Either of two extreme points in a celestial object's orbit
An
apsis
(from
Ancient Greek
?ψ??
(
hapsis
)
'arch, vault';
pl.
apsides
AP
-sih-deez
)
[1]
[2]
is the farthest or nearest point in the
orbit
of a
planetary body
about its
primary body
. The
line of apsides
is the line connecting the two
extreme values
.
Apsides pertaining to orbits around the
Sun
have distinct names to differentiate themselves from other apsides; these names are
aphelion
for the farthest and
perihelion
for the nearest point in the solar orbit.
[3]
The
Moon
's two apsides are the farthest point,
apogee
, and the nearest point,
perigee
, of its orbit around the host
Earth
. Earth's two apsides are the farthest point,
aphelion
, and the nearest point,
perihelion
, of its orbit around the host Sun. The terms
aphelion
and
perihelion
apply in the same way to the orbits of
Jupiter
and the other
planets
, the
comets
, and the
asteroids
of the
Solar System
.
General description
[
edit
]
There are two apsides in any
elliptic orbit
. The name for each apsis is created from the prefixes
ap-
,
apo-
(from
?π(?)
, (ap(o)-)
'away from') for the farthest or
peri-
(from
περ?
(peri-)
'near') for the closest point to the
primary body
, with a suffix that describes the primary body. The suffix for Earth is
-gee
, so the apsides' names are
apogee
and
perigee
. For the Sun, the suffix is
-helion
, so the names are
aphelion
and
perihelion
.
According to
Newton's laws of motion
, all periodic orbits are ellipses. The barycenter of the two bodies may lie well within the bigger body?e.g., the Earth?Moon barycenter is about 75% of the way from Earth's center to its surface.
[4]
If, compared to the larger mass, the smaller mass is negligible (e.g., for satellites), then the
orbital parameters
are independent of the smaller mass.
When used as a suffix?that is,
-apsis
?the term can refer to the two distances from the primary body to the orbiting body when the latter is located: 1) at the
periapsis
point, or 2) at the
apoapsis
point (compare both graphics, second figure). The line of apsides denotes the distance of the line that joins the nearest and farthest points across an orbit; it also refers simply to the extreme range of an object orbiting a host body (see top figure; see third figure).
In
orbital mechanics
, the apsides technically refer to the distance measured between the
center of mass
of the
central body
and the center of mass of the orbiting body. However, in the case of a
spacecraft
, the terms are commonly used to refer to the orbital
altitude
of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).
Terminology
[
edit
]
The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.
- For generic situations where the primary is not specified, the terms
pericenter
and
apocenter
are used for naming the extreme points of orbits (see table, top figure);
periapsis
and
apoapsis
(or
apapsis
) are equivalent alternatives, but these terms also frequently refer to distances?that is, the smallest and largest distances between the orbiter and its host body (see second figure).
- For a body orbiting the
Sun
, the point of least distance is the
perihelion
(
), and the point of greatest distance is the
aphelion
(
);
[5]
when discussing orbits around other stars the terms become
periastron
and
apastron
.
- When discussing a satellite of
Earth
, including the
Moon
, the point of least distance is the
perigee
(
), and of greatest distance, the
apogee
(from
Ancient Greek
: Γ? (
G?
), "land" or "earth").
[6]
- For objects in
lunar orbit
, the point of least distance are called the
pericynthion
(
) and the greatest distance the
apocynthion
(
). The terms
perilune
and
apolune
, as well as
periselene
and
aposelene
are also used.
[7]
Since the Moon has no natural satellites this only applies to man-made objects.
Etymology
[
edit
]
The words
perihelion
and
aphelion
were coined by
Johannes Kepler
[8]
to describe the orbital motions of the planets around the Sun.
The words are formed from the prefixes
peri-
(Greek:
περ?
, near) and
apo-
(Greek:
?π?
, away from), affixed to the Greek word for the Sun, (
?λιο?
, or
h?lios
).
[5]
Various related terms are used for other
celestial objects
. The suffixes
-gee
,
-helion
,
-astron
and
-galacticon
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, but
-saturnium
has very rarely been used in the last 50 years for Saturn. The
-gee
form is also used as a generic closest-approach-to "any planet" term?instead of applying it only to Earth.
