant objects could be in resonance with Neptune. They show a 10% likelihood that 2000 CR105 is in a 20,1 resonance, a 38% likelihood that 2003 QK91 is in a 10,3 resonance, and an 84% likelihood that 2000 YW134 is in an 8,3 resonance. The likely dwarf planet 2005 TB190 appears to have less than a 1% likelihood of being in a 4,1 resonance
14.
Sednoid
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A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. Only two objects are known from this population,90377 Sedna and 2012 VP113, both of which have greater than 75 AU, but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU and they are usually grouped with the detached objects. The sednoids orbits cannot be explained by perturbations from the giant planets and their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt such as the hypothesized Planet Nine. They could have captured from around passing stars, most likely in the Suns birth cluster. The two sednoids, like all of the more extreme detached objects, have an orientation of ≈ 0°.5 Gyr for Sedna. This suggests that one or more undiscovered massive perturbers may exist in the outer Solar System, a super-Earth at 250 AU would cause these objects to librate around ω = 0°±60° for billions of years. There are multiple possible configurations and a low-albedo super-Earth at that distance would have an apparent magnitude below the current all-sky-survey detection limits and this hypothetical super-Earth has been dubbed Telisto and Planet Nine. Larger, more-distant perturbers would also be too faint to be detected, fourteen known objects have a semi-major axis greater than 150 AU, a perihelion beyond Neptune, an argument of perihelion of 340°±55°, and an observation arc of more than 1 year. Each of the mechanisms for Sednas extreme orbit would leave a distinct mark on the structure. If a trans-Neptunian planet was responsible, all such objects would share roughly the same perihelion, if it rotated in the opposite direction, then two populations would form, one with low and one with high inclinations. The perturbations from passing stars would produce a variety of perihelia and inclinations, each dependent on the number. Acquiring a larger sample of objects would therefore help in determining which scenario is most likely. I call Sedna a fossil record of the earliest Solar System, eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and the number of stars that were close to the Sun when it formed. A 2007–2008 survey by Brown, Rabinowitz and Schwamb attempted to locate another member of Sednas hypothetical population, although the survey was sensitive to movement out to 1,000 AU and discovered the likely dwarf planet 2007 OR10, it detected no new sednoids. Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects probably exist in this region, new icy body hints at planet lurking beyond Pluto
15.
Dwarf planet
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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have observed in enough detail to demonstrate that they actually fit the IAUs definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto and they subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. The classification of bodies in other systems with the characteristics of dwarf planets has not been addressed. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for some decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the asteroid for the smaller bodies. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets and it was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was more than ten times as massive as the largest object in the asteroid belt, Ceres. In the 1990s, astronomers began to find objects in the region of space as Pluto. Many of these shared several of Plutos key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects and this led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported and it became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, Eris was discovered in January 2005, it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the became a matter of intense debate during the IAU General Assembly in August 2006. The IAUs initial draft proposal included Charon, Eris, and Ceres in the list of planets, dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. The IAUs final Resolution 5A preserved this three-category system for the bodies orbiting the Sun
16.
Plutoid
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A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i. e. a body orbiting beyond Neptune that is massive enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union working group Committee on Small Bodies Nomenclature, the term plutoid is not widely used by astronomers, though ice dwarf is not uncommon. There are thought to be thousands of plutoids in the Solar System, the IAU developed this category of astronomical objects as a consequence of its 2006 resolution defining the word planet. Satellites of plutoids are not plutoids themselves, on 24 August 2006, the IAU decided to reclassify Pluto as a dwarf planet, requiring that a planet must clear the neighbourhood around its orbit. The General Assembly of the IAU further resolved, Pluto is recognized as the prototype of a new category of Trans-Neptunian Objects and this new category had been proposed under the name pluton or a plutonian object earlier in the General Assembly. The former was rejected, in part because pluton is a geological term, the definition of the category also fluctuated during its early stages. Ultimately, the final resolution left the definition, like the name. The term was announced after the Executive Committee meeting, along with a simplified definition. However, due to an error, the IAU Working Group for Planetary System Nomenclature was not consulted. As of 2009, the term had not been accepted by the scientific community. Prior to the emergence of the term plutoid as an IAU-sanctioned categorization, the term ice dwarf saw some adoption as a near-synonym to the eventual term plutoid. There are large numbers of objects in the Oort cloud. However, it is not clear whether all so-categorized ice dwarfs are actually icier than icy asteroids such as Ceres, nonetheless, Ceres is sometimes called a terrestrial dwarf to distinguish it from Pluto and Eris. This conception foreshadowed the reclassification of Pluto to dwarf planet and plutoid after the discovery of Eris, with the creation of the term dwarf planet, some ambiguity was created as to which of two IAU bodies would be responsible for naming dwarf planets. Eris had been named through the IAU Committee on Small Body Nomenclature, along with announcing the name plutoid, the IAU decision of 11 June 2008 institutionalized this cooperative process involving the two bodies in the naming of new plutoids. In keeping with minor planet naming guidelines, priority will be given to names proposed by the discovery teams, and plutoids may not share a name with a small Solar System body. When the definition of planet was instated at the IAU General Assembly of 2006, Ceres, Pluto. However, precise regulations as to how hydrostatic equilibrium would be measured were left undefined for the time being, without an official procedure for calculating the lower bound of size to be a dwarf planet, no further bodies could be formally recognized as either dwarf planets or plutoids
17.
