# "Official" time for other planets

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.

• 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? Dec 7 '16 at 22:08
• 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. Dec 7 '16 at 22:09
• 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! Dec 7 '16 at 22:10
• @JanDvorak yes, that's probably good point, thanks! Dec 7 '16 at 22:13
• @JanDvorak Actually, astronomers have already defined a "prime meridian" for most planets + have even created a scheme for doing so with future discoveries: astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/…
– user21
Dec 8 '16 at 14:46

Time on Mars is measured in sols.

Venus 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
• Well, us Terrans use sols for Martian time. What the aboriginals on Mars use is as yet unknown. Dec 8 '16 at 12:28
• 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. Jul 23 '19 at 12:35

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...

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

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.

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.