Classical Kuiper belt object
A classical Kuiper belt object called a cubewano, is a low-eccentricity Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune's orbit; that is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets. The name "cubewano" derives from the first trans-Neptunian object found after Pluto and Charon, 15760 Albion, which until January 2018 had only had the provisional designation 1992 QB1. Similar objects found were called "QB1-o's", or "cubewanos", after this object, though the term "classical" is much more used in the scientific literature. Objects identified as cubewanos include: 15760 Albion Makemake, the largest known cubewano and a dwarf planet 50000 Quaoar and 20000 Varuna, each considered the largest TNO at the time of discovery 19521 Chaos, 58534 Logos, 53311 Deucalion, 66652 Borasisi, 88611 Teharonhiawako 1997 CU29, 2002 TX300, 2002 AW197, 2002 UX25 2014 MU69 Haumea was provisionally listed as a cubewano by the Minor Planet Center in 2006, but turned out to be resonant.
There are two basic dynamical classes of classical Kuiper-belt bodies: those with unperturbed orbits, those with markedly perturbed orbits. Most cubewanos are found between the 2:3 orbital resonance with the 1:2 resonance. 50000 Quaoar, for example, has a near-circular orbit close to the ecliptic. Plutinos, on the other hand, have more eccentric orbits bringing some of them closer to the Sun than Neptune; the majority of classical objects, the so-called cold population, have low inclinations and near-circular orbits, lying between 42 and 47 AU. A smaller population is characterised by inclined, more eccentric orbits; the terms'hot' and'cold' has nothing to do with surface or internal temperatures. Instead, the terms'hot and'cold' refer to the orbits of the objects, by analogy to particles in a gas, which increase their relative velocity as they become heated up; the Deep Ecliptic Survey reports the distributions of the two populations. The vast majority of KBOs have inclinations of less than 5° and eccentricities of less than 0.1.
Their semi-major axes show a preference for the middle of the main belt. The'hot' and'cold' populations are strikingly different: more than 30% of all cubewanos are in low inclination, near-circular orbits; the parameters of the plutinos’ orbits are more evenly distributed, with a local maximum in moderate eccentricities in 0.15–0.2 range and low inclinations 5–10°. See the comparison with scattered disk objects; when the orbital eccentricities of cubewanos and plutinos are compared, it can be seen that the cubewanos form a clear'belt' outside Neptune's orbit, whereas the plutinos approach, or cross Neptune's orbit. When orbital inclinations are compared,'hot' cubewanos can be distinguished by their higher inclinations, as the plutinos keep orbits below 20°. In addition to the distinct orbital characteristics, the two populations display different physical characteristics; the difference in colour between the red cold population, such as 2014 MU69, more heterogeneous hot population was observed as early as in 2002.
Recent studies, based on a larger data set, indicate the cut-off inclination of 12° between the cold and hot populations and confirm the distinction between the homogenous red cold population and the bluish hot population. Another difference between the low-inclination and high-inclination classical objects is the observed number of binary objects. Binaries are quite common on low-inclination orbits and are similar-brightness systems. Binaries are less common on high-inclination orbits and their components differ in brightness; this correlation, together with the differences in colour, support further the suggestion that the observed classical objects belong to at least two different overlapping populations, with different physical properties and orbital history. There is no official definition of'cubewano' or'classical KBO'. However, the terms are used to refer to objects free from significant perturbation from Neptune, thereby excluding KBOs in orbital resonance with Neptune; the Minor Planet Center and the Deep Ecliptic Survey do not list cubewanos using the same criteria.
Many TNOs classified as cubewanos by the MPC are classified as ScatNear by the DES. Dwarf planet Makemake is such a borderline classical cubewano/scatnear object. 2002 KX14 may be an inner cubewano near the plutinos. Furthermore, there is evidence that the Kuiper belt has an'edge', in that an apparent lack of low-inclination objects beyond 47–49 AU was suspected as early as 1998 and shown with more data in 2001; the traditional usage of the terms is based on the orbit's semi-major axis, includes objects situated between the 2:3 and 1:2 resonances, between 39.4 and 47.8 AU. These definitions lack precision: in particular the boundary between the classical objects and the scattered disk remains bl
Giant planet
A giant planet is any massive planet. They are primarily composed of low-boiling-point materials, rather than rock or other solid matter, but massive solid planets can exist. There are four known giant planets in the Solar System: Jupiter, Saturn and Neptune. Many extrasolar giant planets have been identified orbiting other stars. Giant planets are sometimes called jovian planets, after Jupiter, they are sometimes known as gas giants. However, many astronomers now apply the latter term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, as ice giants. Both names are misleading: all of the giant planets consist of fluids above their critical points, where distinct gas and liquid phases do not exist; the principal components are hydrogen and helium in the case of Jupiter and Saturn, water and methane in the case of Uranus and Neptune. The defining differences between a low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation.
Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history. The term gas giant was coined in 1952 by the science fiction writer James Blish and was used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form. Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter has metallic hydrogen near its center, but much of its volume is hydrogen and traces of other gases above their critical points; the observable atmospheres of all these planets are quite thin compared to their radii, only extending one percent of the way to the center. Thus the observable portions are gaseous; the rather misleading term has caught on because planetary scientists use rock and ice as shorthands for classes of elements and compounds found as planetary constituents, irrespective of what phase the matter may appear in.
In the outer Solar System and helium are referred to as gases. When deep planetary interiors are considered, it may not be far off to say that, by ice astronomers mean oxygen and carbon, by rock they mean silicon, by gas they mean hydrogen and helium; the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the planets similar to the latter two. With this terminology in mind, some astronomers have started referring to Uranus and Neptune as ice giants to indicate the predominance of the ices in their interior composition; the alternative term jovian planet refers to the Roman god Jupiter—the genitive form of, Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. Objects large enough to start deuterium fusion are called brown dwarfs, these occupy the mass range between that of large giant planets and the lowest-mass stars; the 13-Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance.
Larger objects will burn most of their deuterium and smaller ones will burn only a little, the 13 MJ value is somewhere in between. The amount of deuterium burnt depends not only on the mass but on the composition of the planet on the amount of helium and deuterium present; the Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, the Exoplanet Data Explorer up to 24 Jupiter masses. A giant planet has a thick atmosphere of hydrogen and helium, they may have a dense molten core of rocky elements, or the core may have dissolved and dispersed throughout the planet if the planet is hot enough. In "traditional" giant planets such as Jupiter and Saturn hydrogen and helium constitute most of the mass of the planet, whereas they only make up an outer envelope on Uranus and Neptune, which are instead composed of water and methane and therefore referred to as "ice giants". Extrasolar giant planets that orbit close to their stars are the exoplanets that are easiest to detect; these are called hot Jupiters and hot Neptunes because they have high surface temperatures.
Hot Jupiters were, until the advent of space-borne telescopes, the most common form of exoplanet known, due to the relative ease of detecting them with ground-based instruments. Giant planets are said to lack solid surfaces, but it is more accurate to say that they lack surfaces altogether since the gases that constitute them become thinner and thinner with increasing distance from the planets' centers becoming indistinguishable from the interplanetary medium. Therefore, landing on a giant planet may or may not be possible, depending on the size and composition of its core. Gas giants consist of hydrogen and helium; the Solar System's gas giants and Saturn, have heavier elements making up between 3 and 13 percent of their mass. Gas giants are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable molten core with a rocky composition. Jupiter and Saturn's outermost portion of the hydroge
(84922) 2003 VS2
2003 VS2 is a trans-Neptunian object discovered by the Near Earth Asteroid Tracking program on November 14, 2003. Like Pluto, it is in a 2:3 orbital resonance with Neptune, giving it the orbital properties of a plutino. Mike Brown's website lists it as "likely" a dwarf planet. However, Brown assumed that VS2 was much bigger than it is, the light-curve analysis has questioned whether it would be in the hydrostatic equilibrium. Like Pluto, 2003 VS2 is locked in the 3:2 mean-motion resonance with Neptune, although its orbit is less eccentric than Pluto's, it has smaller orbital inclination. 2003 VS2 has a significant light-curve amplitude of 0.21±0.01. The most value of the rotation period is 7.41±0.02 h. 2003 VS2 has a moderately red surface with a moderately red color indexes B−V=0.93, V−R=0.59. Its geometrical albedo is about 15%. In 2007, its diameter was estimated by the Spitzer Space Telescope at 725±200 km. However, in 2012, this was reduced to 523.0+35.1−34.4 km after new Herschel Space Telescope observations.
