I was taught from Bate Mueller and White, that the proper terms for the closest and furthest points and distances from a body in orbit around another unspecified body are "periapsis" and "apoapsis" as depicted here.

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However, in comments to this question, it is strongly suggested that the usage of these terms is incorrect.

“Apses” are the points closest to and furthest from a primary body—see en.wikipedia.org/wiki/Apsis, among many other sources. These points are called “peri-” and “apo-” with the suffix pertaining to the body (e.g. “-gee” for Earth, thus “perigee”). The generic word for a celestial body is ἄστρον

“Periapsis” would mean “the point closest to the point closest to the body.” Nonsense. The proper word is “periastron”: “periapse” is a misconstruction.

Is the usage of "periapsis" really incorrect or perhaps just a misnomer?

Should we replace "periapsis" with "periastron"?

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    $\begingroup$ Peri- and Apoapsis are universal. "Astron" in turn would mean "star" so only to use for Peri-/Apoapses around stars. $\endgroup$
    – Greenhorn
    Commented Dec 1, 2020 at 18:22
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    $\begingroup$ Both are in common usage and both are well understood. My Greek wouldn't be good enough to say whether periapsis just means the closest apsis, but that is how it is interpreted. Obviously, periastron can only be applied to stars. $\endgroup$
    – ProfRob
    Commented Dec 1, 2020 at 18:47
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    $\begingroup$ This is probably more suited to ELU, but in short "No" We are speaking English not Greek. Etymology is not meaning. $\endgroup$
    – James K
    Commented Dec 1, 2020 at 18:52
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    $\begingroup$ That comment is just wrong. Periapsis and Apoapsis are the generic terms, Periastron and Apoastron are the terms for orbits around a star. $\endgroup$
    – Polygnome
    Commented Dec 1, 2020 at 23:37
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    $\begingroup$ IMHO, discussion of astronomical terminology is on-topic here. Astronomy has accumulated a lot of weird & wonderful terminology over the millennia, some of which can be rather misleading or downright bewildering when you first encounter it (eg eccentric anomaly). So I think it's important to have information about that terminology here. OTOH, I guess discussions about what's on topic belong on meta... ;) $\endgroup$
    – PM 2Ring
    Commented Dec 2, 2020 at 14:00

3 Answers 3


No. These words are English, not Greek. "Periapsis" means the point on the orbit when the two bodies are at their closest. It doesn't matter if this good Greek or bad Greek, it is correct English.

Apsis actually derives from "arch" and indicates the two points where the orbital curve is most "arched": ie the points of greatest curvature. There is the "periapsis": The close point of greatest curvature and the "apoapsis" "the distant point of great curvature".

There is no such thing as a "misconstruction". The idea that a word does not mean what it means is obviously nonsensical


A disclaimer first - I am not an astronomer, but I am a Greek with some ancient Greek language knowledge.

"Periapsis" is definitely ancient Greek and it derives from peri+apsi (περί+αψη). Apsi is the noun of the verb "άπτομαι" which means touch something.

Having said that, this word is only being used in astronomy and there is no literal translation to English, so periapsis and periastron could be used interchangeably. The only difference is that periastron refers explicitly to a star whereas periapsis can be referring to any object that orbits. But on the other hand, wikipedia says that "άστρον" is considered to be any celestial body (probably because ancient Greeks didn't know what an actual star is), so in that sense, the 2 words are completely interchangeable.

However "periastron" does not "sound" like it has anything to do with what you describe. Periastron could mean something around the star (e.g. a planet could be a periastron body). But that's just from a linguistics perspective


I’m the author of the original comment. I was paraphrasing from Jean Meeus, Astronomical Algorithms, Second Edition, p. 409.

Indeed, “periapsis” and “apoapsis” are common in English, but unlike what James K says, there is such a thing as a “misconstruct“ (my mistake for using “misconstruction“ in the first place); however, these do often enter common usage.

A language is define by its use of words, no matter if they’re properly or erroneously constructed. If “periapsis” and “apoapsis” are more common, so be it. The fact remains that they are redundant in their meaning, as the “apse” is the point meant—so a “periapsis” would be “the point closest to the point closest to the main body,” which is repetitive.

Anyhow, this belongs more in a linguistics forum than here.

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    $\begingroup$ I feel like you're reading way too much into the Greek etymology. Neither "peri-", "apo-" nor "apsis" contain any explicit reference to points. Translated literally, "periapsis" and "apoapsis" simply mean "near arch" and "far arch", just as "periastron" and "apoastron" simply mean "near star" and "far (from) star". In fact, if one wanted to avoid abbreviated jargon, it would be perfectly reasonable and non-redundant to explicitly refer to the "periapsis/periastron point" and the "apoapsis/apoastron point". But in common usage the "point" is often omitted for brevity. $\endgroup$ Commented Dec 2, 2020 at 14:53
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    $\begingroup$ "they are redundant in their meaning, as the “apse” is the point meant"...except there are two apsides in any elliptical orbit, a near apsis and a far one. It's not redundant at all. $\endgroup$ Commented Dec 2, 2020 at 16:42

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