<p>You need to find the right coefficients of Equation of Time for other planets, which mainly depends on orbit eccentricty and orbit inclination.
Here you find a full demonstration of how to get a generic EOT for any planet:</p>
<p><a href="http://www.ifpan.edu.pl/firststep/aw-works/fsII/mul/mueller.pdf" rel="nofollow noreferrer">http://www.ifpan.edu.pl/firststep/aw-works/fsII/mul/mueller.pdf</a></p>
<p>It's highly complex math which I have yet to fully understand...</p>
<p>This page provide details on algorithm for timekeeping on Mars:
<a href="https://www.giss.nasa.gov/tools/mars24/help/algorithm.html" rel="nofollow noreferrer">https://www.giss.nasa.gov/tools/mars24/help/algorithm.html</a></p>
<p>This page containts a huge amount of data, formulas and explanations about timekeeping on other planets:</p>
<p><a href="http://pbarbier.com/eqtime/eqtime.html" rel="nofollow noreferrer">http://pbarbier.com/eqtime/eqtime.html</a></p>
<p>And we are also discussing this topic in this other question (without an answer yet):</p>
<p><a href="https://astronomy.stackexchange.com/questions/38362/timekeeping-on-other-celestial-bodies-than-earth/38378?noredirect=1#comment76373_38378">Timekeeping on other celestial bodies than Earth</a></p>
<p>Apart from this, to &quot;keep the time&quot; on any planet you need to know your angular distance from planet prime meridian (passing through Greenwich on Earth), but this is just the last, and easiest, step. The actually changeling thing is determining the angular position of prime meridian: once you know it, there are plenty of sites providing algorithms and libraries to determine the position of a planet around the Sun at a specific time: joining the two data (prime meridian angle and planet angle) you will be able to determine angle of sun w.r.t. prime meridian.</p>
<p>Lacking specific libraries/formulas to calculate local time on any planet, you can temporary use Nasa Horizons to retrieve Altidue and Azimuth of Sun over a specific location on any planet using <a href="https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&amp;COMMAND=%270%27&amp;CENTER=%27coord@499%27&amp;OBJ_DATA=%27yes%27&amp;MAKE_EPHEM=%27yes%27&amp;TABLE_TYPE=%27OBSERVER%27&amp;REF_PLANE=%27ECLIPTIC%27&amp;COORD_TYPE=%27GEODETIC%27&amp;SITE_COORD=%270,0,0%27&amp;START_TIME=%272020/8/20%2014:36%27&amp;STOP_TIME=%272020/8/21%2014:36%27&amp;STEP_SIZE=%271h%27&amp;QUANTITIES=%274%27&amp;FIXED_QUANTITIES=%27Custom%27&amp;REF_SYSTEM=%27J2000%27&amp;OUT_UNITS=%27KM-S%27&amp;VECT_TABLE=%273%27&amp;VECT_CORR=%27NONE%27&amp;CAL_FORMAT=%27CAL%27&amp;ANG_FORMAT=%27HMS%27&amp;APPARENT=%27AIRLESS%27&amp;TIME_TYPE=%27UTC%27&amp;TIME_DIGITS=%27MINUTES%27&amp;RANGE_UNITS=%27AU%27&amp;SUPPRESS_RANGE_RATE=%27no%27&amp;SKIP_DAYLT=%27no%27&amp;EXTRA_PREC=%27yes%27&amp;CSV_FORMAT=%27yes%27&amp;VEC_LABELS=%27yes%27&amp;ELM_LABELS=%27yes%27&amp;TP_TYPE=%27ABSOLUTE%27&amp;R_T_S_ONLY=%27NO%27&amp;CA_TABLE_TYPE=%27STANDARD%27" rel="nofollow noreferrer">this query/URL</a>.</p>
<p>Change &quot;499&quot; to other numbers for other planets: 199 = Mercury, 299 = Venus,.... Change SITE_COORD parameter to desired location (lon, lat, altidue): SITE_COORD='0,0,0'</p>
<p>Note: single quote ' is coded as %27 in the url.</p>

CC BY-SA 4.0
2020-08-20T12:32:37.043Z
2020-08-20T12:40:03.403Z
2020-08-20T12:40:03.403Z
jumpjack
<p>Time on Mars is measured in <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars" rel="nofollow noreferrer">sols</a>.</p>

<p><a href="https://space.stackexchange.com/q/18655/58">Venus</a> could be a little trickier, depending on your application.</p>

<p>I'm not aware of any timekeeping system for any other planets. There hasn't been a need for one yet. So, again depending on your application, you could come up with various different possible schemes.</p>

