Astronomy terms for nearest and furthest distances in established orbits
February 11, 2017 11:46 PM   Subscribe

Is there a generic astronomy term for the least or greatest possible distance between two bodies orbiting the same mass?

For example, Phobos and Deimos both orbit Mars. Is there a word to describe the moment when they are as far away from one another, or as close to one another, as their orbits will allow? Not their apogee or perigee to mars, but their max/min distance to each other.

Another example, both Earth and Halley's Comet orbit the sun. Is there a term for when they are at their theoretical closest or furthest apart in relation to one another?
posted by peeedro to Science & Nature (5 answers total) 3 users marked this as a favorite
 
I believe it's pericenter and apocenter (Wiki link.)
posted by Hardcore Poser at 12:18 AM on February 12, 2017


That's the term for position in respect to the central body.

I'm not sure that there is a specific term for minimum or maximum distance between two bodies orbiting a third central one. I used to be very interested in computational astronomy and taught orbit theory for a while and I've never come across one.

When two planets are in a line (or as close as they come to it, as their orbits are in slightly different planes) with the Sun the term used is syzygy - as that article notes, this includes eclipses, transits, oppositions and inferior/superior conjunction.

If two planets are in coplanar circular orbits then their moments of closest approach and furthest distance would be at syzygy, but that would be describing their relative positions rather than the distance between them. Also, real orbits aren't that simple - as this article on Mars notes, the closest approach of Mars to Earth can happen up to ten days before or after the date that Mars is in opposition.

The most relevant application of this is for satellite tracking and in particular predicting and if possible avoiding collisions or excessively close approaches. This presentation (PDF) indicates that NASA uses the term 'conjunction' for near approaches although it doesn't discuss miss distances as such - rather it refers to Pc, the probability of collision.
posted by Major Clanger at 1:31 AM on February 12, 2017 [2 favorites]


When they are closest, you could say that from Deimos, Phobos is at inferior conjunction with Mars, or from Phobos, Deimos is at opposition to Mars (I.e. they are 180 degrees apart on Phobos's sky).

When they are farthest apart, you could say that Phobos and Deimos are at opposition in Mars's sky, or that from Deimos, Phobos is at superior conjunction with Mars. From Phobos, Deimos woud be in conjunction with Mars.

Here's a handy diagram.

Unlike syzygy, conjunctions don't require the bodies to line up exactly, just gave the same right ascension.
posted by BrashTech at 5:05 AM on February 12, 2017 [1 favorite]


In nautical terminology, the minimum distance would be called a CPA (closest point of approach). Quickly google searching "orbital closest point of approach", I find MOID. Seems relevant, but I'm not sure saying "time of MOID" is any more clear communication than "orbital CPA".
posted by ctmf at 11:29 AM on February 12, 2017


In my opinion, I just think you should invent a new word. This is kind of how new words are invented, and especially in a subject like astronomy where everything kind of follows the same patterns and most words come from the same source (greek), and your audience knows that and can use context clues to extrapolate.

So, peri and apo, we know mean close and far. Just find something to replace "center". TWo suggestions (i googled these to make sure they weren't already words)

• peristance, apostance ex:Phobos' apostance to Deimos occurs when blah blah blah

• The greek word for position is θέση, used as suffix in other places as -thesia. So: apothesia, perithesia
posted by FirstMateKate at 12:17 PM on February 12, 2017


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