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

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17

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).

kim holder

Posted 2014-12-01T19:15:42.050

Reputation: 757

Perhaps this has been asked using terminology i'm not familiar with. I searched but didn't find anything. – kim holder – 2014-12-01T19:16:55.360

According to stellarium, the moon traces out a circle of about 10-15 degrees diameter, but this is probably just libration. – barrycarter – 2014-12-02T02:08:29.983

Sorry, yes, that's what I meant. It was a typo and I can't edit it now (too much time has elapsed). – barrycarter – 2014-12-02T02:19:55.270

@barrycarter But you could delete your original comment and replace it with a correct one! – David Richerby – 2014-12-02T11:21:24.510

@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 – 2014-12-02T11:37:02.677

@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. – David Richerby – 2014-12-02T11:43:56.363

@barrycarter - here, i'll delete my response and then that makes it simple. and then later i'll delete this response. :) – kim holder – 2014-12-02T14:23:29.623

Answers

39

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

stellarium

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

tls

Posted 2014-12-01T19:15:42.050

Reputation: 506

1

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).

– LocalFluff – 2014-12-03T08:34:28.910

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 – tls – 2014-12-03T08:40:22.487

At what point on the moon is this? – kim holder – 2014-12-03T15:19:43.167

Sea of Tranquility. I rotated the camera a bit in order to get the the date into the frames. – tls – 2014-12-03T15:22:12.417

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? – Emilio Pisanty – 2014-12-04T11:38:48.123

1Detectable? 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? – tls – 2014-12-04T12:09:02.043

39

You are correct. The Earth would always appear in approximately the same location in the sky, when viewed from a point on the lunar surface. And it would be seen to spin, the continents coming in and out of view over the course of an Earth day (24 hours). The sun would make it's way across the sky, from one horizon to the other over a period of about two weeks, to rise again in another two weeks.

This might sound odd, because the rock we live on spins pretty quickly relative to the most obvious points of reference. In general (assuming that you are viewing from a place not particularly close to one of the poles) the bodies we see in the sky rise in the East, and set in the West about 12 hours later. But if you take a little bit closer look, you start to see that this is not the whole story. Each night, the moon rises and sets almost an hour later than the night before. The stars that you can see at night shift over a period of one year. And the planets move in such strange patterns that before Copernicus (at least in Europe), they seemed to just be wandering.

And not everything appears to move accross the sky. As you mentioned, Polaris, the north star, seems to hang in one spot all of the time, though you can't see it during the day. But even taking all of this into account, the idea of a large planetary body just hanging in one spot all of the time in the sky, may still seem... um, alien. And to that, I might just say, we've been here on this one planet an awfully long time. It's bound to feel a bit like home.

Mark Bailey

Posted 2014-12-01T19:15:42.050

Reputation: 725

9I bet the moon people have been intrigued that over the last couple hundred years they've seen the dark side of the earth get brighter. – corsiKa – 2014-12-01T23:43:01.230

1@corsiKa follow-up question: has the human impact on earth created a difference in the brightness of the "dark side" of the Earth, viewed from the Moon? – Tim – 2014-12-02T03:44:59.973

@TimGostony That's a really interesting question. I made a new question for it here: http://astronomy.stackexchange.com/q/8105/2934 . I included an image which I think may show artificial lights on Earth as seen from the moon, but I'm not 100% sure that's what I was seeing.

– reirab – 2014-12-02T04:51:07.290

And if you are in the higher southern latitudes of the moon, the Earth is always in the same spot - upside down. :P – kim holder – 2014-12-02T14:27:46.293

I'm confused about the sun rises at moon. Since moon got the same face to sun all the time it seens logical (on the light side of the moon) it just a mater of earth eclipses the sun. Both just hand around about the same spot with the earth just swinging a little in front of the sun – jean – 2014-12-02T17:42:06.203

2@jean No. The "dark" side of the moon is not actually dark all of the time. We just never see it (other than when we fly a spaceship around to the other side). The same side of the moon always faces Earth, but as the moon revolves around the Earth, which takes just over 4 weeks, the sun shines on different sides of the moon. When we on Earth see a full moon, that is because the sun and the moon are on opposite sides of Earth. So the side facing us is fully lit. At new moon, the moon is on the same side as the sun, so our side of the moon is dark, but the "dark" side is actually fully lit. – Mark Bailey – 2014-12-02T20:04:46.883

17

To add to Mark Bailey's answer; the Earth would indeed hang in the sky and rotate, but it would also wax and wane over the course of a lunar day (27.3 Earth-days).

Starting at lunar dawn, the Earth would be half-full. The Earth would then wane (more shadow) towards lunar noon. At lunar noon, the Earth would be all in shadow (New Earth) and quite close to the sun. With a telescope (to eliminate the sun), a lunian would see city lights and atmospheric glow around the edges.