During the
Apollo program
, the terms
pericynthion
and
apocynthion
were used when referring to
orbiting the Moon
; they reference Cynthia, an alternative name for the Greek Moon goddess
Artemis
.
[9]
More recently, during the
Artemis program
, the terms
perilune
and
apolune
have been used.
[10]
Regarding black holes, the term peribothron was first used in a 1976 paper by J. Frank and M. J. Rees,
[11]
who credit W. R. Stoeger for suggesting creating a term using the greek word for pit: "bothron".
The terms
perimelasma
and
apomelasma
(from a Greek root) were used by physicist and science-fiction author
Geoffrey A. Landis
in a story published in 1998,
[12]
thus appearing before
perinigricon
and
aponigricon
(from Latin) in the scientific literature in 2002.
[13]
Terminology summary
[
edit
]
The suffixes shown below may be added to prefixes
peri-
or
apo-
to form unique names of apsides for the orbiting bodies of the indicated host/
(primary)
system. However, only for the Earth, Moon and Sun systems are the unique suffixes commonly used.
Exoplanet
studies commonly use
-astron
, but typically, for other host systems the generic suffix,
-apsis
, is used instead.
[14]
[
failed verification
]
Perihelion and aphelion
[
edit
]
The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct
orbit
around the
Sun
.
Comparing
osculating elements
at a specific
epoch
to effectively those at a different epoch will generate differences. The time-of-perihelion-passage as one of six osculating elements is not an exact prediction (other than for a generic
two-body model
) of the actual minimum distance to the Sun using the
full dynamical model
. Precise predictions of perihelion passage require
numerical integration
.
Inner planets and outer planets
[
edit
]
The two images below show the orbits,
orbital nodes
, and positions of perihelion (q) and aphelion (Q) for the planets of the Solar System
[18]
as seen from above the northern pole of
Earth's ecliptic plane
, which is
coplanar
with
Earth's orbital plane
. The planets travel counterclockwise around the Sun and for each planet, the blue part of their orbit travels north of the ecliptic plane, the pink part travels south, and dots mark perihelion (green) and aphelion (orange).
The first image (below-left) features the
inner
planets, situated outward from the Sun as Mercury, Venus, Earth, and Mars. The
reference
Earth-orbit is colored yellow and represents the
orbital plane of reference
. At the time of vernal equinox, the Earth is at the bottom of the figure. The second image (below-right) shows the
outer
planets, being Jupiter, Saturn, Uranus, and Neptune.
The orbital nodes are the two end points of the
"line of nodes"
where a planet's tilted orbit intersects the plane of reference;
[19]
here they may be 'seen' as the points where the blue section of an orbit meets the pink.
-
The perihelion (green) and aphelion (orange) points of the
inner planets
of the Solar System
-
The perihelion (green) and aphelion (orange) points of the
outer planets
of the Solar System
Lines of apsides
[
edit
]
The chart shows the extreme range?from the closest approach (perihelion) to farthest point (aphelion)?of several orbiting
celestial bodies
of the
Solar System
: the planets, the known dwarf planets, including
Ceres
, and
Halley's Comet
. The length of the horizontal bars correspond to the extreme range of the orbit of the indicated body around the Sun. These extreme distances (between perihelion and aphelion) are
the lines of apsides
of the orbits of various objects around a host body.
Distances of selected bodies of the
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 is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.
Earth perihelion and aphelion
[
edit
]
Currently, the Earth reaches perihelion in early January, approximately 14 days after the
December solstice
. At perihelion, the Earth's center is about
0.983
29
astronomical units
(AU) or 147,098,070 km (91,402,500 mi) from the Sun's center. In contrast, the 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
1.016
71
AU
or 152,097,700 km (94,509,100 mi).
The dates of perihelion and aphelion change over time due to precession and other orbital factors, which follow cyclical patterns known as
Milankovitch cycles
. In the short term, such dates can vary up to 2 days from one year to another.
