How does the Earth move in the sky as seen from the Moon?

  • I just want to be sure I am visualizing this correctly, because it seems odd. The Moon is tidally locked to the Earth but there are wobbles to its motion due to libration. So from a point on the surface of the near side of the Moon, the Earth would always be near the same place in the sky? It would describe a small circle or a side-to-side wobble over the course of a month, but never move far from that point?

    That would seem very strange, like it was a gigantic stage prop or something. We are so conditioned that everything rises and sets (except for a few stars near the poles).

    Perhaps this has been asked using terminology i'm not familiar with. I searched but didn't find anything.

    According to stellarium, the moon traces out a circle of about 10-15 degrees diameter, but this is probably just libration.

    Sorry, yes, that's what I meant. It was a typo and I can't edit it now (too much time has elapsed).

    @barrycarter But you could delete your original comment and replace it with a correct one!

    @DavidRicherby I dislike deleting comments because it disrupts the flow of the thread (eg, responses to a comment that no longer exists). Plus, I have a great deal of self-loathing to vent ;)

    @barrycarter So flag the responses as obsolete and they'll be deleted too. It's *much* more confusing to have an accidentally inaccurate comment corrected later in the thread.

    @barrycarter - here, i'll delete my response and then that makes it simple. and then later i'll delete this response. :)

  • tls

    tls Correct answer

    8 years ago

    Still new at stellarium but here are some quick capture gif lasting one month. Sorry about the quality- limited to 256 colors for smaller gifs. Date on lower left corner. By the way the sun is of course the brightest and i use it as reference for recording (start record when sun is in frame then stop when it appears again in the same position which is roughly one month)

    Location on Moon : Sea of Tranquility

    You are looking straight up

    Yellow lines are azimuth

    enter image description here

    second picture:

    Zoomed view of earth

    One whole day 24 hours (give or take a few minutes)

    Location on Moon: Sea of Tranquility

    You are looking straight up

    Yellow lines are azimuth (gif itself rotated to approximately match the orientation of the 260 degree azimuth line in the first picture)

    enter image description here

    Answer for Emilio Pisanty comment on whether the oscillation is detectable by eye


    Field of view 60 degrees (default view when stellarium first opened)

    Picture 3

    Shows one end of the oscillation near the 200 azimuth line (see yellow arrows)

    other end of ellipse near 185 azimuth line

    you can use Gemini (lower right corner) for reference (see yellow arrows) from head to crotch of one Gemini twin would be a good reference for how wide the oscillation is (not sure if your sundial can detect the difference)

    enter image description here

    picture 4

    shows other end of the oscilation near the 185 azimuth line (see yellow arrows)

    enter image description here

    picture 5 shows my stellarium location settings on the moon

    enter image description here

    Great! Other answers have discussed everything but the movement of Earth across the sky as seen from the Moon. If I understand it correctly, the apparent movement of Earth is due to two libration effects: The eccentricity of the lunar orbit and the small inclination of the lunar axis of rotation relative to the plane of its orbit. (The Moon does however not have any diurnal libration effect).

    Yep. on the bottom picture you can just barely see the earth changing in size. I'm currently slowing down the gif and will update as soon as I can

    At what point on the moon is this?

    Sea of Tranquility. I rotated the camera a bit in order to get the the date into the frames.

    Can you comment on the amplitude of the oscillation? Would it be visible to the naked eye? Would it be detectable by e.g. a sundial (Earth-dial?) during the lunar night?

    Detectable? Probably. I'll try to post another gif, which includes the moon's horizon. Like you are looking at the earth's oscillation with the moon's horizon in the foreground. I would have to change the location of the viewer to somewhere (random) near the northern part of the moon in order to place the Earth's oscillation viewable near the moon's horizon. Would that be ok?

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Content dated before 7/24/2021 11:53 AM