Jan Oort
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Jan Hendrik Oort ForMemRS was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. ”In 1955, Oort’s name appeared in Life Magazine’s list of the 100 most famous living people. He has been described as “putting the Netherlands in the forefront of postwar astronomy. ”Oort determined that the Milky Way rotates and overturned the idea that the Sun was at its center. He discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk. ”The Oort cloud, the Oort constants, and the Asteroid,1691 Oort, were all named after him. Oort was born in Franeker, a town in the Dutch province of Friesland, on April 28,1900. ”Several of Oort’s uncles were pastors. “My mother kept up her interests in that, at least in the years of her marriage. “But my father was interested in Church matters. ”In 1903 Oort’s parents moved to Oegstgeest, near Leiden. Oorts father, “was a medical director in a sanitorium for nervous illnesses. We lived in the house of the sanitorium, in a small forest which was very nice for the children, of course, to grow up in. ”Oort’s younger brother, John. In addition to John, Oort had two sisters and an older brother who died of diabetes when he was a student. Oort attended primary school in Oegstgeest and secondary school in Leiden, Oort later said that he had become interested in science and astronomy during his high-school years, and conjectured that his interest was stimulated by reading Jules Verne. His one hesitation about studying pure science was the concern that it “might alienate one a bit from people in general, ” as a result of which “one might not develop the human factor sufficiently. Oort chose Groningen partly because a well known astronomer, Jacobus Cornelius Kapteyn, was teaching there, after studying with Kapteyn, Oort decided on astronomy. “It was the personality of Professor Kapteyn which decided me entirely, “He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating. ”Oort began working on research with Kapteyn early in his third year. According to Oort one professor at Groningen who had influence on his education was physicist Frits Zernike. At Yale, Oort was responsible for making observations with the Observatory’s zenith telescope. “I worked on the problem of variation, ” he later recalled, “which is quite far away from the subjects I had so far been studying. ”Personally, he “felt somewhat lonesome in Yale, ” but also said that “some of my very best friends were made in these years in New Haven. ”In 1924, Oort returned to the Netherlands to work at Leiden University, where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930. In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars, Oort provided two formulae that described galactic rotation, the two constants that figured in these formulae are now known as “Oort’s constants. He also showed that stars lying in the regions of the galactic disk rotated more slowly than those nearer the center
18.
Scientific theory
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Established scientific theories have withstood rigorous scrutiny and are a comprehensive form of scientific knowledge. It is important to note that the definition of a theory as used in the disciplines of science is significantly different from the common vernacular usage of the word theory. These different usages are comparable to the differing, and often opposing, usages of the prediction in science versus prediction in vernacular speech. The strength of a theory is related to the diversity of phenomena it can explain. In certain cases, the less-accurate unmodified scientific theory can still be treated as an if it is useful as an approximation under specific conditions. Scientific theories are testable and make falsifiable predictions and they describe the causal elements responsible for a particular natural phenomenon, and are used to explain and predict aspects of the physical universe or specific areas of inquiry. Scientists use theories as a foundation to further scientific knowledge. As with other forms of knowledge, scientific theories are both deductive and inductive in nature and aim for predictive power and explanatory capability. Paleontologist, evolutionary biologist, and science historian Stephen Jay Gould said, “. facts and theories are different things, not rungs in a hierarchy of increasing certainty. Theories are structures of ideas that explain and interpret facts. ”The defining characteristic of all scientific knowledge, the relevance and specificity of those predictions determine how potentially useful the theory is. A would-be theory that makes no observable predictions is not a theory at all. Predictions not sufficiently specific to be tested are similarly not useful, in both cases, the term theory is not applicable. A body of descriptions of knowledge can be called a theory if it fulfills the following criteria and it is well-supported by many independent strands of evidence, rather than a single foundation. It is consistent with preexisting experimental results and at least as accurate in its predictions as are any preexisting theories and these qualities are certainly true of such established theories as special and general relativity, quantum mechanics, plate tectonics, the modern evolutionary synthesis, etc. It is among the most parsimonious explanations, economical in the use of proposed entities or explanatory steps as per Occams razor. The United States National Academy of Sciences defines scientific theories as follows and it refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. Such fact-supported theories are not guesses but reliable accounts of the real world, the theory of biological evolution is more than just a theory. It is as factual an explanation of the universe as the theory of matter or the germ theory of disease
19.
