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?

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    $\begingroup$ A fortnight is half a lunar day: en.wikipedia.org/wiki/Fortnight#Astronomy $\endgroup$ Jun 9, 2017 at 12:42
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    $\begingroup$ 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? $\endgroup$
    – Fattie
    Jun 11, 2017 at 13:56

2 Answers 2


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's axis with the
                  planet-moon line    
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    $\begingroup$ Are you indicating durations in solar days or in sideral days? ;) $\endgroup$
    – usernumber
    Mar 23, 2020 at 9:45
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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Mar 23, 2020 at 11:07
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    $\begingroup$ A day (without further qualification) is exactly 60×60×24 = 86400 SI seconds. Similarly, a year is a Julian year of 365.25 days (that's the year used in light-years, although Google Calculator incorrectly uses the mean tropical year). See Table 5 on iau.org/publications/proceedings_rules/units $\endgroup$
    – PM 2Ring
    Sep 28, 2021 at 6:07
  • $\begingroup$ @PM2Ring yes indeed about the day. I'm simply quoting the Wikipedia article I've linked to so I'm not "using" anything (except coffee). I didn't really think that "year" had a single accepted explicit definition for some reason. Live and learn :-) $\endgroup$
    – uhoh
    Sep 28, 2021 at 7:31
  • $\begingroup$ Understood. That Wikipedia article gives impeccable (and reasonably modern) references, so I think it's safe to assume that the quoted numbers are using the IAU definition of day. OTOH, the situation does get complicated when using time data from before the modern SI standard was defined, but that's a topic for another question. ;) $\endgroup$
    – PM 2Ring
    Sep 28, 2021 at 7:43

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