This video depicts NASA's Curiosity rover observing Mars' two moons, then shows one moon passing in front of the other. Phobos and Deimos, the moons of Mars, are thought to be asteroids captured in Mars' gravitational field.
Some facts to help us interpret what we are seeing here:
Phobos' diameter is 22.2 km and Deimos' is only 12.6 km.
The orbital distance from Mars of Phobos is 1.4 Martian diameters and Deimos' is 3.5 Martian diameters.
The orbital periods are 7.6 hours for Phobos and 30.3 hours for Deimos.
So, Phobos is closer to Mars, bigger than Deimos, and faster than Deimos, meaning that we are seeing Phobos passing in front of Deimos in this video.
A Martian day is 24 hours, 39 minutes, and 35 seconds (solar day). This is only about 40 minutes slower than Earth. Mars, like all of the planets except Venus, rotates in a counter clockwise direction (astronomers call this prograde).
Both moons orbit in the same direction that Mars rotates, with Phobos orbiting Mars in less than a Martian day, which means it is orbiting faster than the planet rotates, and thus rises in the West and sets in the East.
Deimos is like our own Moon and takes longer than a Martian day to orbit, so Mars rotates quicker than Deimos orbits, and thus it seems to rise in the East and set in the West.
Hope you aren't confused yet!
Don't forget, the next cool thing going to Mars is MAVEN, launching November 18th, heading out to study the history of Mars' atmosphere and climate. I'm working on getting to that launch in order to see an Atlas V launch. That will be incredible! In the meantime, I'm going to Boulder, CO next week to learn a bit more about the science that MAVEN will accomplish from the scientists at LASP at the University of Colorado. I look forward to telling you all about it!
Image courtesy of Wikimedia Commons