Given that "peri" and "apo" are derived from Greek, it is considered by some purists^{3} 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 case of the Moon, in practice all three forms are used, albeit very infrequently. The "-cynthion" form is, according to some, reserved for artificial bodies, whereas 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) has also been suggested.
- For Jupiter, the "-jove" form is occasionally used by astronomers whereas 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 (related to what are known as Milankovitch cycles).

The day and hour of these events for recent and upcoming years are noted in the table below.^{4}

## Mathematical formulae

A diagram of Keplerian orbital elements. F*Periaps,*H

*Apoapsis*and the red line between them is the

*line of apsides*.

The following mathematical formulae 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 (conservation of angular momentum) and the conservation of energy, these quantities are constant for a given orbit:

- Specific relative angular momentum
- Specific orbital energy

where:

- is the semi-major axis
- is the standard gravitational parameter
- 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 . The geometric mean of the two distances is the length of the semi-minor axis .

The geometric mean 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).

## See also

## Notes

- ↑ Online Etymology Dictionary, Homepage. Retrieved November 14, 2008.
- ↑ Properly pronounced "affelion" because the (neo) Greek is αφήλιον, although the hypercorrection "ap-helion" is commonly heard.
- ↑ National Solar Observatgory, Apsis,
*Glossary of Terms.*Retrieved November 14, 2008. - ↑ U.S. Naval Observatory, Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000-2020, Astronomical Applications Department. Retrieved November 14, 2008.

## References

- Bate, Roger R., Donald D. Mueller, and Jerry E. White. 1971.
*Fundamentals of Astrodynamics.*New York: Dover Publications. ISBN 0486600610. - Montenbruck, Oliver, and Gill Eberhard. 2000.
*Satellite Orbits: Models, Methods, and Applications.*Berlin: Springer. ISBN 978-3540672807. - Open University. 1990.
*Planetary Orbits.*Mathematical Models and Methods, Unit 30. Milton Keynes, UK: Open University. ISBN 0749220368. - Rees, Martin J. (ed.). 2008.
*Universe.*New York, NY: DK. ISBN 978-0756636708. - Seeds, Michael A. 2008.
*The Solar System,*6th ed. Belmont, CA: Thomson Brooks/Cole. ISBN 978-0495387879. - Vallado, David Anthony, and Wayne D. McClain. 2001.
*Fundamentals of Astrodynamics and Applications.*Space Technology Library, 12. Dordrecht, The Netherlands: Kluwer Academic Publishers. ISBN 1881883124.

**·**Geostationary transfer

**·**Gravity assist

**·**Hohmann transfer

**·**Inclination change

**·**Phasing

**·**Rendezvous

**·**Transposition, docking, and extractionOther orbital mechanics topics

**Apsis**

**·**Celestial coordinate system

**·**Delta-v budget

**·**Epoch

**·**Ephemeris

**·**Equatorial coordinate system

**·**Gravity turn

**·**Ground track

**·**Interplanetary Transport Network

**·**Kepler's laws of planetary motion

**·**Lagrangian point

**·**Low energy transfers

**·**n-body problem

**·**Oberth effect

**·**Orbit equation

**·**Orbital speed

**·**Orbital state vectors

**·**Perturbation

**·**Retrograde and direct motion

**·**Specific orbital energy

**·**Specific relative angular momentumList of orbits