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Prima walked out to the observatory's main deck. There, as she had expected, Secunda was sitting in the grass, looking up at the stars.

"Pretty, isn't it?" said Secunda, as she craned her neck backwards to look at her sister. "You can see everything tonight."

"Everything?" replied Prima. Slowly wading through the grass, she glanced up and was instantly spellbound by the celestial garden of steady pinpricks of light, white and yellow and blue and red, smeared into blooming radiant petals by the thick observatory dome.

"Everything," whispered Secunda. "Well," she added, "except for Terra, I suppose." She stood, stretched, sighed, and turned to her starry-eyed sister. "Hey," she said, "I want you to follow me. There's a telescope over there that has a lens outside the dome. I bet we can see Mars from here."

Prima frowned slightly. "Really? Where is Mars from here, anyway?"

Suppose that Prima and Secunda are from our Earth, only a few centuries in the future. Could Mars be visible from Luna's night sky? What else would be visible? Would it be normal for Terra to not appear at night?

I know that, at a minimum, Luna's lack of atmosphere would mean that stars don't twinkle. I suspect that the Milky Way is apparent sometimes, and that the sky's view changes as Terra orbits Sol, so that the seasonal constellations are either similar or analogous in scheduling.

Edit: Thank you to all contributors. I've tried to upvote every helpful response, and will be thinking of tidal locking and thin atmosphere quite a bit now. I need the practice, so have some more:

"Got it! Aw, Jupiter's not as big as I had hoped," whined Secunda, pulling her face away from the goggles. The optical tube trailing the goggles, large and padded, made a sympathetic groan as it sagged slightly.

"What‽ No way, let me see already," Prima said as she groped for the goggles in her sister's hands.

"Okay," said Secunda. Prima's hands met hers on the goggles. She stepped towards Prima, and helped orient the optical tube to a position approaching stability. Prima pressed her face to the goggles, while Secunda giggled quietly.

Prima frowned, then jerked up from the goggles and glared at her sister. "You cheat! That's a photograph, isn't it? That's not real!" she protested, before breaking out into giggles herself. "You're such a joker sometimes," she snorted between laughs.

Secunda frowned. "No, I didn't," she started, then gripped the goggles harder and pushed them back into her sister's face. "Look carefully. Do you see it?"

Prima stared closely at the beautiful storm-ridden gaseous orb for a few moments, then gasped, "It's…moving? Crawling? It's alive! I can only barely see it, but it's moving somehow. Rotating?"

Both sisters were quiet for a moment. Then Secunda poked Prima. "C'mon, it's my turn again!" she said.

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    $\begingroup$ Earth will be visible every night or never visible depending on the side of the moon you are on because 1 side always face earth and the other never does. Although earth will have phases but even the night side of the Earth should be visible from artificial lighting. $\endgroup$
    – A. C. A. C.
    Commented Sep 21, 2017 at 18:28
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    $\begingroup$ Hi Corbin. I edited your question slightly to try to (in my opinion) make the title more clear and improve the formatting to make it more readable. Feel free to edit further, or even roll back, if you disagree. $\endgroup$
    – user
    Commented Sep 21, 2017 at 20:41

4 Answers 4

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At this moment? According to NASA/JPL and their simulator at http://space.jpl.nasa.gov/ , something like

mars from moon

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  • $\begingroup$ How come Sun is so much smaller compared to earth? Is that strictly the effect of no atmosphere? $\endgroup$
    – DRF
    Commented Sep 22, 2017 at 10:04
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    $\begingroup$ @DRF - think the image is due to the options I chose, which included a 120 degree FOV $\endgroup$
    – ivanivan
    Commented Sep 22, 2017 at 12:15
  • $\begingroup$ @DRF The sun in the sky is going to take up about 1/2 of a degree, from either the Earth or the moon. (And from the moon, Earth takes up 2 degrees.) But his ivanivan's answer about the field of view explains the image above. $\endgroup$
    – BrettFromLA
    Commented Sep 22, 2017 at 18:29
  • $\begingroup$ @BrettFromLA - what FOV in degrees would give a realistic view of what the human eye would see? I know I can see more than 120 degrees, but that was the max value available in the option. $\endgroup$
    – ivanivan
    Commented Sep 22, 2017 at 21:49
  • $\begingroup$ @ivanivan That's a good question. I don't know. I think we're used to standard photographs from cameras and phones, without telephoto or wide angle lenses, and I would guess that FOV is ... 90 degrees? $\endgroup$
    – BrettFromLA
    Commented Sep 22, 2017 at 22:10
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Because the moon is relatively close to Earth the night sky will be relatively similar. The two big differences are the lack of an atmosphere and a big planet in the sky instead of a moon.

