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Our moon has been slowly drifting away from Earth over the past 2.5B yrs

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Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#21

Post by aussieshibe »

redact207 wrote: post #12
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

Fair warning: everything I know about orbital mechanics is from Kerbal Space Program.

The moon cannot orbit at a higher speed while keeping the same semi-major axis (average distance to the orbital centre of mass).

If you suddenly doubled the orbital speed of the moon right now, the apoapsis (the highest point in its orbit relative to the orbital centre of mass) would increase significantly.

If you slowly accelerate the moon in the direction of its orbital velocity consistently over a long time period, the moon will slow down relative to the Earth, but it's semi major axis will increase.

Actual scientists of HN: have at me.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#22

Post by [deleted] »

redact207 wrote: post #12
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

[deleted]

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#23

Post by devoutsalsa »

redact207 wrote: post #12
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

The moon is drifting away because the Earth is rotating. Eventually the Earth will be tidally locked with the moon, the same side of the Earth facing the moon at all times. When this happens, the moon will (for the most part) stop moving away from the Earth. Well, that’s what would happen if we didn’t get eaten by the sun when it morphs into a red giant. For further research, check out…

“Will the Moon ever leave the Earth’s orbit?” => https://youtu.be/IM_euz9PUiw

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#24

Post by incompatible »

aussieshibe wrote: post #21
redact207 wrote: post #12

Earlier quoted context omitted.

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

Fair warning: everything I know about orbital mechanics is from Kerbal Space Program. The moon cannot orbit at a higher speed while keeping the same semi-major axis (average distance to the orbital centre of mass). If you suddenly doubled the orbital speed of the moon right now, the apoapsis (the highest point in its orbit relative to the orbital centre of mass) would increase significantly. If you slowly accelerate…

Yeah, the moon isn't "drifting away" on its own, its being forced away as it takes energy from the Earth.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#25

Post by ncallaway »

chowells wrote: post #19
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon's orbit is increasing in radius, doesn't that mean its orbital velocity is decreasing? Which way is energy being transferred? This stuff confuses me.

I haven't done the math, but I strongly suspect that the potential energy (from moving away from the source of gravity) is increased more than the kinetic energy (from the reduced velocity) decreases.

So, I would think when the moon moves away from the earth, its total energy increases. Thus, the earth's energy decreases (in the form of slightly reduced rotational kinetic energy)

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#26

Post by totetsu »

divbzero wrote: post #14

A curious effect of this is that total solar eclipses past and future look different from total solar eclipses today. We live in a time when both the Sun and the Moon are roughly the same size (~32 arcminutes) when viewed from Earth. This coincidence results in the image we associate with total solar eclipses where the Sun’s corona is visible as a ring around the Moon.

A central plot point in Iain M. Banks' novel 'Transition'

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#27

Post by TheSpiceIsLife »

User23 wrote: post #18
divbzero wrote: post #14

A curious effect of this is that total solar eclipses past and future look different from total solar eclipses today. We live in a time when both the Sun and the Moon are roughly the same size (~32 arcminutes) when viewed from Earth. This coincidence results in the image we associate with total solar eclipses where the Sun’s corona is visible as a ring around the Moon.

Roughly the same size is understating it severely. They are uncannily similar in apparent size.

The surface of the Earth is the only place we know of where a natural satellite appears exactly the same size as the primary / only star.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#30

Post by consp »

charcircuit wrote: post #17
divbzero wrote: post #14

A curious effect of this is that total solar eclipses past and future look different from total solar eclipses today. We live in a time when both the Sun and the Moon are roughly the same size (~32 arcminutes) when viewed from Earth. This coincidence results in the image we associate with total solar eclipses where the Sun’s corona is visible as a ring around the Moon.

In even 1000 years that would only be an offset 38 meters. That wouldn't meaningfully change the ratio.

While not overwhelming an orbit change of 38000km per 1b years sounds more than enough to change that image.
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