1.
Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
2.
Triton (moon)
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Triton is the largest natural satellite of the planet Neptune. It was discovered on October 10,1846, by English astronomer William Lassell and it is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planets rotation. At 2,700 kilometres in diameter, it is the seventh-largest moon in the Solar System, because of its retrograde orbit and composition similar to Plutos, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of frozen nitrogen, a mostly water-ice crust, an icy mantle. The core makes up two-thirds of its total mass, Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice. Triton is one of the few moons in the Solar System known to be geologically active, as a consequence, its surface is relatively young with sparse impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a nitrogen atmosphere less than 1/70,000 the pressure of Earths atmosphere at sea level. Triton was discovered by British astronomer William Lassell on October 10,1846, a brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptunes discovery, he wrote to Lassell suggesting he search for possible moons, Lassell did so and discovered Triton eight days later. Lassell also claimed to have discovered rings, although Neptune was later confirmed to have rings, they are so faint and dark that it is doubtful that he actually saw them. Triton is named after the Greek sea god Triton, the son of Poseidon, the name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, and was officially adopted many decades later. Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as the satellite of Neptune. Lassell did not name his own discovery, he successfully suggested the name Hyperion, previously chosen by John Herschel. Triton is unique among all large moons in the Solar System for its orbit around its planet. Most of the irregular moons of Jupiter and Saturn also have retrograde orbits. However, these moons are all much more distant from their primaries, and are small in comparison, Tritons orbit is associated with two tilts, the inclination of Neptunes spin to Neptunes orbit, 30°, and the inclination of Tritons orbit to Neptunes spin, 157°. Tritons orbit precesses forward relative to Neptunes spin with a period of about 678 Earth years and that inclination is currently 130°, Tritons orbit is now near its maximum departure from coplanarity with Neptunes. Tritons rotation is locked to be synchronous with its orbit around Neptune
3.
Voyager 2
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Voyager 2 is a space probe launched by NASA on August 20,1977, to study the outer planets. Part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach Jupiter and Saturn but enabled further encounters with Uranus and it is the only spacecraft to have visited either of the ice giants. Voyager 2 is now in its mission to study the outer reaches of the Solar System and has been operating for 39 years,7 months and 17 days. It remains in contact through the Deep Space Network, at a distance of 114 AU from the Sun as of April 5th,2017, Voyager 2 is one of the most distant human-made objects, along with Voyager 1, New Horizons, Pioneer 10 and Pioneer 11. The probe was moving at a velocity of 15.4 km/s relative to the Sun as of December 2014 and is traveling through the heliosheath, upon reaching interstellar space, Voyager 2 is expected to provide the first direct measurements of the density and temperature of the interstellar plasma. The spacecraft would be designed with redundant systems to ensure survival through the entire tour, by 1972 the mission was scaled back and replaced with two Mariner-derived spacecraft, the Mariner Jupiter-Saturn probes. To keep apparent lifetime program costs low, the mission would include only flybys of Jupiter and Saturn, as the program progressed, the name was changed to Voyager. The primary mission of Voyager 1 was to explore Jupiter, Saturn, Voyager 2 was also to explore Jupiter and Saturn, but on a trajectory that would have option of continuing on to Uranus and Neptune, or being redirected to Titan as a backup for Voyager 1. Upon successful completion of Voyager 1s objectives, Voyager 2 would get an extension to send the probe on towards Uranus. Collectively these instruments are part of the Attitude and Articulation Control Subsystem along with redundant units of most instruments and 8 backup thrusters, the spacecraft also included 11 scientific instruments to study celestial objects as it traveled through space. Built with the intent for eventual interstellar travel, Voyager 2 included a large,3.7 m parabolic, when the spacecraft is unable to communicate with Earth, the Digital Tape Recorder can record about 64 kilobytes of data for transmission at another time. The spacecraft was built with 3 Multihundred-Watt radioisotope thermoelectric generators, each RTG includes 24 pressed plutonium oxide spheres and provides enough heat to generate approximately 157 watts of power at launch. Collectively, the RTGs supply the spacecraft with 470 watts at launch, for more details on the Voyager space probes identical instrument packages, see the separate article on the overall Voyager Program. The Voyager 2 probe was launched on August 20,1977, by NASA from Space Launch Complex 41 at Cape Canaveral, Florida, two weeks later, the twin Voyager 1 probe would be launched on September 5,1977. However, Voyager 1 would reach both Jupiter and Saturn sooner, as Voyager 2 had been launched into a longer, more circular trajectory, Voyager 2s closest approach to Jupiter occurred on July 9,1979. It came within 570,000 km of the cloud tops. It discovered a few rings around Jupiter, as well as volcanic activity on the moon Io, the Great Red Spot was revealed as a complex storm moving in a counterclockwise direction. An array of other storms and eddies were found throughout the banded clouds
4.
Natural satellite
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A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
5.
Greek mythology
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It was a part of the religion in ancient Greece. Greek mythology is explicitly embodied in a collection of narratives. Greek myth attempts to explain the origins of the world, and details the lives and adventures of a variety of gods, goddesses, heroes, heroines. These accounts initially were disseminated in a tradition, today the Greek myths are known primarily from ancient Greek literature. The oldest known Greek literary sources, Homers epic poems Iliad and Odyssey, focus on the Trojan War, archaeological findings provide a principal source of detail about Greek mythology, with gods and heroes featured prominently in the decoration of many artifacts. Geometric designs on pottery of the eighth century BC depict scenes from the Trojan cycle as well as the adventures of Heracles, in the succeeding Archaic, Classical, and Hellenistic periods, Homeric and various other mythological scenes appear, supplementing the existing literary evidence. Greek mythology has had an influence on the culture, arts. Poets and artists from ancient times to the present have derived inspiration from Greek mythology and have discovered contemporary significance and relevance in the themes, Greek mythology is known today primarily from Greek literature and representations on visual media dating from the Geometric period from c. Mythical narration plays an important role in every genre of Greek literature. Nevertheless, the only general mythographical handbook to survive from Greek antiquity was the Library of Pseudo-Apollodorus and this work attempts to reconcile the contradictory tales of the poets and provides a grand summary of traditional Greek mythology and heroic legends. Apollodorus of Athens lived from c, 180–125 BC and wrote on many of these topics. His writings may have formed the basis for the collection, however the Library discusses events that occurred long after his death, among the earliest literary sources are Homers two epic poems, the Iliad and the Odyssey. Other poets completed the cycle, but these later and lesser poems now are lost almost entirely. Despite their traditional name, the Homeric Hymns have no connection with Homer. They are choral hymns from the part of the so-called Lyric age. Hesiods Works and Days, a poem about farming life, also includes the myths of Prometheus, Pandora. The poet gives advice on the best way to succeed in a dangerous world, lyrical poets often took their subjects from myth, but their treatment became gradually less narrative and more allusive. Greek lyric poets, including Pindar, Bacchylides and Simonides, and bucolic poets such as Theocritus and Bion, additionally, myth was central to classical Athenian drama
6.
