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
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–
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
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
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.
SUFFIX -helion -hermion -cytherion -gee -lune -cynthion -selene -areion -zene -jove -chron -krone -saturnium -uranion -poseidon -hadion Origin
of the name
Other objects STARS GALAXIES BARYCENTER BLACK HOLES -astron -galacticon -center -focus -apsis -bothron -nigricon PERIHELION AND APHELION OF THE EARTH This section NEEDS ADDITIONAL CITATIONS FOR VERIFICATION . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed. (January 2017) (Learn how and when to remove this template message ) For the orbit of the
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
Currently, the
The
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
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)
Dwarf planet Ceres 380,951,528 km (236,712,305 mi) 446,428,973 km (277,398,103 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
Distances of selected bodies of the
The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets. *
* 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 *
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 |