Apsis

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Apsis

For Edenbridge
Edenbridge

Edenbridge may mean*Edenbridge, Kent, a town in England*Edenbridge , a symphonic metal band from Austria...
's album, see Aphelion
Aphelion (album)

Aphelion is the third album by Austria's Edenbridge . It was released in 2003....
.

In celestial mechanics

Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motion s of celestial objects. The field applies principles of physics, historically classical mechanics, to astronomical objects such as stars and planets to produce ephemeris data....
, an apsis, plural apsides is the point of greatest or least distance of the elliptical orbit of an object from its center of attraction, which is generally the center of mass

Center of mass

The center of mass of a system of wiktionary:Particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated....
of the system.

The point of closest approach is called the periapsis or pericentre and the point of farthest excursion is called the apoapsis (apó, "from", which becomes , ap- or , aph- before an unaspirated

Spiritus lenis

The spiritus lenis is a diacritic used in the polytonic orthography. In ancient Greek, it indicates the lack of initial aspiration , or the absence of the voiceless glottal fricative from the beginning of a word....
or aspirated vowel

Spiritus asper

The spiritus asper , is a diacritic used in the polytonic orthography. In ancient Greek, it indicates initial aspiration , or the presence of the voiceless glottal fricative at the beginning of a word....
, respectively), apocentre or apapsis (the latter term, although etymologically more correct, is much less used).

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Encyclopedia

In celestial mechanics

Celestial mechanics

Center of mass

Spiritus lenis

Spiritus asper

Ellipse

In mathematics, an ellipse is the apparent shape of a circle viewed obliquely from outside it, as distinct from a hyperbola which is the shape seen from inside....
, the line through the longest part of the ellipse.

Derivative terms are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth (Greek , gê, "earth"), and perihelion and aphelion, referring to orbits around the Sun (Greek , helios, "sun"). During the Apollo program, the terms pericynthion and apocynthion were used when referring to the moon.

Formula

These formula

Formula

In mathematics and in the sciences, a formula is a concise way of expressing information symbolically , or a general relationship between quantities....
e characterize the periapsis and apoapsis of an orbit:

Periapsis: maximum speed at minimum (periapsis) distance
Apoapsis: minimum speed at maximum (apoapsis) distance

while, in accordance with Kepler's laws of planetary motion

Kepler's laws of planetary motion

In astronomy, Kepler's three laws of planetary motion are*"The orbit of every planet is an ellipse with the sun at a Focus ."*"A line joining a planet and the sun sweeps out equal areas during equal intervals of time."...
(conservation of angular momentum

Angular momentum

In physics, the angular momentum of a particle about an origin is a vector quantity related to rotation, equal to the mass of the particle multiplied by the cross product of the position vector of the particle with its velocity vector....
) and the conservation of energy, these quantities are constant for a given orbit:

specific relative angular momentum
Specific relative angular momentum

In astrodynamics, the specific relative angular momentum of an orbiting body with respect to a central body is the relative angular momentum of the first body per Units of measurement mass....
specific orbital energy
Specific orbital energy

In astrodynamics the specific orbital energy of an orbiting body traveling through space under standard assumptions in astrodynamics is the sum of its potential energy and kinetic energy per unit mass....

where:

is the semi-major axis
Semi-major axis

In geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae....
is the standard gravitational parameter
Standard gravitational parameter

In astrodynamics, the standard gravitational parameter of a celestial body is the product of the gravitational constant and the mass :The units of the standard gravitational parameter are km3s-2...
is the eccentricity, defined as

Note that for conversion from heights above the surface to distances between an orbit and its primary, the radius of the central body has to be added, and conversely.

The arithmetic mean

Arithmetic mean

In mathematics and statistics, the arithmetic mean of a list of numbers is the sum of all of the list divided by the number of items in the list....
of the two limiting distances is the length of the semi-major axis

Semi-major axis

In geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae....
. The geometric mean

Geometric mean

The geometric mean, in mathematics, is a type of mean or average, which indicates the central tendency or typical value of a set of numbers. It is similar to the arithmetic mean, which is what most people think of with the word "average," except that instead of adding the set of numbers and then dividing the sum by the count of numbers in the...
of the two distances is the length of the semi-minor axis

Semi-minor axis

In geometry, the semi-minor axis is a line segment associated with most conic sections . One end of the segment is the center of the conic section, and it is at right angles with the semi-major axis....
.

The geometric means of the two limiting speeds is , the speed corresponding to a kinetic energy which, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body to escape (the square root of the product of the two speeds is the local escape velocity

Escape velocity

In physics, escape velocity is the speed where the kinetic energy of an object is equal to the magnitude of its gravitational potential energy, as calculated by the equation,...
). Aphelion also refers to a comet that is far from the Sun. A perihelion is also a comet that is close to the Sun.

Terminology

The words "pericentre" and "apocentre" are occasionally seen, although periapsis/apoapsis are preferred in technical usage.