<hr>

<p>Speculation (based on the OP's input regarding application):</p>

<ul>
<li>Mercury - because of the length of the day, I doubt would-be colonists would measure time in days. They would probably use Earth time (because of communications) and also keep track of Mercury years.</li>
<li>Jupiter, Saturn, Uranus, Neptune - no colonization, no timekeeping</li>
</ul>

CC BY-SA 3.0
2016-12-08T12:28:18.220Z
Well,  us *Terrans*  use sols for Martian time.  What the aboriginals on Mars use is as yet unknown.
Carl Witthoft
CC BY-SA 4.0
2019-07-23T12:35:49.130Z
On the surface of Mercury, 1 day = 2 years = 176 days.   That would likely be defined as seasons.   Earth time, as called2voyage points out, would probably be easiest for Mercury, as there's no celestial time that works on a human schedule, unlike Mars, which has a very convenient, 24 hour and 37 minute days.
userLTK
CC BY-SA 3.0
2016-12-07T21:49:39.973Z
2016-12-07T22:21:05.810Z
2017-04-13T12:58:55.210Z
called2voyage
<p>I want to make clock that shows current time on different solar bodies. For that I need to know how to count time on those planets. Is there any official definition of times on other planets in our solar system? Brief google search did not help.</p>

<p>I am capable of implementing library for this myself but I wonder about current <em>state of the art</em> of time telling on other planets. So basically I want to know these things:</p>

<ul>
<li>Is there any official time standard?</li>
<li>If so, where can I find it?</li>
<li>If not, how should I approach this problem? Should I make something like Unix epoch at some time in the past for every planet?</li>
</ul>

<p>Thanks for any tips.</p>

<p>EDIT: Ok, so on Mars, seconds, minutes and hours were made longer to keep 24-hour day. Is this viable to other planets as well? Venus aside for now :)</p>

<p>I was also thinking about keeping the hours the same, having days as long as many whole hours fits in them plus smaller compensation hours which would have length of (total_minutes_in_solar_day % 60). This concept is from David Weber novels, if you are familiar with them.</p>

<p>EDIT2: Based on discussion in the comments, I'll try to make some basic model that would be if possible applicable to any planetary body (thinking ahead :)). This is obviously not going to have any serious real world application but I would still like it to have some sense.</p>

<p>Thanks for your input, I will leave the question open for some more time if anyone wants to add their opinion.</p>

CC BY-SA 3.0
2016-12-07T22:08:35.793Z
Let's take a look at the other planets: Mercury has only one apparent day every two years from the surface and Jupiter, Saturn, Uranus, and Neptune are gas giants with no surface. Do you want to consider dwarf planets and moons?
called2voyage
CC BY-SA 3.0
2016-12-07T22:09:38.460Z
Really, it is hard to give you much more specific advice without knowing the application for your timekeeping system. Timekeeping is pointless without an application.
called2voyage
CC BY-SA 3.0
2016-12-07T22:10:11.607Z
Unix time is defined using seconds, and seconds are the same everywhere (up to relativity). Seconds are SI units. I would advise against touching seconds, even if it means you don't have an integer number of them in a day. If you really want to, you could have units very close in size, but be wary of naming them in a way that can get a decent SI abbreviation. Also, don't use them in physics!
John Dvorak
CC BY-SA 3.0
2016-12-07T22:13:58.913Z
@JanDvorak yes, that's probably good point, thanks!
lsrom
CC BY-SA 3.0
2016-12-07T22:14:02.517Z
@called2voyage my application is the clock that would show current time on other planets. I think it would be cool thing to have on your wall to baffle visits :) other than that I just want to satisfy my need to code something.
lsrom
CC BY-SA 3.0
2016-12-07T22:15:27.187Z
@lsrom So, assuming we had a colony on each of the planets, you want to know how time would be measured there?
called2voyage
CC BY-SA 3.0
2016-12-07T22:18:05.207Z
@called2voyage Yes. I wanted some "official" standard for this so it wouldn't be just something I made up, but apparently there is no such thing.
lsrom
CC BY-SA 3.0
2016-12-07T22:18:34.957Z
Don't forget there's no single "current time" on Earth either. There is GMT, but not many people actually use it except to determine offsets when converting from one timezone to another.
John Dvorak
CC BY-SA 3.0
2016-12-07T22:20:17.603Z
@JanDvorak Well, I think I will deal with that when I dealt with how many hours are in the day :) When I have working time model for planets, I can easily make timezone functions.
lsrom
CC BY-SA 3.0
2016-12-07T22:21:31.793Z
@lsrom I updated my answer.
called2voyage
CC BY-SA 3.0
2016-12-07T22:21:55.123Z
Rotation periods of individual solar bodies can be found on the Internet. How you draw lines on these barren surfaces is up to you.
John Dvorak
CC BY-SA 3.0
2016-12-08T15:01:34.980Z
@barrycarter nice. Though, do I understand correctly the procedure is essentially just "pick a significant point on the surface that is going to stay there, and draw the prime meridian through it", with no specification of what that object should be except that it's significant?
John Dvorak
CC BY-SA 3.0
2016-12-08T15:27:01.130Z
@barrycarter Understood, thanks. Do you happen to know where the prime meridian would be if determined for Earth this way?
John Dvorak
CC BY-SA 3.0
2016-12-09T02:33:09.983Z
It's a **straightforward** and **good** question.  **"Is there any official definition of times on other planets in our solar system?"**
Fattie
CC BY-SA 4.0
2018-06-08T14:11:20.503Z
Related question: https://astronomy.stackexchange.com/questions/26556/how-are-mars-years-counted
Sir Cornflakes
CC BY-SA 4.0
2019-07-23T10:46:47.557Z
@JohnDvorak From https://en.wikipedia.org/wiki/Greenwich_Mean_Time "English speakers often use GMT as a synonym for Coordinated Universal Time(UTC). For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term GMT should not thus be used for technical purposes".
PM 2Ring
<planet><solar-system><time>
"Official" time for other planets
lsrom
# "Official" time for other planets

