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General description

There are two apsides in any elliptic orbit. The name for each apsis is created from the prefixes ap-, apo- (from ἀπ(ό), (ap(o)-) 'away from') for the farthest or peri- (from περί (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 barycenter of the 2-body system 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

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 (/ˌpɛrɪˈhiːliən/), and the point of greatest distance is the aphelion (/æpˈhiːliən/);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 (/ˈpɛrɪdʒiː/), and of greatest distance, the apogee (from Ancient Greek: Γῆ (Gē), "land" or "earth").⁶
• For objects in lunar orbit, the point of least distance are called the pericynthion (/ˌpɛrɪˈsɪnθiən/) and the greatest distance the apocynthion (/ˌæpəˈsɪnθiən/). 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

The words perihelion and aphelion were coined by Johannes Kepler⁸ to describe the orbital motions of the planets around the Sun. The words are formed from the prefixes peri- (Greek: περί, near) and apo- (Greek: ἀπό, away from), affixed to the Greek word for the Sun, (ἥλιος, or hēlíos).⁹

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.¹⁰ More recently, during the Artemis program, the terms perilune and apolune have been used.¹¹

Regarding black holes, the term peribothron was first used in a 1976 paper by J. Frank and M. J. Rees,¹² 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 1998,¹³ thus appearing before perinigricon and aponigricon (from Latin) in the scientific literature in 2002.¹⁴

Terminology summary

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.¹⁵

Host objects in the Solar System with named/nameable apsides

Astronomicalhost object	Suffix	Originof the name
Sun	-helion	Helios
Mercury	-hermion	Hermes
Venus	-cythe-cytherion	Cytherean
Earth	-gee	Gaia
Moon	-lune16-cynthion-selene17	LunaCynthiaSelene
Mars	-areion	Ares
Ceres	-demeter18	Demeter
Jupiter	-jove	ZeusJupiter
Saturn	-chron19-kronos-saturnium-krone20	CronosSaturn
Uranus	-uranion	Uranus
Neptune	-poseideum21-poseidion	Poseidon

Other host objects with named/nameable apsides

Astronomical host object	Suffix	Originof the name
Star	-astron	Lat: astra; stars
Galaxy	-galacticon	Gr: galaxias; galaxy
Barycenter	-center-focus-apsis
Black hole	-melasma-bothron-nigricon	Gr: melos; blackGr: bothros; holeLat: niger; black

Perihelion and aphelion

"Perihelion" redirects here. For other uses, see Perihelion (disambiguation).

"Aphelion" redirects here. For other uses, see Aphelion (disambiguation).

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 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

The two images below show the orbits, orbital nodes, and positions of perihelion (q) and aphelion (Q) for the planets of the Solar System²² 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;²³ here they may be 'seen' as the points where the blue section of an orbit meets the pink.

Lines of apsides

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

Currently, the Earth reaches perihelion in early January, approximately 14 days after the December solstice. At perihelion, the Earth's center is about 0.98329 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 1.01671 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.²⁴ 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).²⁵

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.²⁶ 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.²⁷

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).²⁸

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°,²⁹ 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:³⁰

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	20:0231
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
2030	January 3	10:12	July 4	12:58
2031	January 4	20:48	July 6	07:10
2032	January 3	05:11	July 5	11:54
2033	January 4	11:51	July 3	20:52
2034	January 4	04:47	July 6	18:49
2035	January 3	00:54	July 5	18:22

Other planets

The following table shows the distances of the planets and dwarf planets from the Sun at their perihelion and aphelion.³²

