[tz] Sol 5000 for Opportunity
marshall.eubanks at gmail.com
Sun Feb 18 05:04:27 UTC 2018
On Sat, Feb 17, 2018 at 11:48 PM, Marshall Eubanks <
marshall.eubanks at gmail.com> wrote:
> On Sat, Feb 17, 2018 at 9:46 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:
>> Tim Parenti wrote:
>>> Today, 16 February 2018, at 01:26:20.092 UTC, it is 01:01:06 Coordinated
>>> Mars Time on Mars Sol Date 51235
>> What's the source for these timestamps? It's an Earth reference frame,
> The short answer is that so far, it effectively is.
> Mars spacecraft keep (so far) an effective Earth time.Their clocks (so
> far) typically aren't that good, and so they are just treated as if they
> were on TAI or even UTC and adjusted from time to time to keep close to
> Earth time.
> Mars Coordinated Time (or MTC) is the Mars Mean Solar Time (the Mars
> equivalent of UT1) for the Mars reference longitude (crater Airy-0).
> However, I believe that all of the lander/rover missions to date use
> actually the mean solar time for their latitude and longitude (roughly, UT0
> + a longitude correction) in setting their sols and the local solar time.
> No spacecraft clock is AFAIK set to run at MTC.
> The Martian local solar time undergoes fairly large variations over a
> Martian year, much as the local solar time does on Earth, but more so, due
> to the larger orbital eccentricity. The wikipedia article describes these
> and provides a formula for getting MST from terrestrial time (TT = TAI +
> 32.184 seconds).
> I helped to determine the mean Mars length of day (LOD) and what little is
> known about its variations from Viking and Pathfinder data. There is a
> fairly large seasonal variation of Mars LOD (due to the seasonal transfer
> of CO2 to and from the poles) but little is known about any long period
> changes in Martian LOD, and thus it is not known whether leap seconds would
> be needed to keep Mars atomic time (once that is set up) and Mars mean
> solar time aligned.
> The Wikipedia article does not discuss the relativistic differences
> between time on the Earth and time on Mars, but these are significant; Mars
> is both traveling slower on its orbit and further out in the Sun's
> gravitational potential. Atomic time on Mars would run faster than AT on
> Earth by about 5.1 x 10^-9, or ~0.16 seconds per year, on average, and
> there are seasonal and other variations. This is taken into account in
> great detail in the Martian ephemeris work, but so far as far as I know the
> people setting the spacecraft clocks ignore it.
> The long awaited arrival of the first Deep Space Atomic Clock into Mars
> orbit will change all of this. That will not be adjusted in rate and thus
> would run fast by 3.4
argh! That's 5.1 not 3.4, of course.
I recommend this paper as an introduction to the theory here :
parts per billion (on average) compared to its cousins back here on or near
> Earth, or roughly 7 orders of magnitude above its hoped-for sensitivity
> How (or even whether) future Mars colonists will align their time with the
> Earth's remains to be seen.
>> Because of relativistic effects, if an observer on Earth sees that Mars's
>> 51235 01:01:06.000 CMT timestamp corresponds to Earth's 2018-02-16
>> 01:26:20.092 UTC timestamp, then I guess that an observer on Mars should
>> see that the same CMT timestamp corresponds to a slightly-different UTC
>> timestamp, even assuming both observers have error-free measurements, and
>> that the difference will be observable with millisecond-precision
>> timestamps. This is due to both the relative velocity of the Earth and Mars
>> and to gravitational-field effects. For millisecond/year precision Pan and
>> Xie write that you need to figure in not only the gravitational effects of
>> the Earth, Mars, and the Sun, but also those of Jupiter, Saturn, the Moon,
>> and Venus (!). See their analytic model of timekeeping for a Yinghuo-1-like
>> mission in:
>> Pan J-Y, Xie Y. Relativistic algorithm for time transfer in Mars missions
>> under IAU resolutions: an analytic approach. RAA. 2015. 15(2):281-92.
>> The whole idea of time transfer between Mars and Earth brings into stark
>> relief what an easy job it is to track civil timekeeping on Earth, compared
>> to how it would be elsewhere. And Mars is an easy case. Someone should
>> alert Elon Musk.
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