Volatiles
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In planetary science, volatiles are the group of chemical elements and chemical compounds with low boiling points that are associated with a planets or moons crust or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane, in astrogeology, these compounds, in their solid state, often comprise large proportions of the crusts of moons and dwarf planets. In contrast with volatiles, elements and compounds with high boiling points are known as refractory substances, the terms gas and ice in this context can apply to compounds that may be solids, liquids or gases. The Moon is very low in volatiles, its crust contains oxygen chemically bound into the rocks, in igneous petrology the term more specifically refers to the volatile components of magma that affect the appearance and explosivity of volcanoes. Volatiles in a magma with a high viscosity, generally felsic with a silica content. Volatiles in a magma with a low viscosity, generally mafic with a silica content, tend to vent. Some volcanic eruptions are explosive because the mixing water and magma reaching the surface, releases energy suddenly. Moreover, in cases, the eruption is caused by volatiles dissolved in the magma. Approaching the surface, pressure decreases and the volatiles evolve creating bubbles that circulate in the liquid, the bubbles are connected together forming a network. This especially increments the fragmentation into small drops or spray or coagulate clots in gas, generally, 95-99% of magma is liquid rock. However, the percentage of gas present, represents a very large volume when it expands on reaching atmospheric pressure. Gas is a preponderant part in a system because it generates explosive eruptions. Magma in the mantle and lower crust have a lot of volatiles within and water, also they leak hydrogen sulfide and sulfur dioxide. Sulfur dioxide is usually possible to find in basaltic and rhyolite rocks, volcanoes also release a high amount of hydrogen chloride and hydrogen fluoride as volatiles. There are three factors that effect the dispersion of volatiles in magma, confining pressure, composition of magma. Pressure and composition are the most important parameters, to understand how the magma behaves rising the surface, the role of solubility within the magma must be known. An empirical law has used for different magma-volatiles combination. For instance, for water in magma the equation is n=0.1078 P where n is the amount of dissolved gas as weight percentage, the value changes for example for water in rhyolite where n=0.4111 P and for the carbon dioxide is n=0.0023 P
20.
Planetesimal
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Planetesimals /plænᵻˈtɛsᵻməlz/ are solid objects thought to exist in protoplanetary disks and in debris disks. When the bodies reach sizes of one kilometer, then they can attract each other directly through their mutual gravity. This is how planetesimals are often defined, bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. It is generally thought that about 3, a few planetesimals may have been captured as moons, such as Phobos and Deimos and many of the small high-inclination moons of the giant planets. Planetesimals that have survived to the current day are valuable to science because they contain information about the formation of the Solar System, although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a time capsule, and their composition might reveal the conditions in the Solar Nebula from which our planetary system was formed, the word planetesimal comes from the mathematical concept infinitesimal and literally means an ultimately small fraction of a planet. This corresponds to objects larger than approximately 1 km in the solar nebula, bodies large enough not only to keep together by gravitation but to change the path of approaching rocks over distances of several radii start to grow faster. These bodies, larger than 100 km to 1000 km, are called embryos or protoplanets, in other words, some planetesimals became other types of body once planetary formation had finished, and may be referred to by either or both names. The above definition is not endorsed by the International Astronomical Union, there is also no exact dividing line between a planetesimal and protoplanet. Discovering the Essential Universe by Neil F. Comins Linda T. Elkins-Tanton, planetesimals - Early Differentiation and Consequences for Planets. Cambridge University Press, Cambridge 2017, ISBN9781107118485
21.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
22.
Circumstellar disc
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A circumstellar disc is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form, around mature stars, they indicate that planetesimal formation has taken place and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways, the infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disc around the young, rotating star. The former is a circumstellar disc of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, the main accretion phase lasts a few million years, with accretion rates typically between 10−7 and 10−9 solar masses per year. The disc gradually cools in what is known as the T Tauri star stage, within this disc, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimals. If the disc is sufficiently massive, the runaway accretions begin, the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form, asteroid belt is a reservoir of small bodies in the Solar System located between the orbit of Mars and Jupiter. It is a source of interplanetary dust, edgeworth-Kuiper belt, beyond the orbit of Neptune Scattered disc, beyond the orbit of Neptune Hills cloud, only the inner Oort cloud has a toroid-like shape. The outer Oort cloud is more spherical in shape, making it a circumstellar envelope, the infall of gas onto a binary system allows the formation of circumstellar and circumbinary discs. The formation of such a disc will occur for any system in which infalling gas which contains some degree of angular momentum. A general progression of disc formation is observed with increasing levels of angular momentum and this type of disc will form through accretion if any angular momentum is present in the infalling gas. Circumsecondary disc is one which orbits around the star of the binary star system. This type of disc will form when a high enough level of angular momentum is present within the infalling gas. The amount of angular momentum required is dependent on the mass ratio. Circumbinary disc is one which orbits about both the primary and secondary stars, such a disc will form at a later time than the circumprimary and circumsecondary discs, with an inner radius much larger than the orbital radius of the binary system. An example of a disc may be seen around the star system GG Tauri. Once a circumstellar disc has formed, spiral density waves are created within the material via a differential torque due to the binarys gravity
23.