Type of body	Body	Distance from Sun at perihelion		Distance from Sun at aphelion		Difference (%)‡	Insolationdifference (%)#
		(km)	(miles)	(km)	(miles)
Planet	Mercury	46,001,009 km	28,583,702 mi	69,817,445 km	43,382,549 mi	34.1%	56.6%
	Venus	107,476,170 km	66,782,600 mi	108,942,780 km	67,693,910 mi	1.3%	2.7%
	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	17.1%	31.2%
	Jupiter	740,679,835 km	460,237,112 mi	816,001,807 km	507,040,016 mi	9.2%	17.6%
	Saturn	1,349,823,615 km	838,741,509 mi	1,503,509,229 km	934,237,322 mi	10.2%	19.4%
	Uranus	2,734,998,229 km	1.699449110×109 mi	3,006,318,143 km	1.868039489×109 mi	9.0%	17.2%
	Neptune	4,459,753,056 km	2.771162073×109 mi	4,537,039,826 km	2.819185846×109 mi	1.7%	3.4%
Dwarf planet	Ceres	380,951,528 km	236,712,305 mi	446,428,973 km	277,398,103 mi	14.7%	27.2%
	Pluto	4,436,756,954 km	2.756872958×109 mi	7,376,124,302 km	4.583311152×109 mi	39.8%	63.8%
	Haumea	5,157,623,774 km	3.204798834×109 mi	7,706,399,149 km	4.788534427×109 mi	33.1%	55.2%
	Makemake	5,671,928,586 km	3.524373028×109 mi	7,894,762,625 km	4.905578065×109 mi	28.2%	48.4%
	Eris	5,765,732,799 km	3.582660263×109 mi	14,594,512,904 km	9.068609883×109 mi	60.5%	84.4%

‡ Defined as 1 − (perihelion distance ÷ aphelion distance)

# Defined as 1 − (perihelion distance ÷ aphelion distance)2

Mathematical formulae

These formulae characterize the pericenter and apocenter of an orbit:

Pericenter Maximum speed, v per = ( 1 + e ) μ ( 1 − e ) a {\textstyle v_{\text{per}}={\sqrt {\frac {(1+e)\mu }{(1-e)a}}}\,} , at minimum (pericenter) distance, r per = ( 1 − e ) a {\textstyle r_{\text{per}}=(1-e)a} . Apocenter Minimum speed, v ap = ( 1 − e ) μ ( 1 + e ) a {\textstyle v_{\text{ap}}={\sqrt {\frac {(1-e)\mu }{(1+e)a}}}\,} , at maximum (apocenter) distance, r ap = ( 1 + e ) a {\textstyle r_{\text{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 ) μ a {\displaystyle h={\sqrt {\left(1-e^{2}\right)\mu a}}} Specific orbital energy ε = − μ 2 a {\displaystyle \varepsilon =-{\frac {\mu }{2a}}}

where:

• r ap {\textstyle r_{\text{ap}}} is the distance from the apocenter to the primary focus
• r per {\textstyle r_{\text{per}}} is the distance from the pericenter to the primary focus
• a is the semi-major axis: a = r per + r ap 2 {\displaystyle a={\frac {r_{\text{per}}+r_{\text{ap}}}{2}}}
• μ is the standard gravitational parameter
• e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 {\displaystyle e={\frac {r_{\text{ap}}-r_{\text{per}}}{r_{\text{ap}}+r_{\text{per}}}}=1-{\frac {2}{{\frac {r_{\text{ap}}}{r_{\text{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 ε = μ a {\displaystyle {\sqrt {-2\varepsilon }}={\sqrt {\frac {\mu }{a}}}}

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

Time of perihelion

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 1997.³³ Using an epoch of 2008 shows a less accurate perihelion date of 30 March 1997.³⁴ 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 2005,³⁵ but using an epoch of 2012 produces a less accurate unperturbed perihelion date of 20 January 2006.³⁶

Two body solution vs n-body solution for 12P/Pons–Brooks time of perihelion passage

Epoch	Date of perihelion (tp)
2010	2024-Apr-19.892
n-body37	2024-Apr-21.139
2018	2024-Apr-23.069

Numerical integration shows dwarf planet Eris will come to perihelion around December 2257.³⁸ Using an epoch of 2021, which is 236 years early, less accurately shows Eris coming to perihelion in 2260.³⁹

4 Vesta came to perihelion on 26 December 2021,⁴⁰ but using a two-body solution at an epoch of July 2021 less accurately shows Vesta came to perihelion on 25 December 2021.⁴¹

Short arcs

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 TH367 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.⁴²

