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# Day length on Deimos

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This might be a simple question:  What are the alternating sunlight and darkness periods on the surface of Deimos?

It is tidally locked to Mars with an orbital period of 7.5 hours.

If you were standing on the outward (non Mars facing side) does this mean you would have "day" 7.5 hours long, followed by 7.5 hours darkness?

On the Mars facing side you have a significant fraction of the sky taken up by the planet that throws Deimos into eclipse every "day".  What portion of the day would be eclipse?  How big is Mars as viewed from Diemos?

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4 hours ago, farmerben said:

It is tidally locked to Mars with an orbital period of 7.5 hours.

7.5 hours total

4 hours ago, farmerben said:

How big is Mars as viewed from Diemos?

That's a solid angle of .065 steradians, pretty big, comparable to your outstretched fist

4 hours ago, farmerben said:

On the Mars facing side you have a significant fraction of the sky taken up by the planet that throws Deimos into eclipse every "day".  What portion of the day would be eclipse?

Assuming everything is in the same plane (which is not true at all but I don't want to do the real math), the angular diameter of Mars is 9% of the arc of the sky, so you'd be in some kind of eclipse 9% of the time. The Sun only has an angular diameter of .006 rads, so basically all of the time would be in a total clipse

Edited by NFUN
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One, you're thinking of Phobos. Deimos has a ~30 hour period.

Two, the math needs to be for Phobos (else the period increased to 30 hours)—see 1.

So from Phobos, Mars  is ~0.68 rad—~39° across. Solid angle is ~0.4 sr.

Inclination is ~1°.

Assuming a cylindrical shadow for simplicity, it's ~6600 km across. Phobos is 20-25km dia, and orbits Mars at ~9300 km, so it spends ~11% of the time in total eclipse, ~51 minutes/period, 2 hours 43 minutes per Earth day.

Partial eclipse more, don't feel like figuring that out.

Edited by tater
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5 hours ago, tater said:

One, you're thinking of Phobos. Deimos has a ~30 hour period.

oops

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For reference, this would put Mars viewed from Deimos at about 33 times wider than the Moon on our sky. For Phobos, it's almost 80 times wider.

The other thing that occurs is that even during the 'night' on either moon, if you were on the side facing Mars you'd have this pretty large thing in the sky reflecting a fair amount of light. So weirdly enough, presumably the darkest time would actually be in the middle of the 'day' during your monthly eclipse.

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