# If the Earth did not rotate?

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• mincam
In summary, the Earth does not rotate, meaning that there is a six month day and night. This situation is not particularly plausible, though. The Earth interacts with the gravity of the Sun and any asymmetry in the Earth would eventually lead to "tidal locking", where the rotation period is the same as the orbital period, in which case the Earth would have a permanent night and day side. This is what has happened to the Moon (it is tidally locked to the Earth), which is why it always presents the same face to us.

#### mincam

TL;DR Summary
No earth rotation = no alternating days and nights?
If the Earth did not rotate, would one side always face the sun and the opposite side always be in darkness?

Or would a day be six months, and a night be six months?

Or something else?

"Does not rotate" is something that could be measured on the surface, by the absence of things like Coriolis forces. It would mean that the Earth maintained its orientation with respect to the fixed stars, and you'd have six month day and night.

This situation is not particularly plausible, though. The Earth interacts with the gravity of the Sun and any asymmetry in the Earth would eventually lead to "tidal locking", where the rotation period is the same as the orbital period, in which case the Earth would have a permanent night and day side. This is what has happened to the Moon (it is tidally locked to the Earth), which is why it always presents the same face to us.

FactChecker
mincam said:
No Earth rotation = no alternating days and nights?
Depends on how you define rotation. If with respect to the fixed stars, then you would the long polar days/nights everywhere on the planet. If you are interested about the consequences, you can look at Venus, which has very long days/nights.

Ibix said:
This situation is not particularly plausible, though. The Earth interacts with the gravity of the Sun and any asymmetry in the Earth would eventually lead to "tidal locking", where the rotation period is the same as the orbital period, in which case the Earth would have a permanent night and day side. This is what has happened to the Moon (it is tidally locked to the Earth), which is why it always presents the same face to us.
But it has NOT happened to Venus - even closer to Sun. How has Venus´ rotation period (currently 243 days retrograde) been evolving recently?

snorkack said:
But it has NOT happened to Venus - even closer to Sun. How has Venus´ rotation period (currently 243 days retrograde) been evolving recently?
The interesting thing about Venus' rotation is that at every inferior conjunction with the Earth the same point of Venus' surface points at the Earth. There is some speculation that this might be due to a tidal resonance between the two, so maybe Venus would have locked to the Sun by now if it hadn't been for Earth's influence. There is also a near resonance between the orbits(almost exactly 13:8), which may indicate a past orbital resonance or a drift towards one.

russ_watters
Just to be clear: It would be six months of day and six months of night.

Nobody is saying that one side would always be day and the opposite side always be night?

And if that is the case, I will ask, "Why not?" Then I will compare it to centrifugal force and say that if I swing a bucket of water in an orbit around me, the bottom side of the bucket will always be away from me, and the surface of the water will always face me, so why is the non-rotating Earth different?

I am not suggesting that 6&6 is the wrong answer, I just want to know why.

mincam said:
if I swing a bucket of water in an orbit around me, the bottom side of the bucket will always be away from me, and the surface of the water will always face me, so why is the non-rotating Earth different?
The bucket is rotating once per orbit, the same as the tidal locked case. Imagine a person watching you from a distance - they would see the base of the bucket then the water then the base again (and so on) as the bucket orbits you. If the bucket were not rotating then the base (or whatever) would always be pointing at them.

Agreed, but in my example I am the sun and the Earth is the bucket. I am not the imaginary person watching from a distance who sees both sides.

mincam said:
Agreed, but in my example I am the sun and the Earth is the bucket. I am not the imaginary person watching from a distance who sees both sides.
But you, as the Sun, would have to keep rotating in order to stay facing the orbiting Earth. So, if you are rotating, and the Earth always maintains the same orientation to you, that means the Earth is also rotating.

mincam said:
Agreed, but in my example I am the sun and the Earth is the bucket. I am not the imaginary person watching from a distance who sees both sides.
Doesn't matter, as @Janus points out. Whether or not the bucket is rotating is independent of who is looking at it. It's just easiest to see that it must be rotating from the "outside" perspective.

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If I am the sun I have eyes in the back of my head and I am always making day on just one side of the bucket.

Imagine that I have a light bulb on the top of my head that is casting light in all directions. I am never lighting the bottom outside of the bucket. Correct?

My problem is that I can visualize no way that I will not shine light on the inside of the bucket.

Tell me how I will shine light on the outside bottom of the bucket.

PeroK
mincam said:
Tell me how I will shine light on the outside bottom of the bucket.
If you are swinging the bucket round on a string you never will, because the bucket is forced to rotate once per orbit so the top will always be facing you. This is the same as the tidal locking case.

If you had a hollow sphere on the end of a string, with another free-to-rotate ball inside it, the inner ball could be made to not rotate. You could paint an arrow on the inner ball that initially points north (for example) and confirm that it always points north. (It won't, because friction will spin up the inner ball over time, but if the balls were truly frictionless it would.)

OK, thanks, I think I am getting it.

Assume that the handles of my bucket are mounted on frictionless bearings and they are vertical as I swing the bucket horizontally. After I swing the bucket 180 degrees, the water will all fly out because the bottom will now be facing me.