Heliosphere
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The heliosphere is the bubble-like region of space dominated by the Sun, which extends far beyond the orbit of Pluto. The solar wind flows outward from the Sun until encountering the termination shock, the overall shape of the heliosphere is controlled by the interstellar medium through which it is traveling, as well as the Sun, and is not perfectly spherical. The limited data available and unexplored nature of structures have resulted in many theories. On September 12,2013, NASA announced that Voyager 1 had exited the heliosphere on August 25,2012, originating at the extremely hot surface of the corona, solar wind particles reach escape velocity, streaming outwards at 300 to 800 km/s. As it begins to interact with the medium, its velocity slows before finally stopping altogether. The termination shock was traversed by Voyager 1 in 2004, and it may be a more gentle bow wave. Voyager data led to a new theory that the heliosheath has magnetic bubbles, the stagnation region within the heliosheath, starting around 113 au, was detected by Voyager 1 in 2010. There the solar wind velocity drops to zero, the field intensity doubles. Starting in May 2012 at 120 au, Voyager 1 detected an increase in cosmic rays. In the summer of 2013 NASA announced that Voyager 1 had reached interstellar space as of August 25,2012, cassini and IBEX data challenged the heliotail theory in 2009. In July 2013, IBEX results revealed a 4-lobed tail on the Solar Systems heliosphere, the solar wind consists of particles and fields. Because the Sun rotates once approximately every 25 days, the magnetic field transported by the wind gets wrapped into a spiral. Variations in the Suns magnetic field are carried outward by the solar wind, the heliospheric current sheet is a ripple in the heliosphere created by the rotating magnetic field of the Sun. Extending throughout the heliosphere, it is considered the largest structure in the Solar System and is said to resemble a ballerinas skirt, the outer structure of the heliosphere is determined by the interactions between the solar wind and the winds of interstellar space. The solar wind away from the Sun in all directions at speeds of several hundred km/s in the Earths vicinity. At some distance from the Sun, well beyond the orbit of Neptune and this takes place in several stages, The solar wind is traveling at supersonic speeds within the Solar System. At the termination shock, a shock wave, the solar wind falls below the speed of sound. However, observations in 2009 showed that model is incorrect
24.
Outer space
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Outer space or just space, is the void that exists between celestial bodies, including Earth. The baseline temperature, as set by the radiation from the Big Bang, is 2.7 kelvins. In most galaxies, observations provide evidence that 90% of the mass is in a form, called dark matter. Data indicates that the majority of the mass-energy in the universe is a poorly understood vacuum energy of space which astronomers label dark energy. Intergalactic space takes up most of the volume of the Universe, there is no firm boundary where outer space starts. However the Kármán line, at an altitude of 100 km above sea level, is used as the start of outer space in space treaties. The framework for international law was established by the Outer Space Treaty. This treaty precludes any claims of sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the uses of outer space. Humans began the exploration of space during the 20th century with the advent of high-altitude balloon flights. Earth orbit was first achieved by Yuri Gagarin of the Soviet Union in 1961, due to the high cost of getting into space, manned spaceflight has been limited to low Earth orbit and the Moon. Outer space represents an environment for human exploration because of the dual hazards of vacuum. Microgravity also has an effect on human physiology that causes both muscle atrophy and bone loss. In addition to health and environmental issues, the economic cost of putting objects, including humans. In 350 BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum and this concept built upon a 5th-century BCE ontological argument by the Greek philosopher Parmenides, who denied the possible existence of a void in space. Based on this idea that a vacuum could not exist, in the West it was held for many centuries that space could not be empty. As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled, in ancient China, there were various schools of thought concerning the nature of the heavens, some of which bear a resemblance to the modern understanding. In the 2nd century, astronomer Zhang Heng became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun, the surviving books of the Hsüan Yeh school said that the heavens were boundless, empty and void of substance
25.
Cosmography
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Cosmography is the science that maps the general features of the cosmos or universe, describing both heaven and Earth. The 14th-century work Ajaib al-makhluqat wa-gharaib al-mawjudat by Arab physician Zakariya al-Qazwini is considered to be a work of cosmography. Traditional Hindu, Buddhist and Jain cosmography schematize a universe centered on Mount Meru surrounded by rivers, continents and these cosmographies posit a universe being repeatedly created and destroyed over time cycles of immense lengths. In 1551, Martín Cortés de Albacar, from Zaragoza, Spain, translated into English and reprinted several times, the work was of great influence in Britain for many years. He proposed spherical charts and mentioned magnetic deviation and the existence of magnetic poles, peter Heylins 1652 book Cosmographie was one of the earliest attempts to describe the entire world in English, and being the first known description of Australia and among the first of California. In 1659, Thomas Porter published a smaller, but extensive Compendious Description of the Whole World and these were all part of a major trend in the European Renaissance to explore the known world. The word was commonly used by Buckminster Fuller in his lectures
26.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
27.