Various related terms are used for other celestial objects. The '-gee', '-helion' and '-astron' and '-galacticon' forms are frequently used in the astronomical literature, while the other listed forms are occasionally used, although '-saturnium' has very rarely been used in the last 50 years. The '-gee' form is commonly (although incorrectly) used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. The term peri/apomelasma (from the Greek root) was used by physicist Geoffrey A. Landis

Geoffrey A. Landis

Geoffrey A. Landis works as a scientist and writer of science fiction.Landis holds undergraduate degrees in physics and electrical engineering from MIT and a Ph.D....
in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in 2002 .

Body	Closest approach	Farthest approach
Galaxy Galaxy A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter....	Perigalacticon	Apogalacticon
Star Star A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....	Periastron	Apastron
Black hole Black hole In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation , can escape its pull after having fallen past its event horizon....	Perimelasma/Peribothra/Perinigricon	Apomelasma/Apobothra/Aponigricon
Sun Sun The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....	Perihelion	Aphelion
Mercury Mercury (planet) Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 88 days. The orbit of Mercury has the highest Orbital eccentricity of all the Solar System planets, and it has the smallest axial tilt....	Perihermion	Apohermion
Venus Venus Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus , the Roman mythology goddess of love....	Pericytherion/Pericytherean/Perikrition	Apocytherion/Apocytherean/Apokrition
Earth Earth Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...	Perigee	Apogee
Moon Moon The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....	Periselene/Pericynthion/Perilune	Aposelene/Apocynthion/Apolune
Mars MARS In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....	Periareion	Apoareion
Jupiter	Perizene/Perijove	Apozene/Apojove
Saturn	Perikrone/Perisaturnium	Apokrone/Aposaturnium
Uranus Uranus Uranus is the seventh planet from the Sun and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus ....	Periuranion	Apouranion
Neptune NEPTUNE =Overview=The project, along with sister project, VENUS, offers a unique approach to ocean science. Traditionally, ocean scientists have relied on infrequent ship cruises or space-based satellites to carry out their research....	Periposeidion	Apoposeidion
Pluto Pluto Pluto , Minor planet names Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun....	Perihadion	Apohadion

Since "peri" and "apo" are Greek, it is considered by some purists more correct to use the Greek form for the body, giving forms such as '-zene' for Jupiter and '-krone' for Saturn. The daunting prospect of having to maintain a different word for every orbitable body in the solar system (and beyond) is the main reason why the generic '-apsis' has become the almost universal norm.

In the Moon's case, in practice all three forms are used, albeit very infrequently. The '-cynthion' form is, according to some, reserved for artificial bodies, whilst others reserve '-lune' for an object launched from the Moon and '-cynthion' for an object launched from elsewhere. The '-cynthion' form was the version used in the Apollo Project, following a NASA decision in 1964.
For Venus, the form '-cytherion' is derived from the commonly used adjective 'cytherean'; the alternate form '-krition' (from Kritias, an older name for Aphrodite
Aphrodite

Aphrodite is the classical Greek mythology goddess of love, sex, and beauty. According to Greek oral poet Hesiod, she was born when Uranus was castrated by his son Cronus....
) has also been suggested.
For Jupiter, the '-jove' form is occasionally used by astronomers whilst the '-zene' form is never used, like the other pure Greek forms ('-areion' (Mars), '-hermion' (Mercury), '-krone' (Saturn), '-uranion' (Uranus), '-poseidion' (Neptune) and '-hadion' (Pluto)).

Earth's perihelion and aphelion

The Earth is closest to the Sun in early January and farthest in early July. The relation between perihelion, aphelion and the Earth's seasons changes over a 21,000 year cycle. This anomalistic precession contributes to periodic climate change

Climate change

Climate change is any long-term significant change in the expected patterns of average weather of a specific region over an appropriately significant period of time....
(see Milankovitch cycles

Milankovitch cycles

Milankovitch cycles are the collective effect of changes in the Earth's movements upon its climate, named after Serbian civil engineering and mathematician Milutin Milankovic....
).

The day and hour UT

Coordinated Universal Time

Coordinated Universal Time is a time standard based on International Atomic Time with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation....
of these events for the next few years are:

Year	Perihelion	Aphelion
2007	January 3 20	July 7 00
2008	January 3 00	July 4 08
2009	January 4 15	July 4 02
2010	January 3 00	July 6 11
2011	January 3 19	July 4 15
2012	January 5 00	July 5 03
2013	January 2 05	July 5 15
2014	January 4 12	July 4 00
2015	January 4 07	July 6 19
2016	January 2 23	July 4 16
2017	January 4 14	July 3 20
2018	January 3 06	July 6 17
2019	January 3 05	July 4 22
2020	January 5 08	July 4 12

Planetary perihelion and aphelion

The images below show the Perihelion and Aphelion points of the inner and outer planets respectively.

Image:Inner Planet Orbits.jpg Image:Outer Planet Orbits.jpg

External links

Photographic Size Comparison
Photographic Size Comparison

Apsis

Formula

Terminology

Earth's perihelion and aphelion

Planetary perihelion and aphelion

See also

External links