Viewing the martian sky can be unsettling. Firstly, because it means that you are on Mars which might have been unexpected. But also because the Martian sky has two moons and one of the moons seems to be going the wrong way.
We are used to seeing Earth’s moon rising in the east and setting in the west, just like the Sun does. Deimos, the smaller of the Martian moons, behaves like this. It rises in the east and sets in the west, though it takes its sweet time–requiring a couple of days to fully set. Phobos, the other moon, seems to be the rule-breaker. It rises in the west and races towards the east.
Retrograde
Most things in the solar system “spin” in the same direction. If we look at the solar system from the north pole, most things will be rotating and orbiting counterclockwise. This means that most moons orbit in the same direction as their parent bodies rotate. For example, the Earth rotates towards the east and the Earth’s Moon orbits towards the east.
Not all objects in the solar system are disciplined like this though. There are degenerates like Triton, Neptune’s moon, which has a retrograde orbit which means it orbits Neptune in the reverse direction. Since Phobos is crossing the sky in the opposite direction to our expectation, Phobos must also have a retrograde orbit. Phobos must be a degenerate like Triton, violating the rules of decorum. So we must do what must be done when rules of decorum are violated: write an angry letter.
To whom it may concern,
As an upstanding citizen, keeping a vigilant gaze across the Martian sky, I was in horror at the sight of Phobos. It’s as if the moon has taken the mantle of its namesake: the god of fear. The moon has chosen to violate nature itself and orbit Mars in the wrong direction.
I request that this be remedied at once.
If we were to send this out though, we likely would get a response like this:
lol what?
See attached.
The attached video shows Phobos and Deimos moving in the same direction around Mars.
So from space, the two moons are moving in the same direction but from Mars, they seem to be rising and setting in opposite directions. How is that possible?
Apparent Motion
Maybe we need to better understand why something rises and sets.
If we look at the apparent motion of the Sun, it’s natural to conclude that the Earth is static and the Sun is orbiting the Earth. But we know that’s not happening. On a day-to-day basis, the Sun is basically in the same place relative to Earth. The Sun rises because the Earth rotates and brings the Sun into view. Then the Sun sets because the Earth rotates past the Sun.
Unlike the Sun, the Moon does orbit around the Earth but it takes a month for the Moon to finish an orbit. But the Moon rises/sets much more frequently than once-a-month. So the Moon, also, rises and sets because the Earth rotates and brings it into view.
This is what Deimos is doing as well. Its orbit is slower than Mars’s rotation so it rises because Mars rotates and brings Deimos into view and Deimos sets because Mars rotates past Deimos.
This doesn’t really resolve our mystery though. It makes it more confusing. If the rising and setting happen because of the rotation of the planet, then how can Mars’s moons rise in different directions? The planet can’t rotate two different ways!
The problem here is that we burnt down our intuition a little too much. Deimos and the Earth’s moon don’t rise because of their own motion but that doesn’t have to be true of everything. Things can still rise because of their true motion.
That’s what Phobos is doing. It is orbiting faster than Mars rotates so Phobos brings itself into view. This is why Phobos rises in the west and moves to the east because that is what its orbit is actually doing.
Cars and Racetracks
Circular motion always feels confusing so maybe thinking about this linearly will help us.
Let’s look at a scene of a racetrack with three cars moving at different speeds. All three cars are moving forward but the red car is moving the fastest and the green car the slowest.
What happens if we look at the same scene from the perspective of the blue car? Nothing changes for the red car, it will still look like it is moving forward. Since the green car is slower, the green car will seem to be going backwards.
Even though the red and the green car seem to be going in opposite directions in the second video, they are both moving forward in reality. The change in appearance happens because of the relative speed of the viewer. When the viewer is moving slower than both cars, both cars look like they are moving forward. When the viewer is moving faster than one but slower than the other, the two cars seem to move in opposite directions.
This is what is happening with Mars. The viewer on Mars is like the blue car, Phobos is the red car and Deimos is the green car. In reality, all three are moving in the same direction but because of the relative speeds, Deimos seems to be going backwards and Phobos seems to be going forward.
In some ways, Phobos is the sane moon. Its apparent motion matches its actual motion. It looks like it’s coming from the west and setting in the east and that is its true orbit. Deimos and Earth’s Moon are the liars. They seem like they come from the east but they only look like that because they are slow and get lapped by the people on the surface.
Missing out
Mars’s two moons rise and set in opposite directions because they rise and set for different reasons. Deimos rises because it is slower than Mars and Phobos rises because it is faster.
But can the same moon rise for one reason and then set for another?
Since the cause of rising and setting is based on the relative speeds of the planet and the moon, the only way for the “reason” for rising and setting to change is if the speed difference can flip. A planet doesn’t have an easy mechanism to speed up and slow down its rotation. But the orbital speed of any body can change pretty significantly! In an elliptical orbit, a body’s orbital velocity can change a lot. It slows down when its furthest away from the parent body and speeds up when it reaches closer.
So if we had a moon in a highly elliptical orbit, it would speed up and slow down as it went around the planet. If the planet’s rotational velocity was close to the moon’s orbital velocity then the moon can switch between being slower and faster. If this happened, then the moon would rise from the east and set right back in the east!
We don’t have such a configuration in the solar system but if we did have such a moon, we would have to write a stronger letter condemning such a violation of decorum.