# Deimos & Phobos **URL:** \ **Category:** Factual Questions\ **Created:** [March 19, 2001, 2:12pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584 "2001-03-19T14:12:41Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Sputnik](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@Sputnik](https://boards.straightdope.com/u/Sputnik)\ **Post date:** [March 19, 2001, 2:12pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/1 "2001-03-19T14:12:41Z")
I am writing a story on Mars.  Can anyone tell me how big and bright its micromoons Deimos and Phobos appear as seen from the surface?  How fast do they move across the Martian sky? Are they usually both visible at the same time? ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [March 19, 2001, 2:46pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/2 "2001-03-19T14:46:52Z")
Here’s a web page that answers most of your questions: [http://www.harmsy.freeuk.com/phobos.html](http://www.harmsy.freeuk.com/phobos.html) ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 19, 2001, 4:38pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/3 "2001-03-19T16:38:48Z")
Gees, Podkayne, you probably could have sent him personal photos. ---
**Author:** ![Fiver](https://avatars.discourse-cdn.com/v4/letter/f/e274bd/32.png) [@Fiver](https://boards.straightdope.com/u/Fiver)\ **Post date:** [March 19, 2001, 6:05pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/4 "2001-03-19T18:05:18Z")
Not mentioned in **Podkayne** ’s link is that since Deimos orbits more slowly than Mars, its apparent motion is retrograde: it moves west to east across the sky, instead of east to west like Phobos and the Sun (I may have that backwards, but you get the idea). Also, Phobos’s orbit is so tight that at very high Martian latitudes, the moon is _below_ the horizon! For this and similar topics about our solar system, I direct the OP to the site [Nine Planets](http://www.deepspace.ucsb.edu/ia/nineplanets/nineplanets.html). It’s a great resource. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [March 20, 2001, 1:31pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/5 "2001-03-20T13:31:02Z")
You rang? ---
**Author:** ![Badtz\_Maru](https://avatars.discourse-cdn.com/v4/letter/b/e36b37/32.png) [@Badtz\_Maru](https://boards.straightdope.com/u/Badtz_Maru)\ **Post date:** [March 20, 2001, 2:04pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/6 "2001-03-20T14:04:39Z")
Um, big error on that page. They have the size of both moons way off, I think they used kilometers when they meant meters. ---
**Author:** ![Sputnik](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@Sputnik](https://boards.straightdope.com/u/Sputnik)\ **Post date:** [March 20, 2001, 2:08pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/7 "2001-03-20T14:08:18Z")
Thanks to all for the responses. Which page has the error - “harmsy” or “Nine Planets”? ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 20, 2001, 2:52pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/8 "2001-03-20T14:52:33Z")
Perhaps he’s talking about the \*orbital[/] radii on the harmsy page? Or maybe the pages are changing as we speak. I couldn’t get that Nine Planets site to come up, but there are other [mirrored sites](http://seds.lpl.arizona.edu/billa/tnp/) and they seem to have the right radii. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [March 20, 2001, 3:32pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/9 "2001-03-20T15:32:06Z")
Check out these essays by Asimov: ([http://www.clark.net/pub/edseiler/WWW/essay\_guide.html](http://www.clark.net/pub/edseiler/WWW/essay_guide.html)) Kaleidoscope in the Sky Subject: satellites of Mars First Published In: Aug-67, The Magazine of Fantasy and Science Fiction Collection(s): 1968 Science, Numbers, and I The Sons of Mars Revisited Subject: satellites of Mars First Published In: Nov-77, The Magazine of Fantasy and Science Fiction Collection(s): 1979 The Road to Infinity Dark and Bright Subject: satellites of Mars First Published In: Dec-77, The Magazine of Fantasy and Science Fiction Collection(s): 1979 The Road to Infinity The Real Finds Waiting Subject: satellites of Mars First Published In: Jan-78, The Magazine of Fantasy and Science Fiction Collection(s): 1979 The Road to Infinity You’ll find lots of good info, including how the moons look as seen from the surface of Mars. ---
**Author:** ![Sputnik](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@Sputnik](https://boards.straightdope.com/u/Sputnik)\ **Post date:** [March 20, 2001, 4:23pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/10 "2001-03-20T16:23:38Z")
One of those sites said that Phobos would appear one third the size of our full moon. With its tight orbit, all those craters, and its irregular shape, it must look pretty freaky against that pink Martian sky as seen from the surface. Have any of our landers ever snapped a photo of the Martian moons from the surface? Maybe during one of those frequent eclipses? ---
**Author:** ![Fiver](https://avatars.discourse-cdn.com/v4/letter/f/e274bd/32.png) [@Fiver](https://boards.straightdope.com/u/Fiver)\ **Post date:** [March 20, 2001, 4:57pm UTC](https://boards.straightdope.com/t/deimos-phobos/58584/11 "2001-03-20T16:57:52Z")
**Sputnik** : > [@](#): > > Have any of our landers ever snapped a photo of the Martian moons from the surface? Yes, the Pathfinder/Sojourner mission snapped a few low-resolution pics of each moon. I think they were all at night, though. I don’t have a link handy but if you search on Pathfinder or Sojourner it should be easy enough to find.