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An APSIS (Greek : ἁψίς; plural APSIDES /ˈæpsᵻdiːz/ , Greek: ἁψῖδες) is an extreme point in an object's orbit . The word comes via Latin from Greek and is cognate with apse . For elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- (from περί (peri), meaning 'near') and ap-, or apo- (from ἀπ(ό) (ap(ó)), meaning 'away from') added to a reference to the thing being orbited.

* For a body orbiting the Sun
Sun
, the point of least distance is the PERIHELION (/ˌpɛrᵻˈhiːliən/ ), and the point of greatest distance is the APHELION (/æpˈhiːliən/ ). * The terms become PERIASTRON and APASTRON when discussing orbits around other stars. * For any satellite of Earth
Earth
including the Moon
Moon
the point of least distance is the PERIGEE (/ˈpɛrᵻdʒiː/ ) and greatest distance the APOGEE. * For objects in Lunar orbit, the point of least distance is the PERICYNTHION (/ˌpɛrɪˈsɪnθiən/ ) and the greatest distance the APOCYNTHION (/ˌæpəˈsɪnθiən/ ). PERILUNE and APOLUNE are also used. * For any orbits around a center of mass, there are the terms PERIAPSIS and APOAPSIS (or apapsis). PERICENTER and APOCENTER are equivalent alternatives.

A straight line connecting the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse , its greatest diameter. For a two-body system the center of mass of the system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other it may be taken to be at this focus. However whether or not this is the case, both bodies are in similar elliptical orbits each having one focus at the system's center of mass, with their respective lines of apsides being of length inversely proportional to their masses. Historically, in geocentric systems , apsides were measured from the center of the Earth. However, in the case of the Moon, the center of mass of the Earth–Moon system, or Earth– Moon
Moon
barycenter , as the common focus of both the Moon's and Earth's orbits about each other, is about 74% of the way from Earth's center to its surface.

In orbital mechanics , the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft , the family of terms are commonly used to refer to the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius).

CONTENTS

* 1 Mathematical formulae

* 2 Terminology

* 2.1 Terminology graph

* 3 Perihelion
Perihelion
and aphelion of the Earth
Earth
* 4 Planetary perihelion and aphelion * 5 See also * 6 References * 7 External links

MATHEMATICAL FORMULAE

Keplerian orbital elements : point F is at the pericenter, point H is at the apocenter, and the red line between them is the line of apsides

These formulae characterize the pericenter and apocenter of an orbit:

* Pericenter: maximum speed v p e r = ( 1 + e ) ( 1 e ) a {displaystyle v_{mathrm {per} }={sqrt {tfrac {(1+e)mu }{(1-e)a}}},} at minimum (pericenter) distance r p e r = ( 1 e ) a {displaystyle r_{mathrm {per} }=(1-e)a!,} * Apocenter: minimum speed v a p = ( 1 e ) ( 1 + e ) a {displaystyle v_{mathrm {ap} }={sqrt {tfrac {(1-e)mu }{(1+e)a}}},} at maximum (apocenter) distance r a p = ( 1 + e ) a {displaystyle r_{mathrm {ap} }=(1+e)a!,}

while, in accordance with Kepler\'s laws of planetary motion (based on the conservation of angular momentum ) and the conservation of energy, these two quantities are constant for a given orbit:

* specific relative angular momentum h = ( 1 e 2 ) a {displaystyle h={sqrt {left(1-e^{2}right)mu a}}} * specific orbital energy = 2 a {displaystyle varepsilon =-{frac {mu }{2a}}}

where:

* a is the semi-major axis , equal to r p e r + r a p 2 {displaystyle {frac {r_{mathrm {per} }+r_{mathrm {ap} }}{2}}} * μ is the standard gravitational parameter * e is the eccentricity , defined as e = r a p r p e r r a p + r p e r = 1 2 r a p r p e r + 1 {displaystyle e={frac {r_{mathrm {ap} }-r_{mathrm {per} }}{r_{mathrm {ap} }+r_{mathrm {per} }}}=1-{frac {2}{{frac {r_{mathrm {ap} }}{r_{mathrm {per} }}}+1}}}

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 of the two limiting distances is the length of the semi-major axis a. The geometric mean of the two distances is the length of the semi-minor axis b.

The geometric mean of the two limiting speeds is 2 = a {displaystyle {sqrt {-2varepsilon }}={sqrt {frac {mu }{a}}}}

which is the speed of a body in a circular orbit whose radius is a {displaystyle a} .

TERMINOLOGY

The words "pericenter" and "apocenter" are often 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 when referring to the Earth, Sun, stars and the Galactic Center respectively. The suffix '-jove' is occasionally used for Jupiter, while '-saturnium' has very rarely been used in the last 50 years for Saturn. The '-gee' form is commonly used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. During the Apollo program
Apollo program
, the terms pericynthion and apocynthion (referencing Cynthia, an alternative name for the Greek Moon
Moon
goddess Artemis
Artemis
) were used when referring to the Moon
Moon
. Regarding black holes, the term peri/apomelasma (from a Greek root) was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon (from a Latin) appeared in the scientific literature in 2002.

TERMINOLOGY GRAPH

The following suffixes are added to peri- and apo- to form the terms for the nearest and farthest orbital distances from these objects.