Whether the earth is visible on the moon is dependant on where on the moon you are. The moon is tidally locked to the Earth so the same side will always be facing the Earth. From the near side of the moon the earth is always visible.

Without an atmosphere there will be less light pollution and twinkling.

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  • $\begingroup$ Would it be safe to say there would be no light pollution or twinkling? There's nothing for light to bounce off (except possible dust from impacts or activities), and nothing to diffract light from stars. $\endgroup$
    – BrettFromLA
    Commented Sep 21, 2017 at 21:32
  • $\begingroup$ The earth is many times brighter than the moon, and there is dust and traces of gasses to scatter light, so there will be light pollution . However, there will be no twinkling, $\endgroup$
    – pojo-guy
    Commented Sep 22, 2017 at 2:28
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    $\begingroup$ @SlippD.Thompson Not quite fixed, there is some Libration. $\endgroup$
    – Taemyr
    Commented Sep 22, 2017 at 8:35
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    $\begingroup$ @BrettFromLA probably a lot less light pollution but electrically charged dust is an issue on the moon in both daylight and at night. space.stackexchange.com/questions/1721/… $\endgroup$
    – Dan Is Fiddling By Firelight
    Commented Sep 22, 2017 at 12:47
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    $\begingroup$ @BrettFromLA -, earthshine will be brighter than moonshine, so even without earth/moon based light sources polluting you'll still loose those stars on the edge of visible brightness $\endgroup$
    – ivanivan
    Commented Sep 22, 2017 at 21:51
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Could Mars be visible from Luna’s night sky? Yes depending on the time of year and the hour of the day, as on earth. Would it be normal for Terra to appear at night? From the far side of Luna no never, from the near side of Luna yes always as Luna is tidally locked to Terra it rotates once per month so always shows the same face to Terra.

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Generally speaking, you can see the same stars from the moon as you can from the Earth, but there are some issues:

1) The moon is tidal locked to the earth, which means the same face always faces the earth. If you are on the far side of the moon, you will always see stars in the same manner as if you were standing on the Earth. If you are on the near side of the moon, you will always be looking past the Earth, which means the Earth will block almost everything (if not everything) in the sky.

Also, if you are on the far side of the moon, you will experience a long day (~13.5 days), and a long night (also ~13.5 days). If you live on the near-side of the moon, you will experience those same long days and nights, but you will also have periods of reflected light from the earth (full moon) and eclipsed light from the earth (new moon). You need only look at a lunar phases to see what "daylight" is on the near face.

This is a big deal as it means a substantial number of stars won't be visible either because the sun is in your eyes or the Earth is blocking the view.

2) The moon's orbit is not on the same orbital plane as the Earth (see pic, below). This means the stars visible on Earth at a particular time won't be the same stars visible to the moon at that same time. (What do I mean by this? If it were the middle of the night and the moon were directly overhead, then someone standing on the Earth's equator will see a slightly different star field than someone standing on the moon's equator.)

enter image description here

3) The moon wobbles something awful (see pic below and relevant discussion on Astronomy.SE). This will also affect what stars are visible at a given moment compared to the same moment on Earth.

enter image description here

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    $\begingroup$ "Earth will block almost everything (if not everything) in the sky": The Earth is not that big in the moon sky. See the many many pictures made by the astronauts of the Apollo missions; it is about 4 times as large as the Moon is on Earth's sky. "The moon wobbles something awful": only for suitably chosen values of "awful"; its movement is regular enough to allow it to be used to determine longitude with remarkable precision. $\endgroup$
    – AlexP
    Commented Sep 21, 2017 at 20:23
  • $\begingroup$ What, exactly, is that second image meant to be showing/demonstrating? $\endgroup$
    – MozerShmozer
    Commented Sep 21, 2017 at 20:23
  • $\begingroup$ @AlexP, that's a great point. The Earth would only reduce the star field by a minor amount. Thanks! $\endgroup$
    – JBH
    Commented Sep 21, 2017 at 20:25
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    $\begingroup$ Adding to @AlexP's initial comment, it's also worth keeping in mind that absent an atmosphere, there is nothing to spread light out in the sky. So the Sun and the Earth are likely to be very bright, and the ground (assuming you're on the daylight side of the Moon) will be awfully bright, but other than that, the sky will be essentially pitch black. If you look away from any of these three bright objects for long enough to allow your eyes to adapt to the lower light levels, the view of the sky will be, pardon the pun, outright stellar. $\endgroup$
    – user
    Commented Sep 21, 2017 at 20:39
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    $\begingroup$ @JBH when someone points out a factual error in your answer which you agree with, then it would be nice when you edit your answer and correct that error. $\endgroup$
    – Philipp
    Commented Sep 23, 2017 at 20:04

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