Can you explain in more detail why Phobos orbits Mars twice a day instead of three times a day?
You are right that Phobos goes around Mars more than 3 times a day (3.136 = 1/.3189) as referenced to the Sun. However, at a point on the surface of Mars, Phobos would be seen just twice a day, since it has to catch up with the planet’s rotation (2.161 = 1/.3189 – 1/1.026). Since its orbit period is so short, Phobos actually appears to move in the direction that it travels, west to east, as seen from the surface of Mars. The real motion of our own moon is slow enough that the Earth’s rotation makes it appear to move east to west.
