I had a discussion with a religious co-worker just now who says that the moon is getting farther away from the earth. I’ve read that before so I don’t deny that part but he went on to say that if you turn back the clock to ten thousand years ago, the moon would only be a few centemeters from the earth. With that, he claimed that the earth and moon can’t be as old as what most scientists say they are. So, my question is, what are the theories on where the moon came from? Has the moon always been getting farther and farther away from Earth and if so, is my co-workers religious theory that it was once only a few centimeters from the Earth true?
I am not an astonomer.
As with many fringe thoughts, there is a grain of truth to it.
Current theory says the moon was caused by a Mars size body striking a glancing blow to the Earth. If the moon had been flung off the other wat, it would have remerged with the Earth.
The Monn is spiraling slowly away from the Earth. At one time it was very near the Earth. That one time was more like 4 billion years ago instead of 10,000.
I’m sure someone with more hard facts will be along shortly
The most generally accepted theory is that the moon was formed from some early proto-planetary cueballing. Earth got the heck smacked out of her from what was believed to be a Mars-sized object. The resulting “explosion” caused a big chunk of the Earth to be blown away, where it later accreted into the moon. I think it’s generally believed.
I can see where the theory of the “proto-planetary cueballing” effect could have created the moon but where is the evidence? If a Mars sized object did indeed smack the Earth, wouldn’t there be a large Mars sized crater to show for it? Wouldn’t there be millions of smaller than moon sized particles orbiting Earth along with the moon? How is it that the moon seems near perfectly round?
To answer your friend’s silly “ten thousand years” argument, the Moon’s rate of escape from the Earth amounts to about 4 centimetres per year. In 10,000 years, that’s 40,000 centimetres - 400 metres, the distance around a track and field oval. Since the Moon is about a quarter of a million miles away, 400 metres is an insignificant distance. You couldn’t even tell the difference to look at it; the Moon looked exactly the same in 8000 BC as it does now.
It’s now generally accepted that the Moon is accreted material from a collision with another planet-like body, but it’s unlikely it ever actually orbited that closely; it probably started off at a reasonable distance and has been floating away at about the same rate ever since.
It would not be possible for two bodies the size of the Earth and the Moon to orbit each other too closely without tearing each other apart, and if the Moon had started off too close, the Earth might have become tidally locked to it already (as it is becoming now, but extremely slowly) and our “day” would be over six houndred hours long, with the same side of the Earth always facing the Moon.
Again, I am not an astonomer…
Mars size crater? I think the Earth was very young and still forming at the time. I think the Earth absorbed the Mars sized body.
I think the millions of smaller particles formed the moon much like the planets formed so instead of millions of smaller particles, you have the moon.
All planets are roundish. I think it is the most stable shape. The effects of gravity greatly favor the spherical shape
IANAPlanetary Astronomer, but some of the evidence is the fact that the moon is very crust-like, and the Earth is very core-like. This is consistent with the outer layers of the Earth getting knocked off to form a Moon.
me neither-
but look at this site- they say the difference in the diameter of the moons orbit is affected by the tides, and sloshing of the liquid seas.
http://www.sunspot.noao.edu/sunspot/pr/answerbook/moon.html#q121
sorry about long quote 
Oh yes, I meant to point that out. The Moon formed from a chip off the old Earth, and the Moon is getting further away due to tidal interactions. But these two things are not really related; orbital mechanics doesn’t work like that. It’s not like you can give the Earth a push away from the Sun and have it spiral out for billions of years thereafter.
This is a common creationist tactic; make a short-term measurement of some change, then extrapolate it as if the rate of change has been constant for all time. (quite apart from RickJay’s valid point)
In regards to there being a Mars Sized crater. I believe it’s understood that an impact that large would liquify the crust, threfore leaving no visible traces.
What ? Ten replies and nobody’s thought to link to Tim Thompson’s excellent Talk.Origins page on the subject?
Will the moon keep receding or will it eventually reach a point of equilibrium?
It will eventually reach a point of equilibrium. The moon is changing earth’s rotational energy into orbital energy, causing the earth to slow down, and the moon to move farther away. Eventually the earth will be tidally locked facing the moon, and the moon will stop moving away. I imagine this will take a very long time though, the sun will likely go red giant long before this happens.
The effect is caused by the tides. Since the earth is spinning faster than the moon orbits, the tidal bulge is always spinning ahead of the center-line between the earth and the moon. This causes the earth to get pulled “back” towards the earth moon centerline, slowing it down. At the same time, the moon is pulled “forwards” towards the tidal bulge. Due to the way orbital mechanics work, being pulled forward in an orbit causes you to move outwards. The effect is very small however, since the angle is very shallow, and the earth doesn’t deform all that much, so the mass of the bulge is relatively small.
The numbers from a Lunar Vital Statistics page gives the Mean Distance of Moon from Earth as 238,712 mi (384,400 km), reaching its closest point (Perigee) as 221,331 mi (356,410 km) from Earth.
As RickJay pointed out, the moon is receding from the earth at a rate of 4 cm/year. At this rate, it would take about 8.9 billion years to cover the minimum distance between the earth and the moon. Whoever argues that the moon would have been very close only 10,000 years ago is forgetting to move the decimal point.
D
There was an interesting program on one of the Discovery channels yesterday about the moon and its probable creation. They mentioned that originally the moon might have been as close as 14,000 miles away when it formed, covering an area in the sky about 15 times larger than today. They even had footage of it!
Has anyone made a graph on the Earth-Moon distance over the entire history of the system?
Quoth Mark Twain, on the subject of extrapolation:
So, the science fiction idea that the Moon will undergo orbital decay and crash into Earth is not valid? Admittedly, the last place I read that was in a Clarke novel that’s probably 30 years old, and I suppose the mechanics that have been worked out now were not known then.
What Clarke novel was that? None of this stuff about the Moon is new information, really, and if Arthur Clarke had the Moon crashing into the Earth, he must have figured a way for it to make sense.
There was also a story like that back in, I think, the 1930’s, called “The Moon Menace”.
An interesting thing about tidal effects (if my memory isn’t tricking me) is that they’re inversely proportional to the cube of the distance between two objects, not the square. So, if the Moon were half as far away, the lunar tides would be eight times as high. If the Moon were, say, 25,000 miles away, the effects would be catastrophic.