Hill sphere
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An astronomical bodys Hill sphere is the region in which it dominates the attraction of satellites. The outer shell of that constitutes a zero-velocity surface. To be retained by a planet, a moon must have an orbit that lies within the planets Hill sphere and that moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, one simple view of the extent of the Solar System is the Hill sphere of the Sun with respect to local stars and the galactic nucleus. In more precise terms, the Hill sphere approximates the gravitational sphere of influence of a body in the face of perturbations from a more massive body. It was defined by the American astronomer George William Hill, based on the work of the French astronomer Édouard Roche, for this reason, it is also known as the Roche sphere. In the example to the right, the Hill sphere extends between the Lagrangian points L1 and L2, which lie along the line of centers of the two bodies. The region of influence of the body is shortest in that direction. When eccentricity is negligible, this becomes r ≈ a m 3 M3, in the Earth example, the Earth orbits the Sun at a distance of 149.6 million km. The Hill sphere for Earth thus extends out to about 1.5 million km, all stable satellites of the Earth must have an orbital period shorter than seven months. The previous formula can be re-stated as follows,3 r 3 a 3 ≈ m M and this is also convenient, because many planetary astronomers work in and remember distances in units of planetary radii. The Hill sphere is only an approximation, and other forces can eventually perturb an object out of the sphere and this third object should also be of small enough mass that it introduces no additional complications through its own gravity. The region of stability for retrograde orbits at a distance from the primary, is larger than the region for prograde orbits at a large distance from the primary. This was thought to explain the preponderance of retrograde moons around Jupiter, however, a sphere of this size and mass would be denser than lead. In fact, in any low Earth orbit, a body must be more dense than lead in order to fit inside its own Hill sphere. A spherical geostationary satellite, however, would only need to be more than 6% of the density of water to support satellites of its own. Within the Solar System, the planet with the largest Hill radius is Neptune, with 116 million km, or 0.775 au, an asteroid from the asteroid belt will have a Hill sphere that can reach 220000 km, diminishing rapidly with decreasing mass. The Hill sphere of 1999 KW4, a Mercury-crosser asteroid that has a moon, a typical extrasolar hot Jupiter, HD209458 b, has a Hill sphere radius of 593,000 km, about 8 times its physical radius of approx 71,000 km
28.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
29.
Comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of collections of ice, dust. The coma may be up to 15 times the Earths diameter, if sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars, hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition, Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma and the tail, however, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. As of November 2014 there are 5,253 known comets, however, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
30.
Short-period comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of collections of ice, dust. The coma may be up to 15 times the Earths diameter, if sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars, hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition, Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma and the tail, however, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. As of November 2014 there are 5,253 known comets, however, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
31.
Giant planet
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A giant planet is any massive planet. They are usually composed of low-boiling-point materials, rather than rock or other solid matter. There are four giant planets in the Solar System, Jupiter, Saturn, Uranus. Many extrasolar giant planets have been identified orbiting other stars, giant planets are also sometimes called jovian planets, after Jupiter. They are also known as gas giants. However, many astronomers apply the term only to Jupiter and Saturn, classifying Uranus and Neptune. Both names are misleading, all of the giant planets consist primarily of fluids above their critical points. The principal components are hydrogen and helium in the case of Jupiter and Saturn, the defining differences between a very low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation, the other, part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history. The term gas giant was coined in 1952 by the fiction writer James Blish and was originally used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form. Other than solids in the core and the layers of the atmosphere, all matter is above the critical point. Fluid planet would be an accurate term. Jupiter also has metallic hydrogen near its center, but much of its volume is hydrogen, helium, the observable atmospheres of all these planets are quite thin compared to their radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous, in the outer Solar System, hydrogen and helium are referred to as gases, water, methane, and ammonia as ices, and silicates and metals as rock. When deep planetary interiors are considered, it may not be far off to say that, by ice astronomers mean oxygen and carbon, by rock they mean silicon, and by gas they mean hydrogen and helium. The many ways in which Uranus and Neptune differ from Jupiter, with this terminology in mind, some astronomers have started referring to Uranus and Neptune as ice giants to indicate the predominance of the ices in their interior composition. The alternative term jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, objects large enough to start deuterium fusion are called brown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-mass stars
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Formation and evolution of the Solar System
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The formation of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. This model, known as the hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant. Its subsequent development has interwoven a variety of disciplines including astronomy, physics, geology. Since the dawn of the age in the 1950s and the discovery of extrasolar planets in the 1990s. The Solar System has evolved considerably since its initial formation, many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later been captured by their planets. Still others, such as Earths Moon, may be the result of giant collisions, collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System. The positions of the planets often shifted due to gravitational interactions and this planetary migration is now thought to have been responsible for much of the Solar Systems early evolution. In the far distant future, the gravity of passing stars will gradually reduce the Suns retinue of planets, some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of tens of billions of years, the first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had developed for millennia, but was not widely accepted until the end of the 17th century, the first recorded use of the term Solar System dates from 1704. The most significant criticism of the hypothesis was its apparent inability to explain the Suns relative lack of momentum when compared to the planets. However, since the early 1980s studies of stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts. Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power, in 1935, Eddington went further and suggested that other elements also might form within stars. Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen, when a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems. The nebular hypothesis says that the Solar System formed from the collapse of a fragment of a giant molecular cloud. The cloud was about 20 parsec across, while the fragments were roughly 1 parsec across, the further collapse of the fragments led to the formation of dense cores 0. 01–0.1 pc in size. One of these fragments formed what became the Solar System. The remaining 2% of the mass consisted of elements that were created by nucleosynthesis in earlier generations of stars
33.
