This answer mentions that on Mars, you can see its satellite get bigger and smaller as it rises and sets, because the satellite orbits much closer than our own Moon does from Earth.

I'm pretty sure some of the rovers and landers on Mars took footage (video or timelapse) of Mars' satellites. Are there any where you can see this phenomena?

  • $\begingroup$ Wondering if there's any video or still photos from Terran telescopes which have sufficient resolution to show a change in apparent size of Deimos over a single orbit. $\endgroup$ – Carl Witthoft Sep 15 '20 at 14:16
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    $\begingroup$ @CarlWitthoft Viewed from earth, the apparent diameter of Deimos varies by maybe +/-0.05%. The telescope should have an angular resolution of 1E-10 rad, or 0.02 milli-arcseconds. For optical wavelengths, that translates to a diameter or baseline around 10 km. $\endgroup$ – Ralf Kleberhoff Sep 15 '20 at 16:15
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    $\begingroup$ Generally, there is no "footage" from Mars because none of the current orbiters, landers, or rovers have the capability/inclination to shoot video. At best, there are timelapses $\endgroup$ – Dragongeek Sep 17 '20 at 11:55
  • $\begingroup$ Wouldn't this effect be even stronger for Phobos? $\endgroup$ – SE - stop firing the good guys Sep 17 '20 at 14:14
  • $\begingroup$ @SE-stopfiringthegoodguys Indeed. I always get the two mixed up -_-" $\endgroup$ – usernumber Sep 17 '20 at 14:16

The longest timelapse of a Martian moon I'm aware of is this 27 minute long timelapse of Phobos rising at June 28, 2013.

The light conditions, and the relatively small size of Phobos in the sky, makes the effect hard to see.

Phobos goes from horizon to horizon in approximately 4 hours 15 minutes, so this timelapse should cover about a ninth of the path.

At zenith, Phobos is 1.76 Mars radii away, and when setting, 2.57 radii away, and as such, Phobos should appear to have a 46% larger diameter. (only 3.6% for Deimos, which is also even smaller in the night sky).

orbit illustration 1

Unfortunately, the video is centred straight up, so it covers a segment where Phobos both grows larger and shrinks again. With a rough back-of-the-envelope calculation I estimate that as only a 1% change in diameter, so count your pixels!

A video covering Phobos near the horizon would give better results, even if it's shorter.

Here's such a clip from Spirit, but the image quality isn't good enough to see much beyond a moving blurry dot. One would probably need Curiosity footage for this.

Update: here's timelapse of Phobos rising over the horizon, and even slightly longer (32 minutes).

The atmosphere makes it hard to judge size,

That's more like a 13% change, so a much better view.

orbit illustration 2

If there are better timelapses, they are almost certain to be of Phobos, since the greater distance to Deimos makes the effect both weaker and less easy to observe:

relative size

(relative size in the sky)


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