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This question arises from "What planet is better than earth to infer solar system configuration?", where one of the answers pointed that it's easier to discover the Moon from Mars than discovering the Galilean satellites of Jupiter from Earth.

Then my question is:

  • How easy (or difficult) would be to see the Moon from Mars?
  • Could it be seen with naked eye?
  • Is actually easier to see the Moon from Mars than to see the Galilean moons of Jupiter from Earth?

I know there are favourable factors (Mars is closer to Earth than Earth to Jupiter, and Earth is smaller than Jupiter, so it's expected to obscure less its moon than Jupiter obscures its) and also unfavourable factors (the Moon is closer to the Earth than the Galilean moons to Jupiter, and Earth and Moon are backlit when seen from Mars). What is the overall result?

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    $\begingroup$ what a great question. $\endgroup$ – Fattie Sep 30 '16 at 13:16
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We have pictures of this, thanks to Curiousity!

The Earth and Moon from the Martian surface (Source)

“A human observer with normal vision, if standing on Mars, could easily see Earth and the moon as two distinct, bright “evening stars,” said NASA in a statement issued today.

So yes, you can see our moon from Mars.

The apparent brightness of our moon from Mars is +0.9. The apparent brightness of Jupiter's 4 biggest moons is between 4.6 and 5.6 under ideal conditions. However, while our moon can be seen visually separate from Earth from Mars, Jupiter's moons can't be distinguished from Jupiter itself with an unaided eye.

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  • $\begingroup$ Why isn't the Moon in the image in the plane of Earth and Mars? Its orbit isn't at all that much inclined. $\endgroup$ – LocalFluff Oct 1 '16 at 6:34
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    $\begingroup$ They are about same plane, but since Curiosity is not in the Martian poles the horizon in the image is not in the same plane. In Earth it's the same: all planets lie in the ecliptic (the same plane) but the ecliptic is not an horizontal line in our sky, except for some observers some days between the poles an the polar circles. $\endgroup$ – Pere Oct 1 '16 at 9:02
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You should be able to answer this easily: just look up the angular subtense of the Galileans from Earth and compare with the angular subtense of our Moon from Mars (and heck, check out the ability of Martians to see their own moons :-) ). You might also want to check out the orbital radii to ensure you can resolve the moons vs. their home planet.

The Galilean angular separation from Jupiter is 2 to 10 arc-minutes.

Ganymede's diameter is 5300 km; under ideal orbital positioning, it's 3.95029 AU , or 590 Gm from Earth, so it subtends about 2 arc-seconds (someone check my math! :-) ).

Our Moon's diameter is 3400 km and its distance from Mars, at closest approach, is about 60e6 km, for a subtense of about 12 arc-seconds.

That suggests our Moon is easily seen from Mars w/ Galileo's original telescope.

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    $\begingroup$ You can see our Moon from Mars with the naked eye. It has an apparent magnitude of about 2. $\endgroup$ – Ginasius Sep 30 '16 at 14:41
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    $\begingroup$ @Ginasius assuming aboriginal Martians have eyesight equivalent to Terran species :-) . But good point! $\endgroup$ – Carl Witthoft Sep 30 '16 at 14:46
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    $\begingroup$ And is separated from the Earth by a substantial fraction of a degree at maximum. Easy to see. $\endgroup$ – Rob Jeffries Sep 30 '16 at 16:06

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