William Lassell
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William Lassell, FRS FRSE FRSL FRAS was an English merchant and astronomer. He is remembered for his improvements to the Reflecting telescope and his ensuing planetary discoveries, william Lassell was born in Bolton, Lancashire, a town west of Manchester. He was educated first in Bolton then at Rochdale Academy, after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a brewer, which enabled him to indulge his interest in astronomy. He built an observatory at his house Starfield in West Derby, there he had a 24-inch reflector telescope, for which he pioneered the use of an equatorial mount for easy tracking of objects as the Earth rotates. He ground and polished the mirror himself, using equipment he constructed, the observatory was later moved further out of Liverpool, to Bradstone. In 1846 Lassell discovered Triton, the largest moon of Neptune, in 1848 he independently co-discovered Hyperion, a moon of Saturn. In 1851 he discovered Ariel and Umbriel, two moons of Uranus, when Queen Victoria visited Liverpool in 1851, Lassell was the only local she specifically requested to meet. In 1855, he built a 48-inch telescope, which he installed in Malta because of the conditions that were better than in often-overcast England. On his return to the UK after several years in Malta he moved to Maidenhead, the 48-inch telescope was dismantled and was eventually scrapped. Lassell was a Fellow of the Royal Astronomical Society from 1839, won the Gold Medal of the Royal Astronomical Society in 1849 and he was elected a Fellow of the Royal Society in 1849 and won their Royal Medal in 1858. Lassel was also a Fellow of the Royal Society of Literature, Lassell died in Maidenhead in 1880. Upon his death, he left a fortune of £80,000 and his telescope was presented to the Royal Observatory in Greenwich. The crater Lassell on the Moon, a crater on Mars, the asteroid 2636 Lassell, Biography and other topics Short biography and pictures Biography and technical detail of telescopes
7.
Nereid (moon)
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Nereid is the third-largest moon of Neptune. It has an eccentric orbit. It was the moon of Neptune to be discovered, by Gerard Kuiper in 1949. Nereid was discovered on 1 May 1949 by Gerard P. Kuiper on photographic plates taken with the 82-inch telescope at the McDonald Observatory and he proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology and it was the second and last moon of Neptune to be discovered before the arrival of Voyager 2. Nereid orbits Neptune in the direction at an average distance of 5,513,400 km. In 1991, a period of Nereid of about 13.6 hours was determined by an analysis of its light curve. In 2003, another period of about 11.52 ±0.14 hours was measured. However, this determination was later disputed, other researchers have so far failed to detect any periodic modulation in Nereids light curve. Nereid is Neptunes third-largest satellite and has a radius of about 170 kilometres. It is rather large for an irregular satellite, the shape of Nereid is unknown. Since 1987 some photometric observations of Nereid have detected large variations of it brightness, which can happen over years and months and they persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations and this means that they may be quite chaotic. To date there is no explanation of the variations, but, if they exist. Nereids rotation can be either in the state of forced precession or even chaotic rotation due to its elliptical orbit. In any case its rotation should be rather irregular, spectrally Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between Uranuss moons Titania and Umbriel, which suggests that Nereids surface is composed of a mixture of water ice and some spectrally neutral material. The spectrum is different from the outer-Solar-System minor planets, centaurs Pholus, Chiron and Chariklo
8.
Neso (moon)
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Neso, also known as Neptune XIII, is the outermost natural satellite of Neptune. It is an Irregular moon discovered by Matthew J. Holman, Brett J. Gladman, et al. on August 14,2002, Neso orbits Neptune at a distance of more than 48 Gm, making it the most distant known moon of any planet. At apocenter, the satellite is more than 72 Gm from Neptune and this distance is of such an order that it exceeds Mercurys aphelion, which is approximately 70 Gm from the Sun. Neso is also the moon with the longest orbital period,26.67 years and it follows a highly inclined and highly eccentric orbit illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity, Neso is about 60 km in diameter based on an assumed albedo, and assuming a mean density of 1.5 g/cm3, its mass is estimated at 2×1017 kg. Given the similarity of the parameters with Psamathe, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Neso is named one of the Nereids. Before the announcement of its name on February 3,2007, Neso was known by its provisional designation, notes Citations MPC, Natural Satellites Ephemeris Service Mean orbital parameters from JPL Matthew Holmans Neptunes page David Jewitts pages Scott Sheppards pages Neso in Fiction
9.
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
10.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
11.
Orbital inclination
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Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky
12.
Saturn
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Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude
13.
Phoebe (moon)
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Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was the first satellite to be discovered photographically, Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassinis trajectory to Saturn and time of arrival were specifically chosen to permit this flyby, after the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus. Phoebe is roughly spherical and has a differentiated interior and it was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is thought to be a captured planetesimal from the Kuiper belt and it was the first satellite to be discovered photographically. Phoebe was named after Phoebe, a Titan in Greek mythology that was associated with the Moon and it is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named characters in the Greek myth of Jason. In 2005, the IAU officially named 24 craters, Phoebes orbit is retrograde, i. e. it orbits Saturn opposite to Saturns rotation. For more than 100 years, Phoebe was Saturns outermost known moon, Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturns regular moons except Iapetus orbit very nearly in the plane of Saturns equator, the outer irregular satellites follow moderately to highly eccentric orbits, and none are expected to rotate synchronously as all the inner moons of Saturn do. The Phoebe ring is one of the rings of Saturn and this ring is tilted 27 degrees from Saturns equatorial plane. It extends from at least 128 to 207 times the radius of Saturn, the ring is about 20 times as thick as the diameter of the planet. Inwardly migrating ring material would thus strike Iapetuss leading hemisphere, created its two-tone coloration, although very large, the ring is virtually invisible—it was discovered using NASAs infrared Spitzer Space Telescope. Phoebe is roughly spherical and has a diameter of 213±1.4 km, Phoebe rotates every nine hours 16 minutes and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K, most of Saturns inner moons have very bright surfaces, but Phoebes albedo is very low, as dark as lampblack. The Phoebean surface is heavily scarred, with craters up to 80 kilometres across. Phoebes dark coloring initially led to scientists surmising that it was a captured asteroid and these are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then. In addition, quantities of carbon dioxide have been detected on the surface and it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturns inner moons
14.