See also

• Distance of closest approach
• Eccentric anomaly
• Flyby (spaceflight)
• Hyperbolic trajectory § Closest approach
• Mean anomaly
• Perifocal coordinate system
• True anomaly

External links

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

References

1. "apsis". Dictionary.com Unabridged (Online). n.d. https://www.dictionary.com/browse/apsis ↩

2. "apsis". The American Heritage Dictionary of the English Language (5th ed.). HarperCollins. https://www.ahdictionary.com/word/search.html?q=apsis ↩

3. 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. https://www.space.com/earth-farthest-from-sun-aphelion-july-2023 ↩

4. "Earth-Moon Barycenter - SkyMarvels.com". www.skymarvels.com. Retrieved April 23, 2024. https://www.skymarvels.com/gallery/Vid%20-%20Earth-Moon%20Barycenter.htm ↩

5. Since the Sun, Ἥλιος in Greek, begins with a vowel (H is the long ē 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 /əˈfiːliən/ [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 ἀπό followed by the 'h' from ἥλιος becomes phi; thus, the Greek word is αφήλιον. (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 https://www.oxforddictionaries.com/definition/english/aphelion ↩

6. Chisholm, Hugh, ed. (1911). "Perigee" . Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 149. /wiki/Hugh_Chisholm ↩

7. "Basics of Space Flight". NASA. Archived from the original on September 30, 2019. Retrieved May 30, 2017. https://solarsystem.nasa.gov/basics/glossary ↩

8. Klein, Ernest, A Comprehensive Etymological Dictionary of the English Language, Elsevier, Amsterdam, 1965. (Archived version) https://archive.org/stream/AComprehensiveEtymologicalDictionaryOfTheEnglishLanguageByErnestKlein/A%20Comprehensive%20Etymological%20Dictionary%20of%20the%20English%20Language%20by%20Ernest%20Klein_djvu.txt ↩

9. Since the Sun, Ἥλιος in Greek, begins with a vowel (H is the long ē 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 /əˈfiːliən/ [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 ἀπό followed by the 'h' from ἥλιος becomes phi; thus, the Greek word is αφήλιον. (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 https://www.oxforddictionaries.com/definition/english/aphelion ↩

10. "Apollo 15 Mission Report". Glossary. Archived from the original on March 19, 2010. Retrieved October 16, 2009. https://history.nasa.gov/alsj/a15/a15mr-f.htm ↩

11. 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. https://ntrs.nasa.gov/citations/20210019019 ↩

12. 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. https://doi.org/10.1093%2Fmnras%2F176.3.633 ↩

13. 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 http://www.infinityplus.co.uk/stories/perimelasma.htm ↩

14. R. Schödel; 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. /wiki/ArXiv_(identifier) ↩

15. "MAVEN » Science Orbit". Archived from the original on November 8, 2018. Retrieved November 7, 2018. http://lasp.colorado.edu/home/maven/science/science-orbit/ ↩

16. "Basics of Space Flight". NASA. Archived from the original on September 30, 2019. Retrieved May 30, 2017. https://solarsystem.nasa.gov/basics/glossary ↩

17. "Basics of Space Flight". NASA. Archived from the original on September 30, 2019. Retrieved May 30, 2017. https://solarsystem.nasa.gov/basics/glossary ↩

18. "Dawn Journal: 11 Years in Space". www.planetary.org. Archived from the original on October 24, 2018. Retrieved October 24, 2018. http://www.planetary.org/blogs/guest-blogs/marc-rayman/20181019-dawn-journal-11-years-in-space.html ↩

19. "Basics of Space Flight". NASA. Archived from the original on September 30, 2019. Retrieved May 30, 2017. https://solarsystem.nasa.gov/basics/glossary ↩

20. Cecconi, B.; Lamy, L.; Zarka, P.; Prangé, 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 (A3): A03215. Bibcode:2009JGRA..114.3215C. doi:10.1029/2008JA013830. Archived from the original on December 9, 2019. Retrieved December 9, 2019 – via ui.adsabs.harvard.edu. https://ui.adsabs.harvard.edu/abs/2009JGRA..114.3215C/abstract ↩

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