The latter measurement is considered more reliable. Assuming a Pluto-like density of 2 g/cm3, one can obtain a mass estimate of about 1.5×1020 kg. Sedna, another big trans-Neptunian object discovered the same day Huge rock-ice body circles Sun 2003 VS2 precovery 2003 VS2 at the JPL Small-Body Database Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters
Astronomy
Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, stars, nebulae and comets. More all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, the study of the Universe as a whole. Astronomy is one of the oldest of the natural sciences; the early civilizations in recorded history, such as the Babylonians, Indians, Nubians, Chinese and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, but professional astronomy is now considered to be synonymous with astrophysics. Professional astronomy is split into theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, analyzed using basic principles of physics.
Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results. Astronomy is one of the few sciences in which amateurs still play an active role in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets. Astronomy means "law of the stars". Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now distinct. Both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties," while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, dynamic processes of celestial objects and phenomena."
In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could be called astrophysics; some fields, such as astrometry, are purely astronomy rather than astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics" depending on whether the department is affiliated with a physics department, many professional astronomers have physics rather than astronomy degrees; some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, Astronomy and Astrophysics. In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye.
In some locations, early cultures assembled massive artifacts that had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye; as civilizations developed, most notably in Mesopotamia, Persia, China and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, the nature of the Sun and the Earth in the Universe were explored philosophically; the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Ptolemaic system, named after Ptolemy.
A important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the astronomical traditions that developed in many other civilizations. The Babylonians discovered. Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and inven
Pluto
Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It is the largest known plutoid. Pluto was discovered by Clyde Tombaugh in 1930 and was considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc, 27% more massive than Pluto, was discovered; this led the International Astronomical Union to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded reclassified it as a dwarf planet. Pluto is the largest and second-most-massive known dwarf planet in the Solar System, the ninth-largest and tenth-most-massive known object directly orbiting the Sun, it is less massive than Eris. Like other Kuiper belt objects, Pluto is made of ice and rock and is small—about one-sixth the mass of the Moon and one-third its volume, it has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun.
This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five known moons: Charon, Nix and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body; the New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, produced from methane and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus.
Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X". By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known precovery observations, with the earliest made by the Yerkes Observatory on August 20, 1909. Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, the search for Planet X did not resume until 1929.
Vesto Melvin Slipher, the observatory director, gave the job of locating Planet X to 23-year-old Clyde Tombaugh, who had just arrived at the observatory after Slipher had been impressed by a sample of his astronomical drawings. Tombaugh's task was to systematically image the night sky in pairs of photographs examine each pair and determine whether any objects had shifted position. Using a blink comparator, he shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Pluto has yet to complete a full orbit of the Sun since its discovery because one Plutonian year is 247.68 years long.
The discovery made headlines around the globe. Lowell Observatory, which had the right to name the new object, received more than 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urged Slipher to suggest a name for the new object before someone else did. Constance Lowell proposed Zeus Percival and Constance; these suggestions were disregarded. The name Pluto, after the god of the underworld, was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, interested in classical mythology, she suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States. Each member of the Lowell Observatory was allowed to vote on a short-list of three potential names: Minerva and Pluto. Pluto received every vote; the name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia £5 as
Kuiper belt
The Kuiper belt called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists of small bodies or remnants from when the Solar System formed. While many asteroids are composed of rock and metal, most Kuiper belt objects are composed of frozen volatiles, such as methane and water; the Kuiper belt is home to three recognized dwarf planets: Pluto and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region; the Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object since Charon. Since its discovery, the number of known KBOs has increased to over a thousand, more than 100,000 KBOs over 100 km in diameter are thought to exist.
The Kuiper belt was thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago. The Kuiper belt is distinct from the theoretical Oort cloud, a thousand times more distant and is spherical; the objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects. Pluto is the largest and most massive member of the Kuiper belt, the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006, it is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as "plutinos", that share the same 2:3 resonance with Neptune.
After the discovery of Pluto in 1930, many speculated. The region now called, it was only in 1992. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it; the first astronomer to suggest the existence of a trans-Neptunian population was Frederick C. Leonard. Soon after Pluto's discovery by Clyde Tombaugh in 1930, Leonard pondered whether it was "not that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined to be detected"; that same year, astronomer Armin O. Leuschner suggested that Pluto "may be one of many long-period planetary objects yet to be discovered." In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth hypothesized that, in the region beyond Neptune, the material within the primordial solar nebula was too spaced to condense into planets, so rather condensed into a myriad of smaller bodies.