Hydrostatic equilibrium
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In fluid mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force, hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, there are 31 observationally confirmed such objects, sometimes called planemos, in the Solar System, seven more that are virtually certain, and a hundred or so more that are likely. Newtons laws of state that a volume of a fluid that is not in motion or that is in a state of constant velocity must have zero net force on it. This means the sum of the forces in a given direction must be opposed by a sum of forces in the opposite direction. This force balance is called a hydrostatic equilibrium, the fluid can be split into a large number of cuboid volume elements, by considering a single element, the action of the fluid can be derived. If the density is ρ, the volume is V and g the standard gravity, then, the volume of this cuboid is equal to the area of the top or bottom, times the height — the formula for finding the volume of a cube. This sum equals zero if the velocity is constant. Dividing by A,0 = P b o t t o m − P t o p − ρ ⋅ g ⋅ h, or, P t o p − P b o t t o m = − ρ ⋅ g ⋅ h. Ptop − Pbottom is a change in pressure, and h is the height of the volume change in the distance above the ground. By saying these changes are small, the equation can be written in differential form. Density changes with pressure, and gravity changes with height, so the equation would be, d P = − ρ ⋅ g ⋅ d h. Note finally that this last equation can be derived by solving the three-dimensional Navier–Stokes equations for the situation where u = v = ∂ p ∂ x = ∂ p ∂ y =0. Then the only equation is the z -equation, which now reads ∂ p ∂ z + ρ g =0. Thus, hydrostatic balance can be regarded as a simple equilibrium solution of the Navier–Stokes equations. M is a foliation of spheres weighted by the mass density ρ, the hydrostatic equilibrium pertains to hydrostatics and the principles of equilibrium of fluids. A hydrostatic balance is a balance for weighing substances in water. Hydrostatic balance allows the discovery of their specific gravities, in any given layer of a star, there is a hydrostatic equilibrium between the outward thermal pressure from below and the weight of the material above pressing inward
15.
Rings of Neptune
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Elicer for a program led by William Hubbard. They were eventually imaged in 1989 by the Voyager 2 spacecraft, Neptunes rings are named after astronomers who contributed important work on the planet, Galle, Le Verrier, Lassell, Arago, and Adams. Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea, three other moons orbit between the rings, Naiad, Thalassa and Despina. The rings of Neptune are made of dark material, likely organic compounds processed by radiation. The proportion of dust in the rings is high, while their optical depth is low to moderate, uniquely, the Adams ring includes five distinct arcs, named Fraternité, Égalité1 and 2, Liberté, and Courage. The arcs occupy a range of orbital longitudes and are remarkably stable. How the arcs are stabilized is still under debate, however, their stability is probably related to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea. The first mention of rings around Neptune dates back to 1846 when William Lassell, however, his claim was never confirmed and it is likely that it was an observational artifact. The first reliable detection of a ring was made in 1968 by stellar occultation, soon after the Uranus discovery, a team from Villanova University led by Harold J. Reitsema began searching for rings around Neptune. On 24 May 1981, they detected a dip in a stars brightness during one occultation, however, later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa, a highly unusual event. In the 1980s, significant occultations were much rarer for Neptune than for Uranus, Neptunes next occultation, on 12 September 1983, resulted in a possible detection of a ring. Over the next six years, approximately 50 other occultations were observed only about one-third of them yielding positive results. Something definitely existed around Neptune, but the features of the system remained a mystery. The Voyager 2 spacecraft made the discovery of the Neptunian rings during its fly-by of Neptune in 1989. It confirmed that occasional occultation events observed before were indeed caused by the arcs within the Adams ring, since Voyager 2s fly-by, the brightest rings have been imaged with the Hubble Space Telescope and Earth-based telescopes, owing to advances in resolution and light-gathering power. They are visible, slightly above background levels, at methane-absorbed wavelengths in which the glare from Neptune is significantly reduced. The fainter rings are far below the visibility threshold. Neptune possesses five distinct rings named, in order of increasing distance from the planet, Galle, Le Verrier, Lassell, Arago and Adams
16.
Proteus (moon)
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Proteus, also known as Neptune VIII, is the second largest Neptunian moon, and Neptunes largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, Proteus orbits Neptune in a nearly equatorial orbit at the distance of about 4.75 equatorial radii of the planet. Despite being an icy body more than 400 km in diameter. It is shaped more like a polyhedron with several slightly concave facets. Its surface is dark, neutral in color, and heavily cratered, Proteuss largest crater is Pharos, which is more than 230 km in diameter. There are also a number of scarps, grooves, and valleys related to large craters, Proteus is probably not an original body that formed with Neptune, it may have accreted later from the debris created when the largest Neptunian satellite Triton was captured. Proteus was discovered from the images taken by Voyager 2 space probe two months before its Neptune flyby in August 1989 and it received the temporary designation S/1989 N1. Stephen P. Synnott and Bradford A. Smith announced its discovery on July 7,1989, speaking only of 17 frames taken over 21 days, on 16 September 1991 S/1989 N1 was named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune at the approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity and is inclined by about 0. 5° to the planets equator, Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously with the motion, which means that one face always points to the planet. Proteus is the second largest moon of Neptune and it is about 420 kilometres in diameter, larger than Nereid, the second to be discovered. It was not discovered by Earth-based telescopes because it is so close to Neptune that it is lost in the glare of reflected sunlight, the surface of Proteus is dark—its geometrical albedo is about 10%. The color of its surface is neutral as the reflectivity does not change appreciably with the wavelength from violet to green, in the near-infrared around 2 μm Proteuss surface becomes less reflective, pointing to a possible presence of complex organic compounds such as hydrocarbons or cyanides. These compounds may be responsible for the low albedo of the inner Neptunian moons, although Proteus is usually thought to contain significant amounts of water ice, it has not been detected spectroscopically on the surface. Saturns moon Mimas has an ellipsoidal shape despite being less massive than Proteus. Proteus is slightly elongated in the direction of Neptune, although its shape is closer to an irregular polyhedron than to a triaxial ellipsoid. The surface of Proteus shows several flat or slightly concave facets measuring from 150 to 200 km in diameter and they are probably degraded impact craters
17.
Irregular moon
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In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites,113 irregular satellites have been discovered, orbiting all four of the giant planets. The largest of each planet are Himalia of Jupiter, Phoebe of Saturn, Sycorax of Uranus, in 1997, the first two Uranian irregulars were discovered, Caliban and Sycorax. It is currently thought that the satellites were captured from heliocentric orbits near their current locations. An alternative theory, that they originated further out in the Kuiper belt, is not supported by current observations, there is no widely accepted precise definition of an irregular satellite. Informally, satellites are considered irregular if they are far enough from the planet that the precession of their plane is primarily controlled by the Sun. The radius of the Hill sphere is given in the adjacent table, the orbits of the known irregular satellites are extremely diverse, but there are certain patterns. Retrograde orbits are far more common than prograde orbits, no satellites are known with orbital inclinations higher than 55°. In addition, some groupings can be identified, in one large satellite shares a similar orbit with a few smaller ones. Given their distance from the planet, the orbits of the satellites are highly perturbed by the Sun. The semi-major axis of Pasiphae, for example, changes as much as 1.5 Gm in two years, the inclination around 10°, and the eccentricity as much as 0.4 in 24 years. Consequently, mean orbital elements are used to identify the groupings rather than osculating elements at the given date, irregular satellites have been captured from heliocentric orbits. This could involve, a collision of a body and a satellite, resulting in the incoming body losing energy. A close encounter between an incoming binary object and the planet, resulting in one component of the binary being captured, such a route has been suggested as most likely for Triton. After the capture, some of the satellites could break up leading to groupings of smaller moons following similar orbits, resonances could further modify the orbits making these groupings less recognizable. The current orbits of the moons are stable, in spite of substantial perturbations near the apocenter. The cause of stability in a number of irregulars is the fact that they orbit with a secular or Kozai resonance. The satellites enter the zone of the moons and are lost or ejected via collision
18.
