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We know that right now, by measurement, the Moon is moving further away from the Earth. So I've heard the conclusion that the Moon was much closer to the earth in the past. How do we know that part? Is there empirical evidence of that or is it an assumption based on the fact the moon is moving away right now, so it's a simple extrapolation backwards in time?

Could it possibly be the Moon distance oscillates? Moving further away until a maximum and then moving towards the Earth until a minimum?

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    $\begingroup$ If you accept that now, by measurement, the Moon is moving away, why do you doubt that the same measurement shows the Moon was in the past closer? $\endgroup$ Aug 6, 2023 at 21:32
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    $\begingroup$ I asked the question for a reason, to hear about theories and evidence that it was much closer. Why? Because without that we're left with pure extrapolation into the past, extrapolation alone is never really any good. $\endgroup$
    – scm
    Aug 6, 2023 at 22:08
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    $\begingroup$ @RobbieGoodwin there is nothing wrong with the question and trying to get "into the head" of question authors like this is not really good practice. We can ask questions when we already know the answer, and in science we always, always challenge the reasons that we believe things are the way they are. We should always maintain some lingering doubt about everything (yes, even conservation laws and detailed balance etc.) and never call someone else out for doing likewise. $\endgroup$
    – uhoh
    Aug 7, 2023 at 6:59
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    $\begingroup$ @RobbieGoodwin cf. What is the Current Status of Measurement of the Gravitational Mass of Antimatter? they're trying to find out if antiprotons fall down like protons, or "fall up". (It's a low resolution experiment, they won't get much info on the magnitude of the mass, only te sign) Nobody really thinks they are repelled by Earth's gravity, and yet these scientists are spending time and money just to make sure. Science! $\endgroup$
    – uhoh
    Aug 7, 2023 at 7:08
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    $\begingroup$ @RobbieGoodwin No. To me the question is perfectly clear. "How do we know that part?" and "Could it possibly be the Moon distance oscillates?" Just because we have fifty years of data by bouncing lasers off of retroreflectors and can fit the noisy data with a straight line does not de facto guarantee the motion is monotonic over 4.5 billion years - a time which is 100,000,000x longer than the time for which measurements have been made. The question is excellent as asked, and as written. $\endgroup$
    – uhoh
    Aug 8, 2023 at 0:23

3 Answers 3

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How do we know the Moon was much closer than it is now?

Theory and evidence.

On the theoretical side, it is hypothesized that the Moon formed from a collision between a Mars-sized object and the proto-Earth. Per this hypothesis, the Moon formed after this collision a handful of Earth radii from the center of the Earth. The Moon is now about 60 Earth radii from the center of the Earth, so if this hypothesis is correct, the Moon has receded from the Earth by quite a bit.

Also on the theoretical side, tidal forces make objects that orbit a planet closer than planet synchronous altitude spiral in while objects that orbit a planet further out from synchronous altitude spiral out. The Moon is spiraling out due to these tidal interactions.

On the evidential side, the Moon is very similar to the Earth in terms of geology and chemistry. We know this as a fact due to observations of the Moon and due to Moon rocks brought back to Earth. This similarity gives incredible credence to the hypothesis that the Moon formed from a giant impact, and formed at a much closer distance that the current separation between the Earth and the Moon.

Also on the evidential side, there are tidal rhythmites frozen in ancient stone. These rhythmites show that the Earth's rotation rate used to be much faster than the current rate of one rotation per 24 hours. Some of these tidal rhythmites date back to over half of the Earth's age. The cause of the slowdown in the Earth's rotation rate is the transfer of rotational angular momentum to the Moon's orbit.

