# How long does a Phobos/Mars solar eclipse take?

A recent video of a Martian solar eclipse of the sun by the moon Phobos is fascinating. (NASA's Perseverance Rover Sees Solar Eclipse on Mars - YouTube) But how long did it actually take? Was the video real time? Seems unlikely that it moves that fast, but…
I can’t find anything that answers that question. Dopers?

Let’s try to get an order of magnitude.

Phobos orbits the planet in 7 hours, 39 minutes. That is 0.0131 degrees per second. The diameter of the sun will be around 1/3 of a degree. So around 25 seconds?

Nicely done. I just watched again and it seemed to take just about that amount of time, hence, real time. Thanks.

I did the same calculation here and came to the same conclusion. It actually takes a bit less than the calculated time because the sun exhibits an apparent motion in the opposite direction due to the rotation of Mars, but yes, it’s real time.

It’s not really an eclipse, is it? shouldn’t it be called a transit?

It is a transit, but also technically an eclipse. An eclipse doesn’t have to be total.

That would be if you were viewing it from the center of Mars. For Terra and Luna, the center versus the surface doesn’t matter much, but Phobos is much closer to Mars than Luna is to Terra. So it’d go even quicker.

I did say “order of magnitude”… however, in the video it does seem to take around 25 seconds, plus an extra 10 seconds if you measure from first contact to when Phobos is no longer visible. Is it worth doing a detailed calculation, or should we accept the video as being real-time?

ETA this Wiki table

has more calculations, and photographs, but interestingly enough there is a discrepancy of a few seconds. E.g., on 12 March 2004, the transit lasts about 30 seconds according to the photo, but the calculation, which is not accompanied by source code, has it as 36 seconds. 7 March is 10 seconds versus 29 seconds. NB it does not take the same amount of time every transit, of course.

Phobos’ orbital radius is only a little more than twice Mars’ own radius, so the eclipse should go by about twice as quickly as from the center of Mars.

I understand how you arrived at the speed that Phobos moves, but how did you come up with the diameter of the sun as viewed from Mars?

From Earth, the Sun is half a degree. Mars is about 1.5 times further from the Sun than Earth is. Ergo, from Mars, the Sun is about a third of a degree.