From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a large number of comparatively small bodies" and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system", becoming a comet. In 1951, in a paper in Astrophysics: A Topical Symposium, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, but he did not think that such a belt still existed today. Kuiper was operating on the assumption, common in his time, that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuiper's hypothesis correct, there would not be a Kuiper belt today; the hypothesis took many other forms in the following decades. In 1962, physicist Al G. W. Cameron postulated the existence of "a tremendous mass of small material on the outskirts of the solar system". In 1964, Fred Whipple, who popularised the famous "dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of Uranus that had sparked the search for Planet X, or, at the least, massive enough to affect the orbits of known comets.
Observation ruled out this hypothesis. In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus, he used a blink comparator, the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before. In 1992, another object, 5145 Pholus, was discovered in a similar orbit. Today, an entire population of comet-like bodies, called the centaurs, is known to exist in the region between Jupiter and Neptune; the centaurs' orbits have dynamical lifetimes of a few million years. From the time of Chiron's discovery in 1977, astronomers have speculated that the centaurs therefore must be replenished by some outer reservoir. Further evidence for the existence of the Kuiper belt emerged from the study of comets; that comets have finite lifespans. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space d
Ecliptic
The ecliptic is the mean plane of the apparent path in the Earth's sky that the Sun follows over the course of one year. This plane of reference is coplanar with Earth's orbit around the Sun; the ecliptic is not noticeable from Earth's surface because the planet's rotation carries the observer through the daily cycles of sunrise and sunset, which obscure the Sun's apparent motion against the background of stars during the year. The motions as described above are simplifications. Due to the movement of Earth around the Earth–Moon center of mass, the apparent path of the Sun wobbles with a period of about one month. Due to further perturbations by the other planets of the Solar System, the Earth–Moon barycenter wobbles around a mean position in a complex fashion; the ecliptic is the apparent path of the Sun throughout the course of a year. Because Earth takes one year to orbit the Sun, the apparent position of the Sun takes one year to make a complete circuit of the ecliptic. With more than 365 days in one year, the Sun moves a little less than 1° eastward every day.
This small difference in the Sun's position against the stars causes any particular spot on Earth's surface to catch up with the Sun about four minutes each day than it would if Earth would not orbit. Again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun; the actual speed with which Earth orbits the Sun varies during the year, so the speed with which the Sun seems to move along the ecliptic varies. For example, the Sun is north of the celestial equator for about 185 days of each year, south of it for about 180 days; the variation of orbital speed accounts for part of the equation of time. Because Earth's rotational axis is not perpendicular to its orbital plane, Earth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about 23.4°, known as the obliquity of the ecliptic. If the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes.
The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. The crossing from south to north is known as the vernal equinox known as the first point of Aries and the ascending node of the ecliptic on the celestial equator; the crossing from north to south is descending node. The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26,000 years, a process known as lunisolar precession, as it is due to the gravitational effect of the Moon and Sun on Earth's equatorial bulge; the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, hence of the ecliptic, known as planetary precession; the combined action of these two motions is called general precession, changes the position of the equinoxes by about 50 arc seconds per year.
Once again, this is a simplification. Periodic motions of the Moon and apparent periodic motions of the Sun cause short-term small-amplitude periodic oscillations of Earth's axis, hence the celestial equator, known as nutation; this adds a periodic component to the position of the equinoxes. Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's equator with respect to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic, it is about 23.4° and is decreasing 0.013 degrees per hundred years due to planetary perturbations. The angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, from these ephemerides various astronomical values, including the obliquity, are derived; until 1983 the obliquity for any date was calculated from work of Newcomb, who analyzed positions of the planets until about 1895: ε = 23° 27′ 08″.26 − 46″.845 T − 0″.0059 T2 + 0″.00181 T3 where ε is the obliquity and T is tropical centuries from B1900.0 to the date in question.
From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated: ε = 23° 26′ 21″.45 − 46″.815 T − 0″.0006 T2 + 0″.00181 T3 where hereafter T is Julian centuries from J2000.0. JPL's fundamental ephemerides have been continually updated; the Astronomical Almanac for 2010 specifies:ε = 23° 26′ 21″.406 − 46″.836769 T − 0″.0001831 T2 + 0″.00200340 T3 − 0″.576×10−6 T4 − 4″.34×10−8 T5 These expressions for the obliquity are intended for high precision over a short time span ± several centuries. J. Laskar computed an expression to order T10 good to 0″.04/1000 years over 10,000 years. All of these expressions are for the mean obliquity, that is, without the nutation of the equator included; the true or instantaneous obliquity includes the nutation. Most of the major bodies of the Solar System o