Does that work?

mincam said:
Assume that the handles of my bucket are mounted on frictionless bearings and they are vertical as I swing the bucket horizontally. After I swing the bucket 180 degrees, the water will all fly out because the bottom will now be facing me.

Does that work?
You need the center of mass of the bucket to be exactly between the places where the handles are mouted, otherwise it will rotate so that the center of mass is "outside" the handle mounts. But with that stipulation, and assuming the handles can turn 360°, yes.

OK, thanks. I asked this question because of my Moon Shadow Question. It's the next thread down.

Just for example, assume that the "axis" of any moon's orbit (of any planet) now points N&S. As this orbit "circles" the Sun, I assume that its orbital axis will always point N&S. And in that case it would seem that it would be impossible for that moon not to cast a shadow on its planet at some time. I further assume that this applies to all moons, regardless of the orientation of their axis of rotation.

OK, I see I need to modify: Assume that the moon leaves a path that makes a shadow. I assume that it would be impossible for that path not to leave a shadow .. yada, yada, yada.

mincam said:
OK, thanks. I asked this question because of my Moon Shadow Question. It's the next thread down.

Just for example, assume that the "axis" of any moon's orbit (of any planet) now points N&S. As this orbit "circles" the Sun, I assume that its orbital axis will always point N&S. And in that case it would seem that it would be impossible for that moon not to cast a shadow on its planet at some time. I further assume that this applies to all moons, regardless of the orientation of their axis of rotation.

While all Moons will, at some point or another, pass between the Sun and the planet, this does not automatically mean that they cast a shadow on the planet. If the Moon is small, far from the planet, or both, they simply won't block the Sun as seen from the surface of the planet, and won't cast a full shadow.
For example, Deimos, one of Mars' moons, is only 1/15 the width of the Sun in Mars' sky, thus when it passes in front of it only covers 1/225 of its surface.

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Janus said:
While all Moons will, at some point or another, pass between the Sun and the planet
I can take that as gospel, right?

No exceptions?

mincam said:
I can take that as gospel, right?

No exceptions?
Well...

Just because the moon's path passes in front of the planet doesn't mean the moon must be there at that time.

It is conceivable, for example, that the moon could just happen to not be at the right point in it orbit every time you checked. But it would be pretty contrived.

Depends on how long you're willing to wait.

Who knows - maybe there is a rare situation where the moon's orbit is synced with another body so that it is in resonance and just never happens to be in the right place over a period of arbitrarily long duration.

hmmm27
Well I mean, like, if I want to put up an Earth satellite that never casts a shadow on earth, I can't do it, because it's impossible.

mincam said:
Well I mean, like, if I want to put up an Earth satellite that never casts a shadow on earth, I can't do it, because it's impossible.
Of course you can.

We thought you were talking about natural bodies - for which we look at stability on the order of aeons. For a satellite, we generally look at scales much MUCH smaller than natural bodies, so you could put it in an orbit as you wish, and it will stay there for the foreseeable future, though not for aeons.

So, as suggested above: how long are you willing to observe it for? A million years? Or a hundred years?

mincam said:
if I want to put up an Earth satellite that never casts a shadow on earth, I can't do it, because it's impossible.

Why? Put it in a polar orbit a few million kilometers out with a 6 month period. Ajust the phase so every time the orbital plane crosses the sun the moon is behind the earth.

Why? Put it in a polar orbit a few million kilometers out with a 6 month period. Ajust the phase so every time the orbital plane crosses the sun the moon is behind the earth.
Which then raises the question: could this come about naturally?

DaveC426913 said:
Which then raises the question: could this come about naturally?

Is the universe infinite?

Is the universe infinite?
Even an infinite universe isn't sufficient to bring about physically impossible configurations.

(For example:

- a moon whose peripasis is less than its parent body's radius.

- or like in the first draft of my story, where my planet had four geostat satellites, each at the vertex of a tetrahedron. )

Do you have a reason to suspect it's physically impossible? It appears unlikely, but that's not the same thing.

Do you have a reason to suspect it's physically impossible? It appears unlikely, but that's not the same thing.
The OP is asking if if he can "take it as Gospel" (post 19).

That puts the onus on respondents to be sure there are no exceptions before stating such - regardless of our suspicions one way or 'tother.

DaveC426913 said:
Of course you can.

It is much simpler and easier than that. It is a very common orbit for many types of Earth observing satellites, a sun synchronous polar orbit that rides above the terminator. Especially RADAR mapping satellites commonly use it.

https://en.wikipedia.org/wiki/Sun-synchronous_orbit

Keith_McClary
Nobody seems to have mentioned Legrange points earth-sun, particularly 4 and 5 which are stable positions.
Question is, would that object even be considered a moon of the earth, or of the sun?
https://en.wikipedia.org/wiki/Lagrange_point

Do you have a reason to suspect it's physically impossible? It appears unlikely, but that's not the same thing.

If you mean the polar orbit, one problem is that it will tend to preces
256bits said:
Nobody seems to have mentioned Legrange points earth-sun, particularly 4 and 5 which are stable positions.