[20]
This significant variation is due to the presence of the Moon: while the Earth?Moon
barycenter
is moving on a stable orbit around the Sun, the position of the Earth's center which is on average about 4,700 kilometres (2,900 mi) from the barycenter, could be shifted in any direction from it?and this affects the timing of the actual closest approach between the Sun's and the Earth's centers (which in turn defines the timing of perihelion in a given year).
[21]
Because of the increased distance at aphelion, only 93.55% of the radiation from the Sun falls on a given area of Earth's surface as does at perihelion, but this does not account for the
seasons
, which result instead from the
tilt of Earth's axis
of 23.4° away from perpendicular to the plane of Earth's orbit.
[22]
Indeed, at both perihelion and aphelion it is
summer
in one hemisphere while it is
winter
in the other one. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun.
In the northern hemisphere, summer occurs at the same time as aphelion, when solar radiation is lowest. Despite this, summers in the northern hemisphere are on average 2.3 °C (4 °F) warmer than in the southern hemisphere, because the northern hemisphere contains larger land masses, which are easier to heat than the seas.
[23]
Perihelion and aphelion do however have an indirect effect on the seasons: because Earth's
orbital speed
is minimum at aphelion and maximum at perihelion, the planet takes longer to orbit from June solstice to September equinox than it does from December solstice to March equinox. Therefore, summer in the northern hemisphere lasts slightly longer (93 days) than summer in the southern hemisphere (89 days).
[24]
Astronomers commonly express the timing of perihelion relative to the
First Point of Aries
not in terms of days and hours, but rather as an angle of orbital displacement, the so-called
longitude of the periapsis
(also called longitude of the pericenter). For the orbit of the Earth, this is called the
longitude of perihelion
, and in 2000 it was about 282.895°; by 2010, this had advanced by a small fraction of a degree to about 283.067°,
[25]
i.e. a mean increase of 62" per year.
For the orbit of the 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 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 (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, called the
apsidal precession
. (This is closely related to the
precession of the axes
.) The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:
[26]
Year
|
Perihelion
|
Aphelion
|
Date
|
Time (
UT
)
|
Date
|
Time (
UT
)
|
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
|
2021
|
January 2
|
13:51
|
July 5
|
22:27
|
2022
|
January 4
|
06:55
|
July 4
|
07:11
|
2023
|
January 4
|
16:17
|
July 6
|
20:07
|
2024
|
January 3
|
00:39
|
July 5
|
05:06
|
2025
|
January 4
|
13:28
|
July 3
|
19:55
|
2026
|
January 3
|
17:16
|
July 6
|
17:31
|
2027
|
January 3
|
02:33
|
July 5
|
05:06
|
2028
|
January 5
|
12:28
|
July 3
|
22:18
|
2029
|
January 2
|
18:13
|
July 6
|
05:12
|
Other planets
[
edit
]
The following table shows the distances of the
planets
and
dwarf planets
from the Sun at their perihelion and aphelion.
[27]
Type of body
|
Body
|
Distance from Sun at perihelion
|
Distance from Sun at aphelion
|
difference (%)
|
insolation
difference (%)
|
Planet
|
Mercury
|
46,001,009 km (28,583,702 mi)
|
69,817,445 km (43,382,549 mi)
|
34%
|
57%
|
Venus
|
107,476,170 km (66,782,600 mi)
|
108,942,780 km (67,693,910 mi)
|
1.3%
|
2.8%
|
Earth
|
147,098,291 km (91,402,640 mi)
|
152,098,233 km (94,509,460 mi)
|
3.3%
|
6.5%
|
Mars
|
206,655,215 km (128,409,597 mi)
|
249,232,432 km (154,865,853 mi)
|
17%
|
31%
|
Jupiter
|
740,679,835 km (460,237,112 mi)
|
816,001,807 km (507,040,016 mi)
|
9.2%
|
18%
|
Saturn
|
1,349,823,615 km (838,741,509 mi)
|
1,503,509,229 km (934,237,322 mi)
|
10%
|
19%
|
Uranus
|
2,734,998,229 km (1.699449110
×
10
9
mi)
|
3,006,318,143 km (1.868039489
×
10
9
mi)
|
9.0%
|
17%
|
Neptune
|
4,459,753,056 km (2.771162073
×
10
9
mi)
|
4,537,039,826 km (2.819185846
×
10
9
mi)
|
1.7%
|
3.4%
|
Dwarf planet
|
Ceres
|
380,951,528 km (236,712,305 mi)
|
446,428,973 km (277,398,103 mi)
|
15%
|
27%
|
Pluto
|
4,436,756,954 km (2.756872958
×
10
9
mi)
|
7,376,124,302 km (4.583311152
×
10
9
mi)
|
40%
|
64%
|
Haumea
|
5,157,623,774 km (3.204798834
×
10
9
mi)
|
7,706,399,149 km (4.788534427
×
10
9
mi)
|
33%
|
55%
|
Makemake
|
5,671,928,586 km (3.524373028
×
10
9
mi)
|
7,894,762,625 km (4.905578065
×
10
9
mi)
|
28%
|
48%
|
Eris
|
5,765,732,799 km (3.582660263
×
10
9
mi)
|
14,594,512,904 km (9.068609883
×
10
9
mi)
|
60%
|
84%
|
Mathematical formulae
[
edit
]
These
formulae
characterize the pericenter and apocenter of an orbit:
- Pericenter
- Maximum speed,
, at minimum (pericenter) distance,
.