Halley's Comet
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Halleys Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is visible to the naked eye from Earth. Halley last appeared in the parts of the Solar System in 1986. Halleys returns to the inner Solar System have been observed and recorded by astronomers since at least 240 BC, clear records of the comets appearances were made by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object at the time. The comets periodicity was first determined in 1705 by English astronomer Edmond Halley, Comet Halley is commonly pronounced /ˈhæli/, rhyming with valley, or /ˈheɪli/, rhyming with daily. Spellings of Edmond Halleys name during his lifetime included Hailey, Haley, Hayley, Halley, Hawley, Halley was the first comet to be recognized as periodic. Until the Renaissance, the consensus on the nature of comets. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon, many were still unconvinced that comets orbited the Sun, and assumed instead that they must follow straight paths through the Solar System. In 1687, Sir Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, in which he outlined his laws of gravity and his work on comets was decidedly incomplete. Halley thus concluded that all three comets were, in fact, the same object returning every 76 years, a period that has since been amended to every 75–76 years. After a rough estimate of the perturbations the comet would sustain from the attraction of the planets. Halley died in 1742 before he could observe this himself, Halleys prediction of the comets return proved to be correct, although it was not seen until 25 December 1758, by Johann Georg Palitzsch, a German farmer and amateur astronomer. It did not pass through its perihelion until 13 March 1759 and this effect was computed prior to its return by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute. The confirmation of the return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of Newtonian physics, the comet was first named in Halleys honour by French astronomer Nicolas Louis de Lacaille in 1759. Some scholars have proposed that first-century Mesopotamian astronomers already had recognized Halleys Comet as periodic and this theory notes a passage in the Bavli Talmud that refers to a star which appears once in seventy years that makes the captains of the ships err. It was necessary to use ancient Chinese comet observations to constrain their calculations, Halleys orbital period over the last 3 centuries has been between 75–76 years, although it has varied between 74–79 years since 240 BC. Its orbit around the Sun is highly elliptical, with an eccentricity of 0.967
34.
Jupiter
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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, Jupiter and Saturn are gas giants, the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity, the Romans named it after their god Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium and it may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rotation, the planets shape is that of an oblate spheroid. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence, a prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere, Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4,2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa, Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun. Astronomers have discovered nearly 500 planetary systems with multiple planets, Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed primarily of gaseous and liquid matter and it is the largest of the four giant planets in the Solar System and hence its largest planet. It has a diameter of 142,984 km at its equator, the average density of Jupiter,1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiters upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, a helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiters atmosphere is approximately 75% hydrogen and 24% helium by mass, the atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, the outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found
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Ecliptic
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The ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system. It also refers to the plane of this path, which is coplanar with the orbit of Earth around the Sun, the motions as described above are simplifications. Due to the movement of Earth around the Earth–Moon center of mass, due to further perturbations by the other planets of the Solar System, the Earth–Moon barycenter wobbles slightly around a mean position in a complex fashion. The ecliptic is actually the apparent path of the Sun throughout the course of a year, because Earth takes one year to orbit the Sun, the apparent position of the Sun also takes the same length of time to make a complete circuit of the ecliptic. With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward every day, again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun. The actual speed with which Earth orbits the Sun varies slightly during the year, for example, the Sun is north of the celestial equator for about 185 days of each year, and south of it for about 180 days. The variation of orbital speed accounts for part of the equation of time, if the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes. The Sun, in its apparent motion along the ecliptic, crosses the equator at these points, one from south to north. The crossing from south to north is known as the equinox, also known as the first point of Aries. The crossing from north to south is the equinox or descending node. Likewise, the ecliptic itself is not fixed, the gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earths orbit, and hence of the ecliptic, known as planetary precession. The combined action of two motions is called general precession, and changes the position of the equinoxes by about 50 arc seconds per year. Once again, this is a simplification, periodic motions of the Moon and apparent periodic motions of the Sun cause short-term small-amplitude periodic oscillations of Earths axis, and hence the celestial equator, known as nutation. Obliquity of the ecliptic is the used by astronomers for the inclination of Earths equator with respect to the ecliptic. It is about 23. 4° and is currently decreasing 0.013 degrees per hundred years due to planetary perturbations, the angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. From 1984, the Jet Propulsion Laboratorys DE series of computer-generated ephemerides took over as the ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated, jPLs fundamental ephemerides have been continually updated. J. Laskar computed an expression to order T10 good to 0″. 04/1000 years over 10,000 years, all of these expressions are for the mean obliquity, that is, without the nutation of the equator included
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Isotropy