Psamathe (moon)
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Psamathe, also known as Neptune X, is a retrograde irregular satellite of Neptune. It is named after Psamathe, one of the Nereids, Psamathe was discovered by Scott S. Sheppard and David C. Jewitt in 2003 using the 8.2 meter Subaru telescope, before the announcement of its name on February 3,2007, it was known by the provisional designation S/2003 N1. Psamathe is about 38 kilometers in diameter and it orbits Neptune at a distance of between 25.7 and 67.7 million km and requires almost 25 Earth years to make one orbit. The orbit of this satellite is close to the theoretical stable separation from Neptune for a body in a retrograde orbit, given the similarity of Psamathes orbital parameters with Neso, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Both are further from their primary than any other known moon in the Solar System, irregular satellites Neptunes Known Satellites S/2003 N1 Neptune Satellite Movie Images MPC, Natural Satellites Ephemeris Service Mean orbital parameters IAUC8193
19.
Extraterrestrial skies
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In astronomy, an extraterrestrial sky is a view of outer space from the surface of a world other than Earth. The only extraterrestrial sky that has been observed and photographed by astronauts is that of the Moon. The skies of Titan, Mars, and Venus have been observed indirectly by space probes designed to land on the surface, characteristics of extraterrestrial skies appear to vary substantially due to a number of factors. An extraterrestrial atmosphere, if present, has a bearing on visible characteristics. The atmospheres density and chemical composition can contribute to differences in colour, opacity, astronomical objects may also be visible and can include natural satellites, rings, star systems and nebulas and other planetary system bodies. Because Mercury has little or no atmosphere, a view of the planets skies would be no different from viewing space from orbit, Mercury has a southern pole star, α Pictoris, a magnitude 3.2 star. It is fainter than Earths Polaris, on average, the visible diameter of the Sun on Mercury is 2.5 times as large as it appears from Earth, and its total brightness is more than 6 times as great. Because of the eccentric orbit, the Suns apparent size in the sky would vary from 2.2 times that from Earth at aphelion. Mercury has a 3,2 spin–orbit resonance and this means that although a sidereal day lasts around 58.7 Earth days, a solar day lasts around 176 Earth days. The on-sky distance over which the Sun appears to be travelling in this direction is slightly larger than the apparent size of the Sun itself. The apparent motion reverses because approximately four days prior to perihelion, at perihelion, Mercurys orbital angular velocity slightly exceeds the rotational velocity, so the Sun appears to be retrograde. Four days after perihelion, the Suns normal apparent motion resumes, because of its spin–orbit resonance, Mercury presents one of two spots of its surface to the Sun on alternate perihelia. After the Sun, the second brightest object in the Mercurian sky is Venus, the reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see only its night side. Indeed, even when Venus is brightest in the Earths sky, for a Mercurian observer, on the other hand, Venus is closest when it is in opposition to the Sun and is showing its full disk. The apparent magnitude of Venus is as bright as −7.7, the Earth and the Moon are also very prominent, their apparent magnitudes being about −5 and −1.2 respectively. The maximum apparent distance between the Earth and the Moon is about 15′, all other planets are visible just as they are on Earth, but somewhat less bright at opposition. The zodiacal light is more prominent than it is from Earth. The atmosphere of Venus is so thick that the Sun is not distinguishable in the sky
20.
Discovery of Neptune
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The planet Neptune was mathematically predicted before it was directly observed. It was a moment of 19th century science and dramatic confirmation of Newtonian gravitational theory. In François Aragos apt phrase, Le Verrier had discovered a planet with the point of his pen and these irregularities could, however, be resolved if the gravity of a farther, unknown planet were disturbing its path around the Sun. In 1845 astronomers Urbain Le Verrier in Paris and John Couch Adams in Cambridge separately began calculations to determine the nature, nevertheless, the Royal Society awarded Le Verrier the Copley medal in 1846 for his achievement, without mention of Adams. The discovery of Neptune led to the discovery of its moon Triton by William Lassell just seventeen days later, Neptune is invisible to the naked eye as it is too dim. Its apparent magnitude is never brighter than 7.7, therefore, the first observations of Neptune were only possible after the invention of the telescope. There is evidence that Neptune was seen and recorded by Galileo Galilei in 1613, Jérôme Lalande in 1795 and John Herschel in 1830 and these pre-discovery observations were important in accurately determining the orbit of Neptune. Neptune would appear prominently even in early telescopes so other pre-discovery observation records are likely, historically it was thought that he believed it to be a fixed blue star, and so he is not credited with its discovery. However, in July 2009 University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was indeed aware that he had discovered something unusual about this star. However, so far there is clear evidence that he identified this moving object as a planet. There is no evidence that he attempted to observe it again. In 1847, Sears C. Walker of the U. S. Naval Observatory searched historical records and he found that observations made by Lalandes staff at the Paris Observatory in 1795 were in the direction of Neptunes position in the sky. In the catalog observations for May 8 and again on May 10 of 1795 a star was observed in the position expected for Neptune. The uncertainty of the position was noted with a colon, the discovery of these records of Neptunes position in 1795 led to a better calculation of the planets orbit. John Herschel almost discovered Neptune the same way his father, William Herschel, had discovered Uranus in 1781, in an 1846 letter to Wilhelm Struve, John Herschel states that he observed Neptune during a sweep of the sky on July 14,1830. Although his telescope was powerful enough to resolve Neptune into a blue disk and show it to be a planet, he did not recognize it at the time. In 1821, Alexis Bouvard had published astronomical tables of the orbit of Uranus, subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. Adams learned of the irregularities while still an undergraduate and became convinced of the perturbation hypothesis, after his final examinations in 1843, Adams was elected fellow of his college and spent the summer vacation in Cornwall calculating the first of six iterations
21.
