The distance from Earth to Mars, during their closest approach, is about 55 million kilometers. At their furthest apart, that distance would be about 401 million kilometers.

Distance at closest approach at ___________
Earth–Mars 55 million km 401 million km
Earth–Jupiter 629 million km 928 million km

Is there a standard astronomical name for "their furthest apart"?

My vague understanding is that if one body orbits around another, then we have words like "at perigee" (closest approach to Earth) "at apogee" (furthest apart from Earth); "perihelion" and "aphelion"; and in general "periapsis" and "apsis." However, it doesn't seem correct to talk about the distance from Mars to Earth "at apsis" because neither body orbits the other.

IIUC, in the specific case of Earth and Mars, we might also say that their closest approach happens (more or less) when Mars is in opposition (relative to the Sun, as seen from Earth); but I don't think I can infer from that any useful terminology for their maximum distance — which would happen more or less when Mars is in opposition relative to the Earth, as seen from the Sun. [eshaya's answer indicates that the phrase I'm looking for here is "...when Mars is in conjunction (relative to the Sun, as seen from Earth)." Contrariwise, for Venus, both extrema occur during conjunctions.]

(I'm interested in names for the distance and/or the position. "Closest approach" applies to both distance and position; "apsis" applies only to the position AFAIK. You wouldn't say "the apsis of the Earth and the Moon is 406,000 km.")

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    $\begingroup$ Farthest apart? $\endgroup$ – David Hammen Feb 6 at 17:42
  • $\begingroup$ in German we can (and do) describe that with the two antonyms like Sonnenferne (max distance to sun) and Sonnennähe (minimum distance to sun). That type of word can be created for any body XXX in the form of XXXferne and XXXnähe. $\endgroup$ – planetmaker Feb 8 at 20:09

The term "maximum separation" is often used, though maximum separation can also refer to the maximum angle between two bodies in the celestial sphere.

Here is an example from Quintana and Lissaur of the usage referring to distance:

close binary stars with maximum separations $Q_B≤0.2 AU$

and here is an example from Nouh of the usage referring to angle:

In this paper, an efficient algorithm is established for computing the maximum (minimum) angular separation ρ max(ρ min) [...] of visual binary stars

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    $\begingroup$ If we don't have a better answer by now, there may not be one. Many obscure concepts lack names. $\endgroup$ – Wayfaring Stranger Feb 7 at 17:17

Historically, astronomers focused on the easier to measure conjunctions, since getting distances is very hard, so knowing exactly when greatest distance occurs was not possible. Planets interior to the one you are on have inferior and superior conjunctions. When it is in front of the star, it is an inferior one, and when it is behind the star, it is a superior one. Superior planets have close and distant conjunctions.

There is a small ambiguous issue in defining when the planet is "aligned with the star" since it is never exactly aligned. When the planet is at the same equatorial right ascension ($\pm 12$ hrs) as the sun it is a conjunction in right ascension. When it is at the same ecliptic longitude ($\pm 180^\circ$) as the star, it is a conjunction in ecliptic longitude.

  • $\begingroup$ This doesn't seem like a general answer, but it does specifically answer the paragraph starting "IIUC, in the specific case of Earth and Mars..." So we might say that for Earth and Venus their closest approach occurs (more or less) during the inferior conjunction of Venus, and their maximum separation occurs (~) during the superior conjunction of Venus. And we might say that for Earth and Mars their closest approach occurs (~) when Mars is in opposition, and their maximum separation occurs (~) when Mars is in conjunction. $\endgroup$ – Quuxplusone Feb 8 at 20:27

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