# Is there a timekeeping word for the orbit of a moon?

For a planet, we can colloquially refer to its period of rotation as a "day" and its period of revolution around its parent star as a "year." Some worlds have unique terms, such as Martian days being referred to as "sols", but the principle is the same: one word for rotation around its axis, another for revolutions around the star.

Is there an equivalent word for the period of a moon's revolution around a planet? On Earth it roughly lines up with a "month," but multi-moon planets would have different periods for each moon.

If I was in charge of timekeeping for a mission to Europa or Ganymede, what term would I use to refer to the period of time it takes for the moon to complete one orbit of Jupiter?

• A fortnight is half a lunar day: en.wikipedia.org/wiki/Fortnight#Astronomy – Wayfaring Stranger Jun 9 '17 at 12:42
• I think a "Europa month" or "Ganymede month" would be reasonable. Looking at the specific technical terms (as Stephen linked to), what other could you say than "Ganymede sidereal period", "Ganymede synodic period" and so on? – Fattie Jun 11 '17 at 13:56

You should use one of the precisely defined terms of specific measures of orbital period, which are described on this Wikipedia page.

You might get away with just "orbital period" is you're not using it in a context that requires precision.

For length of day there's synodic day as well as sidereal period. There's a discussion on the difference between these here.

Adding to @stephenG's answer, the related Wikipedia page Lunar month can be used to get an idea of what the various terms mean.

I have to agree though that one should probably try to get away with just using the word month while distracting the person you're talking to (cough or drop something) and then quickly moving on, because any term that's more precise will have a very specific definition, and if someone says "Wait, don't you mean a blah-blah-blah month?" you'll have to stop figure out if you do or don't!

  kind            frame - or - criteria      Earth's moon (days)

sidereal          inertial wrt stars              27.321661

synodic           rotating with Sun-Earth         29.530588

draconitic        orbital plane (precesses)       27.212220
(or draconic)     time between ascending nodes
(or nodal)

anomalistic       time between periapses          27.554551

tropical          time between alignments of      27.321582
moon'ss axis with the
planet-moon line

• Are you indicating durations in solar days or in sideral days? ;) – usernumber Mar 23 '20 at 9:45
• @usernumber Aaaahhhhhh!!! I did think of that when typing this and did hope that it never came up. Now that it has I'm trying to figure out how to reference Monty Python's bridge of death and swallows here but can't quite make it fit. – uhoh Mar 23 '20 at 11:07