Gerard Kuiper
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Gerard Peter Kuiper was a Dutch–American astronomer, planetary scientist, selenographer, author and professor. He is the namesake of the Kuiper belt. Kuiper is considered by many to be the father of planetary science. As professor at the University of Chicago, he was advisor to Carl Sagan. In 1958, the two worked on the classified military Project A119, the secret Air Force plan to detonate a warhead on the Moon. Kuiper, the son of a tailor in the village of Harenkarspel in North Holland, had an early interest in astronomy. He had extraordinarily sharp eyesight, allowing him to see magnitude 7.5 stars with the naked eye and he went to study at Leiden University in 1924, where at the time a very large number of astronomers had congregated. He befriended fellow students Bart Bok and Pieter Oosterhoff and was taught by Ejnar Hertzsprung, Antonie Pannekoek, Willem de Sitter, Jan Woltjer, Jan Oort and he received his B. Sc. in Astronomy in 1927 and continued straight on with his graduate studies. Kuiper finished his thesis on binary stars with Hertzsprung in 1933. In 1935 he left to work at the Harvard College Observatory where he met Sarah Parker Fuller, whom he married on June 20,1936. Although he had planned to move to Java to work at the Bosscha Observatory, he took a position at the Yerkes Observatory of the University of Chicago, in 1949, Kuiper initiated the Yerkes–McDonald asteroid survey. Kuiper discovered two satellites of planets in the Solar System, namely Uranuss satellite Miranda and Neptunes satellite Nereid. In addition, he discovered carbon dioxide in the atmosphere of Mars, Kuiper also pioneered airborne infrared observing using a Convair 990 aircraft in the 1960s. Kuiper spent most of his career at the University of Chicago, Kuiper was the laboratorys director until his death in 1973 while on vacation with his wife in Mexico. One of the three buildings at Arizona that makes up the LPL is named in his honor, in the 1960s, Kuiper helped identify landing sites on the Moon for the Apollo program. His earlier work on the Moon included the secret Project A119, which was disclosed by Carl Sagan, Kuiper discovered several binary stars which received Kuiper numbers to identify them, such as KUI79. In 1947, Kuiper was awarded the Prix Jules Janssen of the Société astronomique de France, in 1959, Kuiper won the Henry Norris Russell Lectureship of the American Astronomical Society. In 1971, Kuiper received the Kepler Gold Medal from the American Association for the Advancement of Science, besides the minor planet 1776 Kuiper, three craters and the now-decommissioned Kuiper Airborne Observatory was also named after him
22.
Larissa (moon)
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Larissa, also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly and it was first discovered by Harold J. Reitsema, William B. The moon was recovered and confirmed to be the object in its orbit during the Voyager 2 flyby in 1989 after which it received the additional designation S/1989 N2 on August 2,1989. The announcement by Stephen P. Synnott spoke of “10 frames taken over 5 days”, the name was given on September 16,1991. The fourth-largest satellite of Neptune, Larissa is irregular in shape and appears to be heavily cratered, little else is known about it. Larissa has only been visited by Voyager 2, the probe was able to get photographs of Larissa. Larissa Profile by NASAs Solar System Exploration Neptunes Known Satellites
23.
Rings of Uranus
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The rings of Uranus are a system of rings around the planet Uranus, intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10,1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789, some astronomers are skeptical that he could have seen them, as they are very dark. By 1978, nine distinct rings were identified, two additional rings were discovered in 1986 in images taken by the Voyager 2 spacecraft, and two outer rings were found in 2003–2005 in Hubble Space Telescope photos. In the order of increasing distance from the planet the 13 known rings are designated 1986U2R/ζ,6,5,4, α, β, η, γ, δ, λ, ε, ν and μ. Their radii range from about 38,000 km for the 1986U2R/ζ ring to about 98,000 km for the μ ring, additional faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the Bond albedo of the particles does not exceed 2%. They are probably composed of ice with the addition of some dark radiation-processed organics. The majority of Uranuss rings are opaque and only a few kilometers wide, the ring system contains little dust overall, it consists mostly of large bodies 0. 2–20 m in diameter. However, some rings are thin, the broad and faint 1986U2R/ζ, μ and ν rings are made of small dust particles, while the narrow. The relative lack of dust in the system may be due to aerodynamic drag from the extended Uranian exosphere. The rings of Uranus are thought to be young. The Uranian ring system probably originated from the fragmentation of several moons that once existed around the planet. After colliding, the moons probably broke up into many particles, the mechanism that confines the narrow rings is not well understood. Initially it was assumed that every ring had a pair of nearby shepherd moons corralling them into shape. However, in 1986 Voyager 2 discovered only one such shepherd pair around the brightest ring, Herschel drew a small diagram of the ring and noted that it was a little inclined to the red. The Keck Telescope in Hawaii has since confirmed this to be the case, herschels notes were published in a Royal Society journal in 1797. However, in the two centuries between 1797 and 1977 the rings are rarely mentioned, if at all and this casts serious doubt on whether Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing
24.
Naiad (moon)
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Naiad, also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend. Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe, the last moon to be discovered during the flyby, it was designated S/1989 N6. The discovery was announced on September 29,1989, in the IAU Circular No,4867, but the text only talks of 25 frames taken over 11 days, giving a discovery date of sometime before September 18. The name was given on 16 September 1991, Naiad is irregularly shaped and probably has not been modified by any internal geological processes since its formation. Naiad orbits about 23,500 km above Neptunes cloud tops, Naiad orbits Neptune well within its fluid Roche limit, and its density is expected to be low enough that it may be very close to its actual Roche limit already. Since the Voyager 2 flyby, the Neptune system has been studied from ground-based observatories. In 2002–03 the Keck telescope observed the system using adaptive optics, thalassa was found with some image processing, but Naiad was not located. Hubble has the ability to detect all the satellites and possible new satellites even dimmer than those found by Voyager 2. On October 8,2013 the SETI Institute announced that Naiad had been located in archived Hubble imagery from 2004, the suspicion that the loss of positioning was due to considerable errors in Naiads ephemeris proved correct as Naiad was ultimately located 80 degrees from its expected position. Naiad Profile by NASAs Solar System Exploration Neptunes Known Satellites Archival Hubble Images Reveal Neptunes “Lost” Inner Moon
25.
Thalassa (moon)
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Thalassa, also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, a daughter of Aether, Thalassa is also the Greek word for sea. Thalassa was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe and it was given the temporary designation S/1989 N5. The discovery was announced on September 29,1989, but the text only talks of 25 frames taken over 11 days, the name was given on 16 September 1991. Thalassa is irregularly shaped and shows no sign of any geological modification, unusually for irregular bodies, it appears to be roughly disk-shaped. Relatively soon after, the spreading debris may impinge upon Despinas orbit, Thalassa Profile by NASAs Solar System Exploration Neptunes Known Satellites
26.
Despina (moon)
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Despina, also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon, Despina was discovered in late July 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N3, the discovery was announced on August 2,1989, but the text only talks of 10 frames taken over 5 days, giving a discovery date of sometime before July 28. The name was given on 16 September 1991, Despina is irregularly shaped and shows no sign of any geological modification. Despinas orbit lies close to but outside of the orbit of Thalassa, the diameter of Despina is approximately 150 km wide. Despina Profile by NASAs Solar System Exploration Neptunes Known Satellites NASA Astronomy Picture of the Day, Despina and its shadow transiting Neptune
27.
Galatea (moon)
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Galatea, also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend, Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe. The name was given on 16 September 1991 and it is irregularly shaped and shows no sign of any geological modification. Galatea appears to be a shepherd moon for the Adams ring that is 1000 km outside its orbit, resonances with Galatea in the ratio 42,43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring. Galateas mass has been estimated based on the radial perturbations it induces on the ring, Galatea Profile by NASAs Solar System Exploration Neptunes Known Satellites
28.