(Asked by: username_0 on 2016-12-07)

I want to make clock that shows current time on different solar bodies. For that I need to know how to count time on those planets. Is there any official definition of times on other planets in our solar system? Brief google search did not help.

I am capable of implementing library for this myself but I wonder about current *state of the art* of time telling on other planets. So basically I want to know these things:

* Is there any official time standard?
* If so, where can I find it?
* If not, how should I approach this problem? Should I make something like Unix epoch at some time in the past for every planet?

Thanks for any tips.

EDIT: Ok, so on Mars, seconds, minutes and hours were made longer to keep 24-hour day. Is this viable to other planets as well? Venus aside for now :)

I was also thinking about keeping the hours the same, having days as long as many whole hours fits in them plus smaller compensation hours which would have length of (total\_minutes\_in\_solar\_day % 60). This concept is from David Weber novels, if you are familiar with them.

EDIT2: Based on discussion in the comments, I'll try to make some basic model that would be if possible applicable to any planetary body (thinking ahead :)). This is obviously not going to have any serious real world application but I would still like it to have some sense.

Thanks for your input, I will leave the question open for some more time if anyone wants to add their opinion.

**Tags:** planet, solar-system, time

> **Comments:**
> **username_1**: Let's take a look at the other planets: Mercury has only one apparent day every two years from the surface and Jupiter, Saturn, Uranus, and Neptune are gas giants with no surface. Do you want to consider dwarf planets and moons?  
> **username_1**: Really, it is hard to give you much more specific advice without knowing the application for your timekeeping system. Timekeeping is pointless without an application.  
> **username_2**: Unix time is defined using seconds, and seconds are the same everywhere (up to relativity). Seconds are SI units. I would advise against touching seconds, even if it means you don't have an integer number of them in a day. If you really want to, you could have units very close in size, but be wary of naming them in a way that can get a decent SI abbreviation. Also, don't use them in physics!  
> **username_0**: @JanDvorak yes, that's probably good point, thanks!  
> **username_0**: @username_1 my application is the clock that would show current time on other planets. I think it would be cool thing to have on your wall to baffle visits :) other than that I just want to satisfy my need to code something.  
> **username_1**: @username_0 So, assuming we had a colony on each of the planets, you want to know how time would be measured there?  
> **username_0**: @username_1 Yes. I wanted some "official" standard for this so it wouldn't be just something I made up, but apparently there is no such thing.  
> **username_2**: Don't forget there's no single "current time" on Earth either. There is GMT, but not many people actually use it except to determine offsets when converting from one timezone to another.  
> **username_0**: @JanDvorak Well, I think I will deal with that when I dealt with how many hours are in the day :) When I have working time model for planets, I can easily make timezone functions.  
> **username_1**: @username_0 I updated my answer.  
> **username_2**: Rotation periods of individual solar bodies can be found on the Internet. How you draw lines on these barren surfaces is up to you.  
> **username_2**: @barrycarter nice. Though, do I understand correctly the procedure is essentially just "pick a significant point on the surface that is going to stay there, and draw the prime meridian through it", with no specification of what that object should be except that it's significant?  
> **username_2**: @barrycarter Understood, thanks. Do you happen to know where the prime meridian would be if determined for Earth this way?  
> **username_3**: It's a **straightforward** and **good** question.  **"Is there any official definition of times on other planets in our solar system?"**  
> **username_4**: Related question: https://astronomy.stackexchange.com/questions/26556/how-are-mars-years-counted  
> **username_5**: @username_2 From https://en.wikipedia.org/wiki/Greenwich_Mean_Time "English speakers often use GMT as a synonym for Coordinated Universal Time(UTC). For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term GMT should not thus be used for technical purposes".  