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Isotropy is uniformity in all orientations, it is derived from the Greek isos and tropos. Precise definitions depend on the subject area, exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary systematically, Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. Within mathematics, isotropy has a few different meanings, Isotropic manifolds A manifold is isotropic if the geometry on the manifold is the same regardless of direction, a manifold can be homogeneous without being isotropic, but if it is inhomogeneous, it is necessarily anisotropic. Isotropic quadratic form A quadratic form q is said to be if there is a non-zero vector v such that q =0. In complex geometry, a line through the origin in the direction of a vector is an isotropic line. Isotropic coordinates Isotropic coordinates are coordinates on a chart for Lorentzian manifolds. Isotropy group An isotropy group is the group of isomorphisms from any object to itself in a groupoid, Isotropic position A probability distribution over a vector space is in isotropic position if its covariance matrix is the identity. This follows from invariance of the Hamiltonian, which in turn is guaranteed for a spherically symmetric potential. Kinetic theory of gases is also an example of isotropy and it is assumed that the molecules move in random directions and as a consequence, there is an equal probability of a molecule moving in any direction. Thus when there are molecules in the gas, with high probability there will be very similar numbers moving in one direction as any other hence demonstrating approximate isotropy. Fluid dynamics Fluid flow is isotropic if there is no directional preference, an example of anisotropy is in flows with a background density as gravity works in only one direction. The apparent surface separating two differing isotropic fluids would be referred to as an isotrope, thermal expansion A solid is said to be isotropic if the expansion of solid is equal in all directions when thermal energy is provided to the solid. Electromagnetics An isotropic medium is one such that the permittivity, ε, and permeability, μ, of the medium are uniform in all directions of the medium, optics Optical isotropy means having the same optical properties in all directions. The individual reflectance or transmittance of the domains is averaged if the macroscopic reflectance or transmittance is to be calculated, cosmology The Big Bang theory of the evolution of the observable universe assumes that space is isotropic. It also assumes that space is homogeneous and these two assumptions together are known as the Cosmological Principle. As of 2006, the observations suggest that, on scales much larger than galaxies, galaxy clusters are Great features. Here homogeneous means that the universe is the same everywhere and isotropic implies that there is no preferred direction, in the study of mechanical properties of materials, isotropic means having identical values of a property in all directions
37.
Estonia
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Estonia, officially the Republic of Estonia, is a country in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia, across the Baltic Sea lies Sweden in the west and Finland in the north. The territory of Estonia consists of a mainland and 2,222 islands and islets in the Baltic Sea, covering 45,339 km2 of land and water, and is influenced by a humid continental climate. The territory of Estonia has been inhabited since at least 6500 BC, in 1988, during the Singing Revolution, the Estonian Supreme Soviet issued the Estonian Sovereignty Declaration in defiance of Soviet rule, and independence was restored on 20 August 1991. Estonia is a parliamentary republic divided into fifteen counties. Its capital and largest city is Tallinn, with a population of 1.3 million, it is one of the least-populous member states of the European Union, Eurozone, North Atlantic Treaty Organization, OECD and Schengen Area. Estonia is a country with an advanced, high-income economy that is among the fastest growing in the EU. Its Human Development Index ranks very highly, and it performs favourably in measurements of economic freedom, civil liberties, the 2015 PISA test places Estonian high school students 3rd in the world, behind Singapore and Japan. Citizens of Estonia are provided with health care, free education. Since independence the country has developed its IT sector, becoming one of the worlds most digitally advanced societies. In 2005 Estonia became the first nation to hold elections over the Internet, in the Estonian language, the oldest known endonym of the Estonians was maarahvas, meaning country people or people of the land. The land inhabited by Estonians was called Maavald meaning Country Parish or Land Parish, one hypothesis regarding the modern name of Estonia is that it originated from the Aesti, a people described by the Roman historian Tacitus in his Germania. The historic Aesti were allegedly Baltic people, whereas the modern Estonians are Finno-Ugric, the geographical areas between Aesti and Estonia do not match, with Aesti being further down south. Ancient Scandinavian sagas refer to a land called Eistland, as the country is called in Icelandic. Early Latin and other ancient versions of the name are Estia and Hestia, esthonia was a common alternative English spelling prior to 1921. Human settlement in Estonia became possible 13,000 to 11,000 years ago, the oldest known settlement in Estonia is the Pulli settlement, which was on the banks of the river Pärnu, near the town of Sindi, in south-western Estonia. According to radiocarbon dating it was settled around 11,000 years ago, the earliest human inhabitation during the Mesolithic period is connected to Kunda culture, which is named after the town of Kunda in northern Estonia. At that time the country was covered with forests, and people lived in communities near bodies of water
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Dutch people
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The Dutch, occasionally referred to as Netherlanders—a term that is cognate to the Dutch word for Dutch people, Nederlanders—are a Germanic ethnic group native to the Netherlands. They share a culture and speak the Dutch language. The high degree of urbanization characteristic of Dutch society was attained at an early date. During the Republic the first series of large scale Dutch migrations outside of Europe took place, despite the small size of the Netherlands, the Dutch left behind a legacy in excess of their mere numbers. The traditional art and culture of the Dutch encompasses various forms of music, dances, architectural styles and clothing. Internationally, Dutch painters such as Rembrandt, Vermeer and Van Gogh are held in high regard, the dominant religion of the Dutch is Christianity, although in modern times the majority is no longer religious. Significant percentages of the Dutch are adherents of humanism, agnosticism, atheism or individual spirituality, as with all ethnic groups the ethnogenesis of the Dutch has been a lengthy and complex process. The text below hence