Solar System
Solar System
objects ASTRONOMICAL OBJECT The Sun Mercury Venus Earth Moon Mars Jupiter Saturn Uranus Neptune Pluto

SUFFIX -helion -hermion -cytherion -gee -lune -cynthion -selene -areion -zene -jove -chron -krone -saturnium -uranion -poseidon -hadion

Origin of the name Helios
Helios
Hermes
Hermes
Cytherea Gaia Selene Ares
Ares
Zeus
Zeus
Jupiter
Jupiter
Cronos Saturn
Saturn
Uranus
Uranus
Poseidon
Poseidon
Hades
Hades

Other objects STARS GALAXIES BARYCENTER BLACK HOLES

-astron -galacticon -center -focus -apsis -bothron -nigricon

PERIHELION AND APHELION OF THE EARTH

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For the orbit of the Earth
Earth
around the Sun, the time of apsis is often expressed in terms of a time relative to seasons, since this determines the contribution of the elliptical orbit to seasonal variations. The variation of the seasons is primarily controlled by the annual cycle of the elevation angle of the Sun, which is a result of the tilt of the axis of the Earth
Earth
measured from the plane of the ecliptic . The Earth's eccentricity and other orbital elements are not constant, but vary slowly due to the perturbing effects of the planets and other objects in the solar system. See Milankovitch cycles .

On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth
Earth
that is called the apsidal precession . (This is closely related to the precession of the axis .)

Currently, the Earth
Earth
reaches perihelion in early January, approximately 14 days after the December Solstice
December Solstice
. At perihelion, the Earth's center is about 6999983290000000000♠0.98329 astronomical units (AU) or 147,098,070 km (91,402,500 mi) from the Sun's center.

The Earth
Earth
reaches aphelion currently in early July, approximately 14 days after the June Solstice . The aphelion distance between the Earth's and Sun's centers is currently about 7011152097651119397♠1.01671 AU or 152,097,700 km (94,509,100 mi).

Astronomers commonly express the timing of perihelion relative to the vernal equinox not in terms of days and hours, but rather as an angle of orbital displacement, the so-called longitude of the pericenter . For the orbit of the Earth, this is called the longitude of perihelion, and in 2000 was about 282.895°. By the year 2010, this had advanced by a small fraction of a degree to about 283.067°.

The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:

YEAR PERIHELION APHELION

DATE TIME (UT ) DATE TIME (UT )

2007 January 3 19:43 July 6 23:53

2008 January 2 23:51 July 4 07:41

2009 January 4 15:30 July 4 01:40

2010 January 3 00:09 July 6 11:30

2011 January 3 18:32 July 4 14:54

2012 January 5 00:32 July 5 03:32

2013 January 2 04:38 July 5 14:44

2014 January 4 11:59 July 4 00:13

2015 January 4 06:36 July 6 19:40

2016 January 2 22:49 July 4 16:24

2017 January 4 14:18 July 3 20:11

2018 January 3 05:35 July 6 16:47

2019 January 3 05:20 July 4 22:11

2020 January 5 07:48 July 4 11:35

PLANETARY PERIHELION AND APHELION

The following table shows the distances of the planets and dwarf planets from the Sun
Sun
at their perihelion and aphelion.

TYPE OF BODY BODY DISTANCE FROM SUN AT PERIHELION DISTANCE FROM SUN AT APHELION

Planet Mercury 46,001,009 km (28,583,702 mi) 69,817,445 km (43,382,549 mi)

Venus
Venus
107,476,170 km (66,782,600 mi) 108,942,780 km (67,693,910 mi)

Earth
Earth
147,098,291 km (91,402,640 mi) 152,098,233 km (94,509,460 mi)

Mars
Mars
206,655,215 km (128,409,597 mi) 249,232,432 km (154,865,853 mi)

Jupiter
Jupiter
740,679,835 km (460,237,112 mi) 816,001,807 km (507,040,016 mi)

Saturn
Saturn
1,349,823,615 km (838,741,509 mi) 1,503,509,229 km (934,237,322 mi)

Uranus
Uranus
2,734,998,229 km (1.699449110×109 mi) 3,006,318,143 km (1.868039489×109 mi)

Neptune
Neptune
4,459,753,056 km (2.771162073×109 mi) 4,537,039,826 km (2.819185846×109 mi)

Dwarf planet Ceres 380,951,528 km (236,712,305 mi) 446,428,973 km (277,398,103 mi)

Pluto
Pluto
4,436,756,954 km (2.756872958×109 mi) 7,376,124,302 km (4.583311152×109 mi)

Makemake 5,671,928,586 km (3.524373028×109 mi) 7,894,762,625 km (4.905578065×109 mi)

Haumea 5,157,623,774 km (3.204798834×109 mi) 7,706,399,149 km (4.788534427×109 mi)

Eris 5,765,732,799 km (3.582660263×109 mi) 14,594,512,904 km (9.068609883×109 mi)

The following chart shows the range of distances of the planets, dwarf planets and Halley\'s Comet
Comet
from the Sun.

Distances of selected bodies of the Solar System
Solar System
from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity . The radius of the Sun
Sun
is 0.7 million km, and the radius of Jupiter
Jupiter
(the largest planet) is 0.07 million km, both too small to resolve on this image.

The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.

* Perihelion
Perihelion
and aphelion points

*

The perihelion and aphelion points of the inner planets of the Solar System *

The perihelion and aphelion points of the outer planets of the Solar System

SEE ALSO

* Eccentric anomaly
Eccentric anomaly
* Perifocal coordinate system * Solstice

REFERENCES

* ^ "the definition of apsis". Dictionary.com. * ^ Since the Sun, Ἥλιος in Greek, begins with a vowel (H is considered a vowel in Greek), the final o in "apo" is omitted from the prefix. The pronunciation "Ap-helion" is given in many dictionaries , pronouncing the "p" and "h" in separate syllables. Howeve