<p>You should use one of the precisely defined terms of specific measures of orbital period, which are described on <a href="https://en.wikipedia.org/wiki/Orbital_period" rel="noreferrer">this Wikipedia page</a>.</p>

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

<p>For length of day there's <a href="https://en.wikipedia.org/wiki/Synodic_day" rel="noreferrer">synodic day</a> as well as sidereal period.  There's a discussion on the difference between these <a href="http://cseligman.com/text/sky/rotationvsday.htm" rel="noreferrer">here</a>.</p>

CC BY-SA 3.0
2017-06-09T05:42:40.050Z
2017-06-09T05:42:40.050Z
StephenG - Help Ukraine
<p>Adding to <a href="https://astronomy.stackexchange.com/a/21239/7982">@stephenG's answer</a>, the related Wikipedia page <a href="https://en.wikipedia.org/wiki/Lunar_month" rel="nofollow noreferrer">Lunar month</a> can be used to get an idea of what the various terms mean.</p>
<p>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 &quot;Wait, don't you mean a <em>blah-blah-blah</em> month?&quot; you'll have to stop figure out if you do or don't!</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>kind</th>
<th>frame - or - criteria</th>
<th>Earth's moon (days)</th>
</tr>
</thead>
<tbody>
<tr>
<td>sidereal</td>
<td>inertial wrt stars</td>
<td>27.321661</td>
</tr>
<tr>
<td>synodic</td>
<td>rotating with Sun-Earth</td>
<td>29.530588</td>
</tr>
<tr>
<td>draconitic (or draconic) (or nodal)</td>
<td>orbital plane (precesses) time between ascending nodes</td>
<td>27.212220</td>
</tr>
<tr>
<td>anomalistic</td>
<td>time between periapses</td>
<td>27.554551</td>
</tr>
<tr>
<td>tropical</td>
<td>time between alignments of Moon's axis with the planet-moon line</td>
<td>27.321582</td>
</tr>
</tbody>
</table>
</div>
CC BY-SA 4.0
2020-03-23T09:45:53.810Z
Are you indicating durations in solar days or in sideral days? ;)
usernumber
CC BY-SA 4.0
2020-03-23T11:07:44.617Z
@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
CC BY-SA 4.0
2021-09-28T06:07:15.197Z
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 https://www.iau.org/publications/proceedings_rules/units/
PM 2Ring
CC BY-SA 4.0
2021-09-28T07:31:30.810Z
@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 :-)
uhoh
CC BY-SA 4.0
2021-09-28T07:43:45.237Z
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. ;)
PM 2Ring
CC BY-SA 4.0
2020-01-02T13:47:23.630Z
2022-10-07T16:14:21.710Z
2022-10-07T16:14:21.710Z
uhoh
<p>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.</p>

<p>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. </p>

<p>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?</p>

CC BY-SA 3.0
2017-06-09T12:42:10.780Z
A fortnight is half a lunar day: https://en.wikipedia.org/wiki/Fortnight#Astronomy
Wayfaring Stranger
CC BY-SA 3.0
2017-06-11T13:56:35.410Z
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
<the-moon><natural-satellites><time><standards>
Is there a timekeeping word for the orbit of a moon?
Nerrolken
# Is there a timekeeping word for the orbit of a moon?

(Asked by: username_0 on 2017-06-08)

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?

**Tags:** the-moon, natural-satellites, time, standards

> **Comments:**
> **username_1**: A fortnight is half a lunar day: https://en.wikipedia.org/wiki/Fortnight#Astronomy  
> **username_2**: 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?  

---

## 2 answers

### Answer by username_3 on 2017-06-09. Score: 5

You should use one of the precisely defined terms of specific measures of orbital period, which are described on [this Wikipedia page](https://en.wikipedia.org/wiki/Orbital_period).

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](https://en.wikipedia.org/wiki/Synodic_day) as well as sidereal period. There's a discussion on the difference between these [here](http://cseligman.com/text/sky/rotationvsday.htm).


---

### Answer by username_4 on 2020-01-02. Score: 4

Adding to [@stephenG's answer](https://astronomy.stackexchange.com/a/21239/7982), the related Wikipedia page [Lunar month](https://en.wikipedia.org/wiki/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 (or draconic) (or nodal) | orbital plane (precesses) time between ascending nodes | 27.212220 |
| anomalistic | time between periapses | 27.554551 |
| tropical | time between alignments of Moon's axis with the planet-moon line | 27.321582 |

> **Comments:**
> **username_5**: Are you indicating durations in solar days or in sideral days? ;)  
> **username_4**: @username_5 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.  
> **username_6**: 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 https://www.iau.org/publications/proceedings_rules/units/  
> **username_4**: @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 :-)  
> **username_6**: 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. ;)  

---

