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Comment by soliton4 | Hacker News Reader

-- quote -- These have shown that the moon is currently moving 3.8 cm away from the Earth every year.

If we take the moon's current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. -- end quote --

3.8 cm * 1.500.000.000 = 5.700.000.000 cm = 57.000.000 meter = 57.000 km

distance between earth and moon = 384,399 km according to https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)

what is wrong here?

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> we end up with a collision between the Earth and moon

My understanding is that it isn’t that the moon and earth collided, but rather that the moon was formed from earth stuff when another body collided into earth.

the "collision" between earth and moon is entirely hypothetical in this context. yes there was probably a colision involved in the creation of the moon, but that is an entirely different storry that has no relevance to this thread besides the fact that it also involves the earth, the moon and the word colision. if you must continue a conversation about the origin of the moon please do it in a new thread, as any further exploration in this context will lead to more confusion and will ultimately do more harm than good

The rate at which the moon moves away from the Earth isn't constant. I can't find the details right now so I could be totally wrong.

thats true, however the author explicitly stated that the collision point of earth and moon at 1.5 billion years was calculated by assuming the moons CURRENT rate of recession

At no point does the author make any assertion that the rate was constant. Presumably, the current rate of recession is just one variable that gets plugged into an equation.

"If we take the moon's current rate of recession and project it back in time" is a very straight forward way of saying, lets assume the moon receeded from the earth at the current rate.

No it’s not because you are projecting backward in time in the presence of a gravitational field that varies with square of radius.

The path of the moon away from the Earth is not a straight line, it is a tightly wound spiral.

given your insight in the matter, would you care to post the correct calculation then?

Yes, but the rate of energy transfer is related to the ratio of an earth day (which used to be shorter) and the lunar month (which also used to be shorter).

Taken in the opposite direction that moon will stop moving away once the a earth day = lunar month. At which point it will start getting closer, until it hits the roche limit and becomes a ring.

I guess that assumes that the moon's starting point was the surface of the Earth and then started floating upward very slowly to where it is now.

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