- Apocenter
- Minimum speed,
, at maximum (apocenter) distance,
.
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
- Specific orbital energy
where:
- is the distance from the apocenter to the primary focus
- is the distance from the pericenter to the primary focus
- a
is the
semi-major axis
:
- μ
is the
standard gravitational parameter
- e
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
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
which is the speed of a body in a circular orbit whose radius is
.
Time of perihelion
[
edit
]
Orbital elements
such as the
time of perihelion passage
are defined at the
epoch
chosen using an unperturbed
two-body solution
that does not account for the
n-body problem
. To get an accurate time of perihelion passage you need to use an epoch close to the perihelion passage. For example, using an epoch of 1996,
Comet Hale?Bopp
shows perihelion on 1 April 1997.
[28]
Using an epoch of 2008 shows a less accurate perihelion date of 30 March 1997.
[29]
Short-period comets
can be even more sensitive to the epoch selected. Using an epoch of 2005 shows
101P/Chernykh
coming to perihelion on 25 December 2005,
[30]
but using an epoch of 2012 produces a less accurate unperturbed perihelion date of 20 January 2006.
[31]
Numerical integration
shows
dwarf planet
Eris
will come to perihelion around December 2257.
[33]
Using an epoch of 2021, which is 236 years early, less accurately shows Eris coming to perihelion in 2260.
[34]
4 Vesta
came to perihelion on 26 December 2021,
[35]
but using a two-body solution at an epoch of July 2021 less accurately shows Vesta came to perihelion on 25 December 2021.
[36]
Short arcs
[
edit
]
Trans-Neptunian objects
discovered when 80+ AU from the Sun need dozens of observations over multiple years to well constrain their orbits because they move very slowly against the background stars. Due to statistics of small numbers, trans-Neptunian objects such as
2015 TH
367
when it had only 8 observations over an
observation arc
of 1 year that have not or will not come to perihelion for roughly 100 years can have a
1-sigma
uncertainty of 77.3 years (28,220 days) in the perihelion date.
[37]
See also
[
edit
]
References
[
edit
]
- ^
"apsis"
.
Dictionary.com Unabridged
(Online). n.d.
- ^
"apsis"
.
The American Heritage Dictionary of the English Language
(5th ed.). HarperCollins.
- ^
Joe Rao (July 6, 2023).
"Happy Aphelion Day! Earth is at its farthest from the sun for 2023 today"
.
Space.com
. Retrieved
April 22,
2024
.
- ^
"Earth-Moon Barycenter - SkyMarvels.com"
.
www.skymarvels.com
. Retrieved
April 23,
2024
.