---

## 2 answers

### Answer by username_1 on 2016-12-07. Score: 5 | Accepted answer

Time on Mars is measured in [sols](https://en.wikipedia.org/wiki/Timekeeping_on_Mars).

[Venus](https://space.stackexchange.com/q/18655/58) could be a little trickier, depending on your application.

I'm not aware of any timekeeping system for any other planets. There hasn't been a need for one yet. So, again depending on your application, you could come up with various different possible schemes.

---

Speculation (based on the OP's input regarding application):

* Mercury - because of the length of the day, I doubt would-be colonists would measure time in days. They would probably use Earth time (because of communications) and also keep track of Mercury years.
* Jupiter, Saturn, Uranus, Neptune - no colonization, no timekeeping

> **Comments:**
> **username_7**: Well,  us *Terrans*  use sols for Martian time.  What the aboriginals on Mars use is as yet unknown.  
> **username_8**: On the surface of Mercury, 1 day = 2 years = 176 days.   That would likely be defined as seasons.   Earth time, as username_1 points out, would probably be easiest for Mercury, as there's no celestial time that works on a human schedule, unlike Mars, which has a very convenient, 24 hour and 37 minute days.  

---

### Answer by username_6 on 2020-08-20. Score: 0

You need to find the right coefficients of Equation of Time for other planets, which mainly depends on orbit eccentricty and orbit inclination.
Here you find a full demonstration of how to get a generic EOT for any planet:

<http://www.ifpan.edu.pl/firststep/aw-works/fsII/mul/mueller.pdf>

It's highly complex math which I have yet to fully understand...

This page provide details on algorithm for timekeeping on Mars:
<https://www.giss.nasa.gov/tools/mars24/help/algorithm.html>

This page containts a huge amount of data, formulas and explanations about timekeeping on other planets:

<http://pbarbier.com/eqtime/eqtime.html>

And we are also discussing this topic in this other question (without an answer yet):

[Timekeeping on other celestial bodies than Earth](https://astronomy.stackexchange.com/questions/38362/timekeeping-on-other-celestial-bodies-than-earth/38378?noredirect=1#comment76373_38378)

Apart from this, to "keep the time" on any planet you need to know your angular distance from planet prime meridian (passing through Greenwich on Earth), but this is just the last, and easiest, step. The actually changeling thing is determining the angular position of prime meridian: once you know it, there are plenty of sites providing algorithms and libraries to determine the position of a planet around the Sun at a specific time: joining the two data (prime meridian angle and planet angle) you will be able to determine angle of sun w.r.t. prime meridian.

Lacking specific libraries/formulas to calculate local time on any planet, you can temporary use Nasa Horizons to retrieve Altidue and Azimuth of Sun over a specific location on any planet using [this query/URL](https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%270%27&CENTER=%27coord@499%27&OBJ_DATA=%27yes%27&MAKE_EPHEM=%27yes%27&TABLE_TYPE=%27OBSERVER%27&REF_PLANE=%27ECLIPTIC%27&COORD_TYPE=%27GEODETIC%27&SITE_COORD=%270,0,0%27&START_TIME=%272020/8/20%2014:36%27&STOP_TIME=%272020/8/21%2014:36%27&STEP_SIZE=%271h%27&QUANTITIES=%274%27&FIXED_QUANTITIES=%27Custom%27&REF_SYSTEM=%27J2000%27&OUT_UNITS=%27KM-S%27&VECT_TABLE=%273%27&VECT_CORR=%27NONE%27&CAL_FORMAT=%27CAL%27&ANG_FORMAT=%27HMS%27&APPARENT=%27AIRLESS%27&TIME_TYPE=%27UTC%27&TIME_DIGITS=%27MINUTES%27&RANGE_UNITS=%27AU%27&SUPPRESS_RANGE_RATE=%27no%27&SKIP_DAYLT=%27no%27&EXTRA_PREC=%27yes%27&CSV_FORMAT=%27yes%27&VEC_LABELS=%27yes%27&ELM_LABELS=%27yes%27&TP_TYPE=%27ABSOLUTE%27&R_T_S_ONLY=%27NO%27&CA_TABLE_TYPE=%27STANDARD%27).

Change "499" to other numbers for other planets: 199 = Mercury, 299 = Venus,.... Change SITE\_COORD parameter to desired location (lon, lat, altidue): SITE\_COORD='0,0,0'

Note: single quote ' is coded as %27 in the url.


---

