QUESTION:
It appears that the Martian Sol has been defined as 24 hours 37 minutes
which is the sidereal rotational period for Mars.  Why are you not using the
Martian synodic rotational period of 24 hours 39 minutes?  The synodic
period is the time from sunrise to sunrise (or midday to midday) and thus is
actually what we tend to think of as the "day." If events are scheduled
according to the sidereal period, it would appear that you would eventually
end up scheduling activities during the Martian night when you wanted the
day.

ANSWER from Mary Urquhart on October 6, 1997:
The Martian Sol is defined as a solar day (the motion of Mars in orbit
around the Sun which increases the length of a solar day is considered)
rather than a sidereal day (actual rotation period).  I'm not sure where
your number came from.  The Pathfinder homepage lists a Sol as 24.6 hours
which could be either a sidereal day or a solar day, and the Mars Global
Surveyor site lists the actual rotation period (not defined as a Sol).

From http://www-pdsimage.jpl.nasa.gov/PDS/public/vikingl/viklnav.html

"Mars solar day The Mars solar day (sol) is considered to have a length of
24 hours, 39 minutes, and 35.25 seconds; it is usually abbreviated as sol.
Sol 0 is the particular spacecraft's landing day. Sol 0 for VL-1 is July 20,
1976, and sol 0 for VL-2 is September 3, 1976."

The length of a Sol changes (very) slightly during a Mars year due to the
eccentricity of the orbit of Mars (how elliptical it is).

Even with the use of a solar day to define a Sol, the Sun will rise and set
at different times during the Martian year for different latitudes (seasonal
changes).  The people in charge of sending commands to the spacecraft must
be aware of not only when the Sun rises on a particular day, but also when
the Earth is above the Martian horizon.  As long as the spacecraft operator
knows which definition of a day is being used, the definition itself really
doesn't matter in scheduling events.