QUESTION:
Does Phobos eclipse Deimos?

ANSWER from Peter Thomas on May 17, 1997:

	This could be either: Phobos eclipsing Deimos as seen from Mars (or
even the earth), or Phobos eclipsing the sun as seen from Deimos.

	To see Phobos eclipse Deimos from the surface of Mars, one would have
to be essentially on the Martian equator.  Phobos orbits quickly enough (every
7 hours 42 minutes) that it rises in the west and sets in the east; it is not
even visible from the poles of Mars because its orbit is so low.  It would 
take less than a minute to pass in front of Deimos as seen from the ground.
Deimos orbits about every 30 hours, so it moves slowly in the sky as
seen from the surface of Mars.

	Phobos can certainly get between the Sun and Deimos even though it is
closer to Mars than Deimos.  The satellites and the sun can line up
about twice a Martian year when the sun is nearly lined up on Mars' equator
(as the sun is twice a year on the earth's equator: begining of spring and
of fall).  Because the satellites are small (Phobos 27 km, or 17 miles
across, Deimos: 13 km or 8 miles across) and far from each other (the
closest they get is 8600 miles), neither one would block out the sun
completely from the other.  From Deimos, Phobos would at best cover 1/3 the
diameter of the sun..so it wouldn't cut down the light very much.  Also,
this alignment has to occur when Mars is not in the way, which certainly can
happen.  
	When an object passes in front of the sun, but is not large enough
to cover much of the sun's disk, it is called a "transit".