# How long does a Phobos/Mars solar eclipse take? **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2022, 9:04pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153 "2022-05-08T21:04:10Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [May 8, 2022, 9:04pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/1 "2022-05-08T21:04:10Z")
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](https://www.youtube.com/watch?v=aKK7vS2CHC8&t=35s)) 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? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 8, 2022, 9:18pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/2 "2022-05-08T21:18:30Z")
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? ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [May 8, 2022, 9:21pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/3 "2022-05-08T21:21:26Z")
Nicely done. I just watched again and it seemed to take just about that amount of time, hence, real time. Thanks. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [May 8, 2022, 9:56pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/4 "2022-05-08T21:56:46Z")
I did the same calculation **[here](https://boards.straightdope.com/t/perseverance-rover-on-mars-was-mars-lander-set-for-feb-18th-landing/932068/253)** 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. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [May 8, 2022, 10:03pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/5 "2022-05-08T22:03:26Z")
It’s not really an eclipse, is it? shouldn’t it be called a transit? ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [May 8, 2022, 10:24pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/6 "2022-05-08T22:24:49Z")
It is a transit, but also technically an eclipse. An eclipse doesn’t have to be total. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 8, 2022, 10:35pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/7 "2022-05-08T22:35:24Z")
> [@DPRK](#): > > 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? 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. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 8, 2022, 10:52pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/8 "2022-05-08T22:52:11Z")
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 > **[Transit of Phobos from Mars](https://en.wikipedia.org/wiki/Transit_of_Phobos_from_Mars)** > > Pages for logged out editors learn more > > A transit of Phobos across the Sun as seen from Mars takes place when Phobos passes directly between the Sun and a point on the surface of Mars, obscuring a large part of the Sun's disc for an observer on Mars. During a transit, Phobos can be seen from Mars as a large black disc rapidly moving across the face of the Sun. At the same time, the shadow (antumbra) of Phobos moves across the Martian surface. > The event could also be regarded as a parti... 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 9, 2022, 9:11pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/9 "2022-05-09T21:11:56Z")
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. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [May 9, 2022, 9:38pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/10 "2022-05-09T21:38:38Z")
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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 9, 2022, 10:23pm UTC](https://boards.straightdope.com/t/how-long-does-a-phobos-mars-solar-eclipse-take/964153/11 "2022-05-09T22:23:04Z")
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.