focuses on the history of the Dutch ethnic group, for Dutch national history, for Dutch colonial history, see the article on the Dutch Empire. Following the end of the period in the West around 500, with large federations settling the decaying Roman Empire. In the Low Countries, this began when the Franks, themselves a union of multiple smaller tribes. Eventually, in 358, the Salian Franks, one of the three main subdivisions among the Frankish alliance settled the areas Southern lands as foederati, Roman allies in charge of border defense. On a political level, the Frankish warlords abandoned tribalism and founded a number of kingdoms, however, the population make-up of the Frankish Empire, or even early Frankish kingdoms such as Neustria and Austrasia, was not dominated by Franks. Though the Frankish leaders controlled most of Western Europe, the Franks themselves were confined to the Northwestern part of the Empire, the current Dutch-French language border has remained virtually identical ever since, and could be seen as marking the furthest pale of gallicization among the Franks. The medieval cities of the Low Countries, which experienced major growth during the 11th and 12th century, were instrumental in breaking down the already relatively loose local form of feudalism, as they became increasingly powerful, they used their economical strength to influence the politics of their nobility. While the cities were of political importance, they also formed catalysts for medieval Dutch culture. The various city guilds as well as the necessity of water boards in the Dutch delta and it is also around this time, that ethnonyms such as Diets and Nederlands emerge. This process marked a new episode in the development of the Dutch ethnic group, as now political unity started to emerge, consolidating the strengthened cultural, despite their linguistic and cultural unity, and economic similarities, there was still little sense of political unity among the Dutch people. However, the centralist policies of Burgundy in the 14th and 15th centuries, at first violently opposed by the cities of the Low Countries, had a profound impact and changed this
39.
Perturbation (astronomy)
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In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body. The other forces can include a body, resistance, as from an atmosphere. The study of perturbations began with the first attempts to predict planetary motions in the sky, in ancient times the causes were a mystery. Newton, at the time he formulated his laws of motion and of gravitation, applied them to the first analysis of perturbations, the complex motions of gravitational perturbations can be broken down. The hypothetical motion that the body follows under the effect of one other body only is typically a conic section. This is called a problem, or an unperturbed Keplerian orbit. The differences between that and the motion of the body are perturbations due to the additional gravitational effects of the remaining body or bodies. If there is one other significant body then the perturbed motion is a three-body problem. A general analytical solution exists for the problem, when more than two bodies are considered analytic solutions exist only for special cases. Even the two-body problem becomes insoluble if one of the bodies is irregular in shape, most systems that involve multiple gravitational attractions present one primary body which is dominant in its effects. The gravitational effects of the bodies can be treated as perturbations of the hypothetical unperturbed motion of the planet or satellite around its primary body. In methods of general perturbations, general differential equations, either of motion or of change in the elements, are solved analytically, usually by series expansions. The result is expressed in terms of algebraic and trigonometric functions of the orbital elements of the body in question. This can be applied generally to many different sets of conditions, historically, general perturbations were investigated first. The classical methods are known as variation of the elements, variation of parameters or variation of the constants of integration, in these methods, it is considered that the body is always moving in a conic section, however the conic section is constantly changing due to the perturbations. General perturbations is applicable if the perturbing forces are about one order of magnitude smaller, or less. In the Solar System, this is usually the case, Jupiter, general perturbation methods are preferred for some types of problems, as the source of certain observed motions are readily found. In effect, the positions and velocities are perturbed directly, special perturbations can be applied to any problem in celestial mechanics, as it is not limited to cases where the perturbing forces are small
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Electromagnetic radiation
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In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, light, ultraviolet, X-, classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to other and perpendicular to the direction of energy and wave propagation. The wavefront of electromagnetic waves emitted from a point source is a sphere, the position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves are produced whenever charged particles are accelerated, and these waves can interact with other charged particles. EM waves carry energy, momentum and angular momentum away from their source particle, quanta of EM waves are called photons, whose rest mass is zero, but whose energy, or equivalent total mass, is not zero so they are still affected by gravity. Thus, EMR is sometimes referred to as the far field, in this language, the near field refers to EM fields near the charges and current that directly produced them, specifically, electromagnetic induction and electrostatic induction phenomena. In the quantum theory of electromagnetism, EMR consists of photons, quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation. The energy of a photon is quantized and is greater for photons of higher frequency. This relationship is given by Plancks equation E = hν, where E is the energy per photon, ν is the frequency of the photon, a single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light. The effects of EMR upon chemical compounds and biological organisms depend both upon the power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules, the effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons. In contrast, high ultraviolet, X-rays and gamma rays are called ionizing radiation since individual photons of high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the speed of light. Maxwell’s equations were confirmed by Heinrich Hertz through experiments with radio waves, according to Maxwells equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the field are always accompanied by a wave in the magnetic field in one direction
[astro-ph].
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