During the lunar afternoon, the Earth would wax (less shadow) and be half-full again (on the other side) at lunar sunset. Full Earth would occur at lunar midnight. This would be about 16 times as bright as a full Moon on Earth, so it never gets really dark on the lunar Near Side. The Earth would then wane again until it was half-full at lunar dawn.

Oscar Bravo

Posted 2014-12-01T19:15:42.050

Reputation: 398

3And a Full Earth is when the moon werewolves come out – Jon Story – 2014-12-02T12:31:47.650

2Not to mention the solar eclipses would be incredible, while the Earthian (nomenclature?) eclipses would not be near as cool as lunar eclipses. – Benjam – 2014-12-02T17:35:15.050

2@Benjam I think that terrestrial is the word you're looking for. Earth's solar eclipse/moon's terrestrial eclipse is Sun > Moon > Earth; moon's solar eclipse/earth's lunar eclipse is Sun > Earth > Moon; – Tim S. – 2014-12-02T18:45:37.293

@TimS. Good call. Still feels wrong, though. Even though I know it's correct. Seems like maybe it should end in -ar to be consistent with the others. – Benjam – 2014-12-02T18:55:09.273

2Terran eclipse? – kim holder – 2014-12-03T21:05:06.700

2@briligg That seems more like it, but then eventually, we'll need a Zerg eclipse, and a Protoss eclipse. ;) – Benjam – 2014-12-05T18:19:03.457

10

To expand a little more, yes the Earth would hang in the same spot in the sky, moving around in a small circle as the moon rotated around it over the course of each of its 28 day orbits. It would have phases, full Earth when the moon is between it and the sun, new Earth when the Earth is between the moon and the sun, and wax and wane between these two points. Each time the Earth was full, a different part of it would be visible from the moon, depending on the season and how it happened to line up when full Earth occurred.

The Earth would roll in the sky according to the seasons on Earth and the precession of the moon's orbit. The moon tilts only 1.5 degrees from the ecliptic and so in essence has no seasons. The orbit of the moon is tilted 5 degrees from the ecliptic, so when it is farthest to the south of the plane of the solar system, more of the south of our planet is visible, and vice-versa when it is farthest north. But the Earth is tilted 23 degrees from the ecliptic, so this is what would mostly determine what part of it was visible from the moon. Here is a good summary of the relationship of the Earth and Moon. The distance between the two bodies is not to scale, but the relative size of the one to the other is to scale.

the orbit of the moon around the Earth

In December, Antarctica would be visible - as it was during the famous Blue Marble photo from Apollo 17, taken on Dec. 7, 1972.

Apollo 17 photograph of Earth, showing Africa, Arabia, and Antarctica.

(This version of the photo is supposed to be as it was in the original photograph. They took it on the way to the moon, so the orientation of the camera was the only thing that determined which way was 'up'. If you were on the moon at far southern latitudes, this is in fact how it would look from your point of view.)

In July, the Arctic would be visible. Apollo 11 took this photo on July 16, 1969, when they were half-way to the moon.

Apollo 11 photo of Earth, showing Africa, Europe, and the Middle East

Here actually you don't see much of the Arctic. The moon's orbit had carried it a bit south of the Earth at that point. Heading to it meant going south a bit, so when they looked back at the Earth, they saw less of the far north.

Just to be clear, something you would NEVER see is this, which has been widely misunderstood as a photo of the whole planet from space. Actually it is a super-wide-angle photo taken from low earth orbit - the horizon line is for the part of the world the satellite could see, it is not one hemisphere of the Earth.

enter image description here

If you think about it for a second you realize this can't possibly be true. The United States is gigantic, if it was really this size Canada would occupy the whole arctic and roll over onto the other side of the world, displacing most of Russia. Argentina and Chile would take the place of Antarctica.

kim holder

Posted 2014-12-01T19:15:42.050

Reputation: 757

In the July 11th 1969 photo we can't see the Artic because of the position of the Moon. If the Moon were between Sun and Earth we would see the full Earth and the Artic - or at least a bigger part of it. – Pere – 2017-02-14T15:49:54.250

This is why Earth with a moon could not have a defined planetary ring. The wobble and tilt of the Moon it to great. – Muze – 2018-09-29T20:23:20.437

My be not visible but the Earth would also increase and decrease in size. During new moon it would be dark but with the Sun reflecting off the Earth onto the Moon it should be 80 times brighter then what the full moon looks like from Earth? – Muze – 2018-09-29T20:29:46.307

The last picture reminds me of the opening to _Moral Orel_ where one half of the world just has the US on it, not even Canada or Mexico!

– CJ Dennis – 2014-12-03T03:14:41.417