QUESTION:
I was wondering, if the time for Mars to revolve around the sun is
twice as long as the earth, how would you determine what date or
day it is on Mars?

ANSWER from Geoff Briggs on February 12, 1997:
This is an excellent question which points out the need to develop a
calendar that works for Mars.  This will be especially important when we
have astronauts working on Mars.  The NASA Ames Center for Mars Exploration
is beginning to work on this problem which turns out to be quite complex.

Some relevant points: The Mars day is less than an hour longer than our own
and, since 1976 when Viking mission began at Mars, has been called a "sol".
It takes about 687 Earth days for Mars to orbit the Sun.  At present, the
martian year has no specific "January 1st" to conveniently identify its
beginning.

There is a quite convenient way of tracking the seasons, however.  The four
seasons on Mars are quite Earth-like in that the tilt of the martian axis
(the angle from being perpendicular to the plane of its orbit about the
Sun) is almost the same as the Earth's (about 25 degrees versus about 23
degrees for Earth).  However, the martian orbit is much more elliptical
than Earth's (which is almost circular) and this creates additional
seasonal effects. The seasons are tracked by measuring the longitude (zero
to 360 degrees) of Mars in its orbit relative to the Sun using a convenient
reference direction.  This longitude angle is zero at the northern Spring
equinox (when day and night are equally long in both northern and southern
hemispheres), 90 degrees at the northern summer solstice, 180 degrees at
the northern autumn equinox and 270 degrees at the northern winter solstice.

In developing a practical Mars calendar there will be many considerations
including when each year should begin, when year zero should be, how many
"months" there should be in a year, how many days in a "month" and in a
"week", and so on.