Halimede (moon)
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Halimede, or Neptune IX, is a retrograde irregular satellite of Neptune. It was discovered by Matthew J. Holman, John J. Kavelaars, Tommy Grav, fraser and Dan Milisavljevic on August 14,2002. Halimede has the second most eccentric and third most inclined orbit around Neptune and this is illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity, Halimede is about 62 kilometers in diameter and appears neutral in the visible light. Halimede like many of the satellites of Neptune, is named after one of the Nereids. Before the announcement of its name on February 3,2007, Matthew Holmans Neptunes page David Jewitts pages Neptunes Known Satellites MPC, Natural Satellites Ephemeris Service Mean orbital parameters from JPL
29.
Sao (moon)
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Sao is a prograde irregular satellite of Neptune. It was discovered by Matthew J. Holman et al. on August 14,2002, Sao orbits Neptune at a distance of about 22.4 million km and is about 44 kilometers in diameter. Sao follows an exceptionally inclined and moderately eccentric orbit illustrated on the diagram in relation to other satellites of Neptune. The satellites above the axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity, the satellite is in Kozai resonance, i. e. its inclination and eccentricity are coupled. Sao, or Neptune XI, like many of the satellites of Neptune, is named after one of the Nereids. Before the announcement of its name on February 3,2007, ephemeris from MPC Mean orbital parameters from JPL Matthew Holmans Neptunes page David Jewitts pages Scott Sheppards pages
30.
Laomedeia
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Laomedeia, also known as Neptune XII, is a prograde irregular satellite of Neptune. It was discovered by Matthew J. Holman, et al. on August 13,2002, before the announcement of its name on February 3,2007, it was known as S/2002 N3. It orbits Neptune at a distance of about 23,571,000 km and is about 42 kilometers in diameter and it is named after Laomedeia, one of the 50 Nereids. Neptunes Known Satellites, by Scott S. Sheppard David Jewitt pages MPC, Natural Satellites Ephemeris Service Mean orbital parameters, NASA
31.
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
32.
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
33.
Camille Flammarion
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Nicolas Camille Flammarion FRAS was a French astronomer and author. He was an author of more than fifty titles, including popular science works about astronomy, several notable early science fiction novels. He also published the magazine LAstronomie, starting in 1882 and he maintained a private observatory at Juvisy-sur-Orge, France. Camille Flammarion was born in Montigny-le-Roi, Haute-Marne, France and he was the brother of Ernest Flammarion, founder of the Groupe Flammarion publishing house. He was a founder and the first president of the Société astronomique de France, in January,1895, after 13 volumes of LAstronomie and 8 of BSAF, the two merged, making L’Astronomie the Bulletin of the Societé. The Flammarion engraving first appeared in Flammarion’s 1888 edition of L’Atmosphère, in 1907, he wrote that he believed that dwellers on Mars had tried to communicate with the Earth in the past. He also believed in 1907 that a comet was heading toward Earth. In 1910, for the appearance of Halleys Comet, he believed the gas from the tail would impregnate atmosphere. He has been described as an astronomer, mystic and storyteller who was obsessed by life after death, and on other worlds, and seemed to see no distinction between the two. He was influenced by Jean Reynaud and his Terre et ciel and he was convinced that souls after the physical death pass from planet to planet, progressively improving at each new incarnation. In Real and Imaginary Worlds and Lumen, he describes a range of species, including sentient plants which combine the processes of digestion and respiration. ”His psychical studies also influenced some of his science fiction. Other than that, his writing about other worlds adhered fairly closely to then current ideas in evolutionary theory, among other things, he believed that all planets went through more or less the same stages of development, but at different rates depending on their sizes. Flammarion’s influence was great, not just on the thought of his day. Both George Griffith and Edgar Rice Burroughs are referring to him in their writing, in the English translation of Lumen, Brian Stapleford argues that both Olaf Stapledon and William Hope Hodgson have likely been influenced by Flammarion. Arthur Conan Doyles The Poison Belt, published 1913, also have a lot in common with Flammarions worries that the tail of Halleys Comet would be poisonous for earth life, Camille was a brother of Ernest Flammarion and Berthe Martin-Flammarion and uncle of a woman named Zelinda. His first wife was Sylvie Petiaux-Hugo Flammarion, and his wife was Gabrielle Renaudot Flammarion. Camille, a contemporary of Giovanni Schiaparelli, extensively researched the so-called canals during the 1880s and 1890s and he assumed the planet was in an advanced stage of its habitability, and the canals were the product of an intelligent species attempting to survive on a dying world. Religious belief must not take the place of impartial analysis and we must be constantly on our guard against illusions
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Neptune (mythology)
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Neptune was the god of freshwater and the sea in Roman religion. He is the counterpart of the Greek god Poseidon, in the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, the brothers presided over the realms of Heaven, the earthly world, and the Underworld. Depictions of Neptune in Roman mosaics, especially those of North Africa, are influenced by Hellenistic conventions, Neptune was likely associated with fresh water springs before the sea. Like Poseidon, Neptune was worshipped by the Romans also as a god of horses, under the name Neptunus Equester, Neptune is the creator of horses and is the god of the sea as well as the owner of a powerful weapon, the Trident. Poseidon is the greek Neptune and is one of the big three gods Zeus, Hades, and Poseidon and he accidentally created horses when he had an affair with Medusa, and also created a boy who roams the seas as a pirate past the pillar of Hercules. The etymology of Latin Neptunus is unclear and disputed, the ancient grammarian Varro derived the name from nuptus i. e. covering, with a more or less explicit allusion to the nuptiae, marriage of Heaven and Earth. Among modern scholars Paul Kretschmer proposed a derivation from IE *neptu- moist substance, similarly Raymond Bloch supposed it might be an adjectival form in -no from *nuptu-, meaning he who is moist. Georges Dumézil though remarked words deriving root *nep- are not attested in IE languages other than Vedic and he proposed an etymology that brings together Neptunus with Vedic and Avestan theonyms Apam Napat, Apam Napá and Old Irish theonym Nechtan, all meaning descendant of the waters. By using the approach the Indo-Iranian, Avestan and Irish figures would show common features with the Roman historicised legends about Neptune. Dumézil thence proposed to derive the nouns from IE root *nepot-, descendant, IE root *nebh-, having the original meaning of damp, wet, has given Sanskrit nábhah, Hittite nepis, Latin nubs, nebula, German Nebel, Slavic nebo etc. The concept would be close to that expressed in the name of Greek god Όυράνος, derived from IE root *h2wórso-, to water, irrigate and *h2worsó- and this etymology would be more in accord with Varros. The district was connected to the cult of the god, Messapus and Halesus. Messapus led the Falisci and others to war in the Aeneid, nepi and Falerii have been famed since antiquity for the excellent quality of the water of their springs, scattered in meadows. Nepet is considered a hydronymic toponym of preIndoeuropean origin widespread in Europe and from an appellative meaning damp wide valley, plain, cognate with pre-Greek νάπη, such an identification may well be grounded in the strict relationship between the Latin and Greek theologies of the two deities. Servius the grammarian also explicitly states Neptune is in charge of all the rivers, springs and he also is the lord of horses because he worked with Minerva to make the chariot. He may find a parallel in Irish god Nechtan, master of the well from all the rivers of the world flow out. Poseidon on the other hand underwent the process of becoming the main god of the sea at an earlier time. For a time he was paired with Salacia, the goddess of the salt water, Neptune was also considered the legendary progenitor god of a Latin stock, the Faliscans, who called themselves Neptunia proles
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Poseidon
