PDA

View Full Version : question about Martian orbits



crosscountry
2010-May-08, 01:55 PM
I was giving a talk last week about Mars, and I realized about halfway through a sentence that I don't know the words for when an orbiting sattelitte reaches highest or lowest distance from Mars. On earth it's called Apogee and Perigee. Around the sun it is Aphelion and Perihelion.

I also can't remember the words for Lunar orbits either. Can you help me?

grant hutchison
2010-May-08, 02:13 PM
Periareion and apareion or apoareion. Conventionally and pedantically, one uses the equivalent Greek name (in this case, Ares) so as to avoid mixing Greek and Latin roots. That said, perimars and apomars seem to be in common use, and are much more easily understood.
As general terms, not tied to any particular body, you can use periapsis and apoapsis, periapse and apoapse, or (mixed roots again) pericentre and apocentre.

Edit: For the moon it's pericynthion and apocynthion, though perilune and apolune are also used.
Edit: Oh, and I forgot about periselene and aposelene, for the moon.

Grant Hutchison

grant hutchison
2010-May-08, 03:09 PM
Interestingly, the Oxford English Dictionary reports a technical usage difference between -cynthion and -lune. Pericynthion is the closest approach to the moon by a spacecraft not launched from the moon; perilune is the corresponding word if the spacecraft was launched from the moon. It seems to be a difference that has never been in consistent use, but the OED mentions it.
Probably not news to BAUT's Apollo buffs, but news to me.

Grant Hutchison

AndreasJ
2010-May-08, 03:22 PM
Wikipedia has a page (http://en.wikipedia.org/wiki/Apsis) with a list of such terms.

grant hutchison
2010-May-08, 03:40 PM
Wikipedia has a page (http://en.wikipedia.org/wiki/Apsis) with a list of such terms.The origin of some of the Wikipedia entries is unclear; I wonder if some may even have come from an old thread (http://www.bautforum.com/showthread.php/222-A-terminology-question) here at BAUT, which I happened on a few minutes ago.
Wikipedia is incorrect that periareion and apoareion are "never used". A quick search reveals a trickle of usage in technical literature, sometimes with the alternative spelling "-areon". The Oxford English Dictionary provides citations for perijove, apojove, apojovium, perisaturnium and periuranium, but not for any of the specific Greek compounds beyond sun, moon and Earth.

Grant Hutchison

grant hutchison
2010-May-08, 03:59 PM
Oh. Puzzling over that Wikipedia table reminds me that the MESSENGER team uses periherm and apoherm for Mercury orbits.

Grant Hutchison

AndreasJ
2010-May-08, 06:38 PM
It mildly surprises me, incidentally, that people nerdy enough to use the Greek roots don't insist on h loss with aspiration (eg. apo + herm- -> apherm-, cf "aphelion" from apo and Helios),

grant hutchison
2010-May-08, 06:48 PM
It mildly surprises me, incidentally, that people nerdy enough to use the Greek roots don't insist on h loss with aspiration ...You can find both apoherm and apherm in technical use. But I recall apoherm as being the chosen version for the MESSENGER team.

Edit: Ah, ToSeeked (http://www.bautforum.com/showthread.php/10664-Quick-question-perigee-and-apogee?p=202068#post202068), literally.

Grant Hutchison

crosscountry
2010-May-10, 01:44 PM
Thanks for that guys. Now I have to figure out which one to use in my talks. Periareion is not easily said and probably less understandable.

Peter B
2010-May-10, 01:53 PM
Thanks for that guys. Now I have to figure out which one to use in my talks. Periareion is not easily said and probably less understandable.

I'd suggest periapsis and apoapsis are the easiest - both to say and to apply generally.

Jeff Root
2010-May-11, 11:01 PM
I didn't learn about the terms "periapsis" and "apoapsis" until relatively
recently, but I think it would be good to make them familiar so that they
can be used in all situations.

-- Jeff, in Minneapolis

crosscountry
2010-Jun-30, 05:37 PM
to add to this question, how do we use Geoid and Aeroid generically? For instance, on a Jovian moon would you use the term Geoid?