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    $\begingroup$ are you saying, the moon is spiraling out -- so eventually it escapes, or eventually it stops receding? $\endgroup$
    – scm
    Aug 6, 2023 at 14:01
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    $\begingroup$ @scm yes, that is the case. The recession will eventually stop, when the tidal forces have slowed down earth's rotation so much that it is also locked to the moon (which will stop the tides altogether). The angular momentum earth loses due to tides is transferred to the moon, which moves further away from us. The moon currently recedes by approx. 1 cm per year. $\endgroup$
    – Chieron
    Aug 6, 2023 at 22:09
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    $\begingroup$ @Flater, yes. The tidal bulge of the oceans isn't quite aligned with the Moon, producing a constant pull. $\endgroup$
    – Mark
    Aug 7, 2023 at 2:17
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    $\begingroup$ @scm Note that due to time scales it is possible for the moon to continue spiraling out without ever escaping or stopping to recede. If we just linearly extrapolate the 1 cm per year (which is surely wrong but hopefully a reasonable first order approximation) that would correspond to 10.000 km per billion years. As the moon is around 400.000 km away from the Earth right now, this would not lead to any major changes before the heat death of our sun. $\endgroup$
    – quarague
    Aug 7, 2023 at 11:27
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    $\begingroup$ @Chieron The current lunar recession rate is an anomalously high 3.8 cm per year rather than 1 cm per year. The average over the last 400 million years, for which good records do exist, is about 2 cm per year. The reason for the current high recession rate is most likely the shapes and depths of the ocean basins, in particular the North Atlantic, which is where much of the tidal energy is lost. $\endgroup$ Aug 7, 2023 at 12:17
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Due to tidal forces between Earth and the moon, the natural satellite slowly spirals away from Earth at a rate of about 1.5 inches (3.82 centimeters) per year, causing Earth to rotate more slowly around its axis.

This has been predicted way before and has been calculated by a statistical method called astrochronology. Through this method, scientists were able to peer into Earth's deep geologic past and reconstructed its history through a model. It revealed that the moon was way closer to the Earth which made Earth spin faster. As a result, a day on Earth lasted just over 18 hours.

Source: https://www.space.com/40802-earth-days-longer-moon-movement.html

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We got to know that the moon was much closer than it is know because of the tidal interaction where in the action-reaction pair the tidal friction is the action and the reaction is that the moon is moving away from us at an increasing rate of 3.8 cm, by this we can estimate that the Moon was much closer before (There the 1 day lasted just 4 hours because the Moon did not put much of an tidal friction), approximately when the Moon was formed by the Great impact hypothesis, theia collided to form the moon and the moon did not move away because the day lasted 4 hours just like we got to know that because the Universe is expanding know just like FLRW metric, the universe must be a point.

But we also got to know it by ancient geological tidal markings (there were layers of underwater submerged rock and dunes whose thickness varied by the strength of the tide between different parts of the tidal cycle like spring and neap tide, then they applied kepler's 3rd law to formulate the distance at which the Moon was situated. One example of this is Moodies Group sandstone in South Africa) which showed that the tides were much higher before therefore the moon must be closer earlier .

As for the second part of the question, The moon's orbit does not oscillate back and forth, though the moon's radius vector oscillates in it's orbit by periapsis and apoapsis, because the tidal interaction is just a one way ticket, it speeds it away and it does not come back because gravity is an inverse square force field and the gravitational linking is not strong enough to make it decelerate and come back towards it before it's too late and it escapes it's hill sphere because the mass of the Earth moon system the bodies is too low.

Thanks!

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  • $\begingroup$ I like your answer because my idea about it oscillating would be because tidal forces transferring momentum would diminish with distance, and gravity would still be attracting, and so I expected it to oscillate until the two forces balance. However in your answer you point out gravity is inverse square. Thanks. I was also looking to see if there was any evidence of greater ocean tides pre-historically. $\endgroup$
    – scm
    Aug 6, 2023 at 13:42
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    $\begingroup$ Thank you, @scm for telling me that my answer was helpful. I try to improve it further $\endgroup$
    – Arjun
    Aug 6, 2023 at 14:26
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    $\begingroup$ @scm I have also added about what are those tidal markings and how they formed $\endgroup$
    – Arjun
    Aug 6, 2023 at 14:33
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    $\begingroup$ The fact that the earth rotation is slowing down is also confirmed by ancient eclipse reports which should have been at different places if the rotation was constant - latimes.com/science/sciencenow/…, space.com/ancient-eclipse-records-earth-rotation-history $\endgroup$ Aug 7, 2023 at 7:18
  • $\begingroup$ Regarding the tidal aspect, you may find this post interesting: astronomy.stackexchange.com/questions/39109/… (cc @scm) $\endgroup$ Aug 7, 2023 at 8:26

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