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Poseidon was one of the twelve Olympian deities of the pantheon in Greek mythology. His main domain was the ocean, and he is called the God of the Sea, additionally, he is referred to as Earth-Shaker due to his role in causing earthquakes, and has been called the tamer of horses. He is usually depicted as a male with curly hair. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology, both were sea gods analogous to Poseidon. According to some folklore, he was saved by his mother Rhea, who concealed him among a flock of lambs and pretended to have birth to a colt. There is a Homeric hymn to Poseidon, who was the protector of many Hellenic cities, according to the references from Plato in his dialogues Timaeus and Critias, the island of Atlantis was the chosen domain of Poseidon. The form Ποτειδάϝων appears in Corinth, the origins of the name Poseidon are unclear. Walter Burkert finds that the second element da- remains hopelessly ambiguous, another theory interprets the second element as related to the word *δᾶϝον dâwon, water, this would make *Posei-dawōn into the master of waters. There is also the possibility that the word has Pre-Greek origin, Plato in his dialogue Cratylus gives two alternative etymologies, either the sea restrained Poseidon when walking as a foot-bond, or he knew many things. If surviving Linear B clay tablets can be trusted, the name occurs with greater frequency than does di-u-ja. A feminine variant, po-se-de-ia, is found, indicating a lost consort goddess. Poseidon carries frequently the title wa-na-ka in Linear B inscriptions, as king of the underworld, the chthonic nature of Poseidon-Wanax is also indicated by his title E-ne-si-da-o-ne in Mycenean Knossos and Pylos, a powerful attribute. In the cave of Amnisos Enesidaon is related with the cult of Eileithyia and she was related with the annual birth of the divine child. During the Bronze Age, a goddess of nature, dominated both in Minoan and Mycenean cult, and Wanax was her companion in Mycenean cult. It is possible that Demeter appears as Da-ma-te in a Linear B inscription, in Linear B inscriptions found at Pylos, E-ne-si-da-o-ne is related with Poseidon, and Si-to Po-tini-ja is probably related with Demeter. Tablets from Pylos record sacrificial goods destined for the Two Queens, the Two Queens may be related with Demeter and Persephone, or their precursors, goddesses who were not associated with Poseidon in later periods. The violated Demeter was Demeter Erinys, in Arcadia, Demeters mare-form was worshiped into historical times. Her xoanon of Phigaleia shows how the local cult interpreted her, a Medusa type with a horses head with snaky hair, holding a dove and a dolphin, probably representing her power over air and water
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Naiad
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In Greek mythology, the Naiads are a type of female spirit, or nymph, presiding over fountains, wells, springs, streams, brooks and other bodies of fresh water. They are distinct from gods, who embodied rivers. Arethusa, the nymph of a spring, could make her way through subterranean flows from the Peloponnesus, the Greek word is Ναϊάς, plural Ναϊάδες It derives from νάειν, to flow, or νᾶμα, running water. Naiad has several English pronunciations, /ˈneɪæd/, /ˈneɪəd/, /ˈnaɪæd/, /ˈnaɪəd/ and they were often the object of archaic local cults, worshipped as essential to humans. Boys and girls at coming-of-age ceremonies dedicated their childish locks to the local naiad of the spring, in places like Lerna their waters ritual cleansings were credited with magical medical properties. Oracles might be situated by ancient springs, Naiads could be dangerous, Hylas of the Argos crew was lost when he was taken by naiads fascinated by his beauty. The naiads were also known to exhibit jealous tendencies, salmacis forced the youth Hermaphroditus into a carnal embrace and, when he sought to get away, fused with him. The loves and rapes of Zeus, according to Graves readings, so, in the back-story of the myth of Aristaeus, Hypseus, a king of the Lapiths, married Chlidanope, a naiad, who bore him Cyrene. Aristaeus had more than ordinary mortal experience with the naiads, when his bees died in Thessaly and his aunt Arethusa invited him below the waters surface, where he was washed with water from a perpetual spring and given advice. St. Charles Avenue in New Orleans was formerly known as Nyades Street, and is parallel to Dryades Street. Crinaeae Eleionomae Limnades or Limnatides Pegaeae Potameides Bibliotheca 2.95,2.11,2.21,2.23,1.61,1.81,1.7.6 Homer. Odyssey 13.355,17.240, Iliad 14.440,20.380 Ovid, theogony Burkert, Walter, Greek Religion, Harvard University Press,1985. Robert Graves, The Greek Myths 1955 Edgar Allan Poe, Sonnet to Science 1829 The Naiades Naiad Nymphs
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Nereid
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In Greek mythology, the Nereids are sea nymphs, the 50 daughters of Nereus and Doris, sister to Nerites. They often accompany Poseidon, the god of the sea, and can be friendly and helpful to sailors, Nereids are particularly associated with the Aegean Sea, where they dwelt with their father Nereus in the depths within a golden palace. The most notable of them are Thetis, wife of Peleus and mother of Achilles, Amphitrite, wife of Poseidon, and Galatea and they symbolized everything that is beautiful and kind about the sea. Their melodious voices sang as they danced around their father and they are represented as very beautiful girls, crowned with branches of red coral and dressed in white silk robes trimmed with gold, but who went barefoot. They were part of Poseidons entourage and carried his trident, in Homers Iliad XVIII, when Thetis cries out in sympathy for the grief of Achilles for the slain Patroclus, her sisters appear. The Nereid Opis is mentioned in Virgils Aeneid and she is called by the goddess Diana to avenge the death of the Amazon-like female warrior Camilla. Diana gives Opis magical weapons for revenge on Camillas killer, the Etruscan Arruns, Opis sees and laments Camillas death and shoots Arruns in revenge as directed by Diana. In modern Greek folklore, the term nereid has come to be used for all nymphs, fairies, or mermaids, Nereid, a moon of the planet Neptune, is named after the Nereids. This list is correlated from four sources, Homers Iliad, Hesiods Theogony, the Bibliotheca, because of this, the total number of names goes beyond fifty
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Moons of Jupiter
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There are 67 known moons of Jupiter. This gives Jupiter the largest number of moons with reasonably stable orbits of any planet in the Solar System. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 63 moons, of Jupiters moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiters equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, the other four regular satellites are much smaller and closer to Jupiter, these serve as sources of the dust that makes up Jupiters rings. The remainder of Jupiters moons are irregular satellites whose prograde and retrograde orbits are farther from Jupiter and have high inclinations. These moons were probably captured by Jupiter from solar orbits, sixteen irregular satellites have been discovered since 2003 and have not yet been named. The physical and orbital characteristics of the moons vary widely, all other Jovian moons are less than 250 kilometres in diameter, with most barely exceeding 5 kilometres. Their orbital shapes range from perfectly circular to highly eccentric and inclined. Orbital periods range from seven hours, to three thousand times more. Jupiters regular satellites are believed to have formed from a circumplanetary disk and they may be the remnants of a score of Galilean-mass satellites that formed early in Jupiters history. Simulations suggest that, while the disk had a high mass at any given moment. However, only 2% the proto-disk mass of Jupiter is required to explain the existing satellites, thus there may have been several generations of Galilean-mass satellites in Jupiters early history. Each generation of moons might have spiraled into Jupiter, due to drag from the disk, by the time the present generation formed, the disk had thinned to the point that it no longer greatly interfered with the moons orbits. The current Galilean moons were still affected, falling into and being protected by an orbital resonance with each other, which still exists for Io, Europa. Ganymedes larger mass means that it would have migrated inward at a faster rate than Europa or Io, many broke up due to the mechanical stresses of capture, or afterward by collisions with other small bodies, producing the moons we see today. The first claimed observation of one of Jupiters moons is that of Chinese astronomer Gan De around 364 BC, however, the first certain observations of Jupiters satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 30× magnification telescope, no additional satellites were discovered until E. E. Barnard observed Amalthea in 1892. With the aid of photography, further discoveries followed quickly over the course of the twentieth century
39.
Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers
40.
S/2004 N 1
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S/2004 N1 is a small moon of Neptune, about 18 km in diameter, which orbits the planet in just under one Earth day. Its discovery on 1 July 2013 increased Neptunes retinue of known satellites to fourteen, the moon is so dim that it was not observed when Voyager 2 flew by in 1989. Mark Showalter of the SETI Institute found it by analyzing archived Neptune photographs the Hubble Space Telescope captured between 2004 and 2009, the designation S/2004 N1 is provisional,2004 refers to the year the data was first acquired, not the year of discovery. Mark Showalter discovered S/2004 N1 on 1 July 2013 while examining Hubble Space Telescope images of Neptunes ring arcs from 2009 and he used a technique similar to panning to compensate for orbital motion and allow stacking of multiple images to bring out faint details. After deciding on a whim to expand the area to radii well beyond the rings. He then found it repeatedly in other archival HST images going back to 2004, Voyager 2, which had observed all of Neptunes other inner satellites, did not detect it during its 1989 flyby, due to its dimness. Given that the relevant images have long been available to the public, S/2004 N1 is the fourteenth known moon of Neptune, and the first to be discovered since September 2003, when Psamathe was discovered. Neptunes largest moon, Triton, has a retrograde and inclined orbit and it is hypothesized that Neptune captured it from the Kuiper belt well after Neptunes original satellite system formed. The pre-existing moons orbits would have shifted by this event, leading to the ejection of some moons. Neptunes present inner satellites are thought to have accreted from the resulting rubble after Tritons orbit was circularized by tidal deceleration. S/2004 N1 is assumed to resemble Neptunes other inner satellites in having a surface as dark as dirty asphalt and their geometrical albedos range from 0.07 to 0.10. S/2004 N 1s apparent magnitude of 26.5 would then give it a diameter of 16 to 20 kilometres, the near-infrared spectra of Neptunes rings and inner moons have been examined with the HST NICMOS instrument. Similar dark, reddish material, characteristic of small outer Solar System bodies, the data is consistent with organic compounds containing C−H and/or C≡N bonds, but spectral resolution was inadequate to identify the molecules. Water ice, abundant in the outer Solar System, is believed to be present, both its inclination and eccentricity are close to zero. It orbits between Larissa and Proteus, making it the second outermost of Neptunes regular satellites and its small size at this location runs counter to a trend among the other regular Neptunian satellites of increasing diameter with increasing distance from the primary. The periods of Larissa and S/2004 N1 are within one percent of a 3,5 orbital resonance, while S/2004 N1. Larissa and Proteus are thought to have passed through a 1,2 mean-motion resonance a few hundred years ago. Among the names under consideration is Polyphemus, the gigantic one-eyed son of Poseidon, media related to S/2004 N1 at Wikimedia Commons
41.
Tidal acceleration
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Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite, and the primary planet that it orbits. The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, the process eventually leads to tidal locking, usually of the smaller first, and later the larger body. The Earth–Moon system is the best studied case, the similar process of tidal deceleration occurs for satellites that have an orbital period that is shorter than the primarys rotational period, or that orbit in a retrograde direction. The naming is somewhat confusing, because the speed of the relative to the body it orbits is decreased as a result of tidal acceleration. Laplaces initial computation accounted for the effect, thus seeming to tie up the theory neatly with both modern and ancient observations. It took some time for the community to accept the reality. But eventually it became clear that three effects are involved, when measured in terms of solar time. Beside the effects of changes in Earths orbital eccentricity, as found by Laplace and corrected by Adams. First there is a real retardation of the Moons angular rate of orbital motion and this increases the Moons angular momentum around Earth. Secondly there is an apparent increase in the Moons angular rate of orbital motion and this arises from Earths loss of angular momentum and the consequent increase in length of day. Because the Moons mass is a fraction of that of Earth. The mass of the Moon is sufficiently large, and it is sufficiently close, in particular, the water of the oceans bulges out towards and away from the Moon. The average tidal bulge is synchronized with the Moons orbit, however, Earths rotation drags the position of the tidal bulge ahead of the position directly under the Moon. As a consequence, there exists a substantial amount of mass in the bulge that is offset from the line through the centers of Earth and the Moon. Because of this offset, a portion of the pull between Earths tidal bulges and the Moon is not perpendicular to the Earth–Moon line, i. e. there exists a torque between Earth and the Moon. This boosts the Moon in its orbit, and slows the rotation of Earth, as a result of this process, the mean solar day, which is nominally 86,400 seconds long, is actually getting longer when measured in SI seconds with stable atomic clocks. The small difference accumulates over time, which leads to a difference between our clock time on the one hand, and Atomic Time and Ephemeris Time on the other hand. This led to the introduction of the second in 1972 to compensate for differences in the bases for time standardization
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Shepherd moon
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A shepherd moon is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the herd of the particles as a shepherd. Due to their effect, they pick up particles and deflect them from their original orbits through orbital resonance. This causes gaps in the system, such as the particularly striking Cassini Division, as well as other characteristic bands. Several of Jupiters small innermost moons, namely Metis and Adrastea, are within Jupiters ring system and are also within Jupiters Roche limit. It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiters tidal forces, possibly facilitated by impacts of ring material on their surfaces, the complex ring system of Saturn has several such satellites. These include Prometheus, Daphnis, Pan, Janus, and Epimetheus, uranus also has shepherd moons on its ε ring, Cordelia and Ophelia. They are interior and exterior shepherds, respectively, both moons are well within Uranuss synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration. Neptunes rings are unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations. It is thought that the influence of the shepherd moon Galatea. Rings around some centaurs have been identified, chariklos rings are remarkably well-defined and are suspected to either be very young or kept in place by a shepherd moon similar in mass to the rings. Chiron is also thought to have similar in form to those of Chariklo. Kirkwood gap Arnold Hanslmeier, Einführung in Astronomie und Astrophysik
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