My interest was piqued by this question and answer. For several reasons NASA needed to refer frequently and extensively to a Martian solar day, and "sol" became the nom du jour. There have been remotely controlled rovers on the Moon as well, but I don't know in the Russian or Chinese if the corresponding word for month was used for convenience, or if a new term for Lunar day was coined for the occasion.

Solar days are important for exploring bodies. Oblique lighting (coming from a direction other than the direction of the camera) is extremely important for 3D reconstruction of the landscape for navigation. Surface conditions (liquids, slushes, venting, boiling) may be very important for a rovers mission. Except for water's freakishly high melting and boiling points (273K, 373K) the materials on the surfaces of many solar system bodies can often have phase changes at the much lower temperatures the sun produces far beyond earth's orbit. Potential ionospheric phenomena may also be of interest, and of course on some bodies it can get prohibitively cold at night.

While some landers have used and will use solar power, this is usually used to charge batteries which could easily deliver power at night. Of course some landers will cary other sources of power, including radioisotope based thermo-electric.

So my question is, besides body_name_as_adjective + "day" (e.g. Lunar day, Martian day, etc.) has any other term been used in space exploration to refer to a solar day on a solar system body besides sol on Mars?

  • $\begingroup$ In what way is water's melting and boiling point freakishly high? $\endgroup$ – gerrit Jan 27 '16 at 11:15
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    $\begingroup$ Look at the freezing points of other molecules with atomic weights under 20, or even 30! $\endgroup$ – uhoh Jan 27 '16 at 11:17
  • $\begingroup$ I see, I get your point now! $\endgroup$ – gerrit Jan 27 '16 at 11:20

"Lunation" is informally used as a "calendar" for lunar days. However I'm not sure it can be said to be an equivalent of "sol" because it's more properly known as the lunation number.

So probably the martian "sol" is the only commonly used example.

By the way, I believe people working on the current probes started to use "sol" because the Martian day is so close to the Earth day; there is always a good chance there will be confusion over exactly which day type is being referred to...

  • $\begingroup$ Interesting! If they were not so close, how would "day" be any less ambiguous - wouldn't they still need two terms? $\endgroup$ – uhoh Jan 27 '16 at 12:25
  • $\begingroup$ Uh, good question and I'm embarrassed to say I'm stumped. I remember in the early days of Spirit & Opportunity someone in their team explaining why they used "Sol" instead of "Day" to avoid confusing the two; but I don't remember all of the reasoning... :( $\endgroup$ – Andy Jan 27 '16 at 12:37
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    $\begingroup$ I remember there was a problem with usage "daytime" and "nighttime" or "day" and "night" being different for the teams on mars time vs everyone else. The closeness of the two periods meant the schedule creeped slowly, but just fast enough that it was difficult to keep track of. $\endgroup$ – uhoh Jan 27 '16 at 12:44
  • $\begingroup$ Yes, even if a Martian day were not close to 24 hours, we would still need to use "sol" or somesuch to identify a Martian solar day. There are many reasons that rover or lander operations are tied to the local solar time (in the next comment) ... $\endgroup$ – Mark Adler Jan 27 '16 at 16:35
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    $\begingroup$ ... imaging is best done during the day, actuators (wheels, arms, etc.) require much less heating to operate during the day and in fact is usually done closer to the afternoon, driving that uses camera images in real-time for navigation is done during the day, relay orbiters come overhead around the same two local solar times each day and night, and four out of seven Mars landers have been solar powered, which combined with the thermal control of moving parts strongly favors operations during the day. $\endgroup$ – Mark Adler Jan 27 '16 at 16:35

There are only 2 "regular" bodies that have had long term landers visit them, Mars and the Moon. Small bodies, asteroids and comets, have unpredictable light cycles. Titan and Venus, which have both had landers, had very short term landers. Bottom line is, if this was going to apply, it would only apply to either Mars or the Moon.

The Moon is somewhat difficult, because few spacecraft survive the lunar night. Also, it is so long that it doesn't mean as much. As Andy mentioned in his answer, it is called Lunation, the day/month of the Moon.


"Sol" is the word for "Sun" in several languages. Since ancient times, humans measured the time by counting how many times the Sun rised and setted in the sky, which is what is known as day.

In other planets, measuring time using Earth-days would be complicated due to not being in sync with the planet's rotation, so a simple solution is to count how many times the Sun rised and setted.


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