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In astronomy, an apsis (plural apsides) is either of the two points in an orbit at which the orbiting body makes its closest or farthest approach to the primary.[1][2]

The point of closest approach is called the periapsis (from the Greek peri around).

The point of farthest approach is called the apoapsis or apapsis (from the Greek apo away from).


Astronomers often change the root word apsis to a root specific to the primary under discussion. Most of these roots are Greek-derived, although some Latin-derived roots have occasionally appeared in the technical literature or even on the Internet.

The following table lists the proper roots, including some Latin-derived roots:

Primary Greek Latin
Galaxy galacticon
Star astron[3]
Black hole melasma nigricon
Sun helion[3]
Mercury hermion
Venus cytherion[4]
Earth gee
Moon selene[5] lune
Mars areion
Jupiter zene[6] jove
Saturn krone saturnion
Uranus uranion
Neptune poseidion
Pluto hadion

References and notes

  1. "Entry for 'Apsis'." Wiktionary. Accessed January 16, 2008.
  2. Dooling, Dave, and Kneale, Ruth A., eds. "Entry for 'Apsis'," Glossary of Terms, National Solar Observatory, February 21, 2005. Accessed January 16, 2008.
  3. 3.0 3.1 When combining the apo- prefix with the roots astron and helion, the letter o is elided.
  4. An alternative root krition (from the Greek Kritias, an older form of Aphrodite, the Greek equivalent of Venus) sometimes appears.
  5. In Project Apollo the root cynthion was in official use. The rationale is that cynthion was the proper root for an artificial body in orbit around the Moon.
  6. This root never appears in the astronomical literature. Astronomers prefer to use jove or else the generic apsis. The root zene derives from a more ancient genitive, zenos, of the name Zeus.