- ^
a
b
Since the Sun, ?λιο? in Greek, begins with a vowel (H is the long ? vowel in Greek), the final o in "apo" is omitted from the prefix. =The pronunciation "Ap-helion" is given in many dictionaries
[1]
Archived
December 22, 2015, at the
Wayback Machine
, pronouncing the "p" and "h" in separate syllables. However, the pronunciation
[2]
Archived
July 29, 2017, at the
Wayback Machine
is also common (
e.g.,
McGraw Hill Dictionary of Scientific and Technical Terms,
5th edition, 1994, p. 114), since in late Greek, 'p' from ?π? followed by the 'h' from ?λιο? becomes phi; thus, the Greek word is αφ?λιον. (see, for example, Walker, John,
A Key to the Classical Pronunciation of Greek, Latin, and Scripture Proper Names
, Townsend Young 1859
[3]
Archived
September 21, 2019, at the
Wayback Machine
, page 26.) Many
[4]
dictionaries give both pronunciations
- ^
Chisholm, Hugh
, ed. (1911).
"Perigee"
.
Encyclopædia Britannica
. Vol. 21 (11th ed.). Cambridge University Press. p. 149.
- ^
a
b
c
d
"Basics of Space Flight"
. NASA.
Archived
from the original on September 30, 2019
. Retrieved
May 30,
2017
.
- ^
Klein, Ernest,
A Comprehensive Etymological Dictionary of the English Language
, Elsevier, Amsterdam, 1965. (
Archived version
)
- ^
"Apollo 15 Mission Report"
.
Glossary
.
Archived
from the original on March 19, 2010
. Retrieved
October 16,
2009
.
- ^
R. Dendy; D. Zeleznikar; M. Zemba (September 27, 2021).
NASA Lunar Exploration ? Gateway's Power and Propulsion Element Communications Links
. 38th International Communications Satellite Systems Conference (ICSSC). Arlington, VA.
Archived
from the original on March 29, 2022
. Retrieved
July 18,
2022
.
- ^
Frank, J.; Rees, M.J. (September 1, 1976).
"Effects of massive black holes on dense stellar systems"
.
MNRAS
.
176
(6908): 633?646.
Bibcode
:
1976MNRAS.176..633F
.
doi
:
10.1093/mnras/176.3.633
.
- ^
Perimelasma
Archived
February 25, 2019, at the
Wayback Machine
, by Geoffrey Landis, first published in
Asimov's Science Fiction
, January 1998, republished at
Infinity Plus
- ^
R. Schodel; T. Ott; R. Genzel; R. Hofmann; M. Lehnert; A. Eckart; N. Mouawad; T. Alexander; M. J. Reid; R. Lenzen; M. Hartung; F. Lacombe; D. Rouan; E. Gendron; G. Rousset; A.-M. Lagrange; W. Brandner; N. Ageorges; C. Lidman; A. F. M. Moorwood; J. Spyromilio; N. Hubin; K. M. Menten (October 17, 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way".
Nature
.
419
(6908): 694?696.
arXiv
:
astro-ph/0210426
.
Bibcode
:
2002Natur.419..694S
.
doi
:
10.1038/nature01121
.
PMID
12384690
.
S2CID
4302128
.
- ^
"MAVEN ≫ Science Orbit"
.
Archived
from the original on November 8, 2018
. Retrieved
November 7,
2018
.
- ^
"Dawn Journal: 11 Years in Space"
.
www.planetary.org
.
Archived
from the original on October 24, 2018
. Retrieved
October 24,
2018
.
- ^
Cecconi, B.; Lamy, L.; Zarka, P.; Prange, R.; Kurth, W. S.; Louarn, P. (March 4, 2009).
"Goniopolarimetric study of the revolution 29 perikrone using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver"
.
Journal of Geophysical Research: Space Physics
.
114
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Bibcode
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{{
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: CS1 maint: bot: original URL status unknown (
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External links
[
edit
]
Look up
apsis
in Wiktionary, the free dictionary.
- Apogee ? Perigee
Photographic Size Comparison, perseus.gr
- Aphelion ? Perihelion
Photographic Size Comparison, perseus.gr
- Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000?2020
Archived
October 13, 2007, at the
Wayback Machine
, usno.navy.mil
- Dates and times of Earth's perihelion and aphelion, 2000?2025
Archived
October 13, 2007, at the
Wayback Machine
from the
United States Naval Observatory
- List of asteroids currently closer to the Sun than Mercury
(These objects will be close to perihelion)
- JPL SBDB
list of Main-Belt Asteroids (H<8) sorted by perihelion date