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Thread: The moon being much closer to the earth in the past?

  1. #1
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    The moon being much closer to the earth in the past?

    hi

    We are told that the moon is increasing its distance to the earth by a few centimetres each year. Adding up since the time of the dinosaurs (or even longer ago), the moon must have been much, much closer to the earth. I have heard on BBC that the moon could not have been closer than 40.000 miles to the earth, before being caught by earths gravity in a colision.
    Back then, would the tidal forces not have been much stronger? More powerful tidal waves on the sea, more vulcanic activities, maybe increased tectonic activity etc? Some states that if the moon was 20 times closer to the earth, then the gravitational forces would be 400 times larger, which would result in devistating effects on our planet. Could this have had effect on the life on earth back then?

    Kind regards

    Karsten

  2. #2
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    You don't say how many centimetres a "few" are, and I don't know the answer myself. But one centimetre per year for a billion years is 10,000 km, which is about 2.5% of the current distance. So that's not tiny, but it's not huge either. If it's three centimetres per year for four billion years, that's more like 30% of the current distance. This all assumes the rate of increase has not been changing over time.

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    I'll try to think about it soon, but now it seems like a good time to drag out some classic Mark Twain thinking:

    In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the lower Mississippi will be only a mile and three-quarters long. . . . There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    His thinking far surpasses my own.
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  4. #4
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    Ask an Astronomer: How close was the Moon to the Earth when it formed?

    It is not easy to estimate how far away from the Earth the Moon was when it formed, but simulations suggest is was about 3-5 times the radius of the Earth, or about 20 to 30 thousand kilometers.
    Estimates of the great impact put it hundreds of millions of years before initial life, and eventual early life, likely residing in water, had much bigger problems to conquer than those a nearby moon had to offer.

    Later, more complicated life evolved in the presence, under the influence, of the moon, and just adapted to it.
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  5. #5
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    Quote Originally Posted by cretaceous View Post
    You don't say how many centimetres a "few" are, and I don't know the answer myself. But one centimetre per year for a billion years is 10,000 km, which is about 2.5% of the current distance. So that's not tiny, but it's not huge either. If it's three centimetres per year for four billion years, that's more like 30% of the current distance. This all assumes the rate of increase has not been changing over time.
    3.820.07 centimetres a year.*

    *From http://rationalwiki.org/, which might not be the best source.
    Last edited by Solfe; 2016-Oct-13 at 03:44 AM.
    Solfe, Dominus Maris Pavos.

  6. #6
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    The Moon is currently moving away from the Earth at about 4 cm per year (3.78 cm to be precise). As 01101001's quote suggests, it's not a good idea to assume that rate has always been constant. But at that rate, if we jump back to the time of the earliest dinosaurs, 230 million years ago or so, that would be a total change of about 8,700 km, compared to the current distance of 385,000 km. So perhaps noticeable if you paying attention closely, but not enough to make a huge difference in tides.

    Certainly, the 20,000 to 30,000 km distance that is talked about as the original distance when it was formed would lead to very large tides. Tidal force scales roughly as the inverse cube of the distance, so being 10 times closer would mean tides roughly 1,000 times (!) as strong. But since tidal friction is what pushes the Moon farther away, it would have been receding rather faster during that initial era, and the tidal effects would have been smaller than that by the time life formed. As the BBC article linked above notes, that also changed the length of Earth's day, and we can indeed see the effect of that on some of the earliest life (daily growth layers in corals).
    Conserve energy. Commute with the Hamiltonian.

  7. #7
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    Talking about the distance to the moon one should not forget that the distance change all the time and on all timescales. Even if it's true that the avarge distance increase with about 4 cm per year seen over long time it is not true that this is a smooth change one got the impression from by many talks I have seen on Internet. Here is a simulation one million yers back in time and in the future over how the orbital elements change for the moon. This is a simulation I did a few years ago by integrating the laws of motion for the main bodies of the solar system and using about the same forumals as JPL's Horizon.

    Obviously the distance to the moon changes over the month between 365000 to 405000 km because the moon orbit is elliptical. But the mean distance also changes over the year because the earth-sun distance changes from the elliptical orbit of the earth. The orbital elements for the bodies in the solar system are dynamic and change all the time and influence the actual mean distance. So when you say that the moon leave the earth by 4 cm per year it is correct, but only as a mean value over a million year or so. On shorter timescales the local variations of the mean distance is much larger.

  8. #8
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    Quote Originally Posted by ThomasK View Post
    Talking about the distance to the moon one should not forget that the distance change all the time and on all timescales. Even if it's true that the avarge distance increase with about 4 cm per year seen over long time it is not true that this is a smooth change one got the impression from by many talks I have seen on Internet. Here is a simulation one million yers back in time and in the future over how the orbital elements change for the moon. This is a simulation I did a few years ago by integrating the laws of motion for the main bodies of the solar system and using about the same forumals as JPL's Horizon.

    Obviously the distance to the moon changes over the month between 365000 to 405000 km because the moon orbit is elliptical. But the mean distance also changes over the year because the earth-sun distance changes from the elliptical orbit of the earth. The orbital elements for the bodies in the solar system are dynamic and change all the time and influence the actual mean distance. So when you say that the moon leave the earth by 4 cm per year it is correct, but only as a mean value over a million year or so. On shorter timescales the local variations of the mean distance is much larger.
    My bold. Don't forget the variation that occurs when the Moon's line of apsides goes from conjunction to quadrature and back over a period of some 7 months. The perigee is over 10,000km closer at conjunction, and yes indeed, this perturbation by the Sun is stronger at Earth's perihelion. Teasing out this 4cm per year mean distance increase from the periodic variations on the order of a billion times larger requires careful observation and orbit fitting over a long period.

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