Refractory |
Moon moving awayAt a rate of 3.8 cm a year. Currently, the moon is an average of 84,401 km from earth 26,315 years ago the moon would have been 1 km closer to earth, and be 84,400 km away The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 1.5 milliseconds longer than it is now. The fact that the Moon is moving away from the Earth at a rate of around 3.8 centimeters per year might mean that we will have no more total solar eclipses in 500 million years or a bit longer, but that is a totally different matter. As you rightly suspect, changing the Moon's distance by a few percent won't have any significant effect on our ability to see it. In fact, the Moon's distance from us changes by a few percent every month as it orbits us and that certainly doesn't affect our ability to see it! Changing the Moon's average distance by a few percent (which is what will happen over the next 500 million years or so) will similarly not prevent us from being able to see the Moon, and to see it quite easily with the unaided eye. To get an appropriate perspective on this, consider the fact that the amount of light we now receive from a full Moon is tens of thousands of times more than what we get from even the brightest star in the night sky, Sirius. The Moon would have to move to many times its current distance for it to even drop below that level, let alone be so faint that we couldn't see it at all. The only real difference we can expect to notice 500 million years from now is that the average size of the Moon will appear a few percent smaller. The average "surface brightness" of the Moon (basically how bright it appears when we look at it) will be the same as it is now -- this is because although we will be receiving less total light from the Moon since it is farther from Earth, that light will get concentrated into a smaller region of our field of view, and the two effects cancel out. (This is true for any object that's big enough for your eyes to resolve -- for example, the computer screen doesn't look fainter when you move your head farther away from it.) --- How close was the Moon to the Earth when it formed? The Moon is thought to have formed when an object roughly the size of Mars hit the Earth. The impact was so violent that it threw large amounts of the Earth's mantle into orbit. This material evenually coalesced and formed the Moon. 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 19-30 thousand km. (The Moon is currently about 84,000 km away from Earth or 3-4 thousand times further away than this.) The Moon probably couldn't have formed closer than 3 Earth radii because tidal forces from the Earth would just pull it apart again, and it is unlikely that the impact could have ejected material further than 5 Earth radii. It's not a totally easy question to answer though as it depends a lot on the (unknown) details of the impact and how the hot material behaved in space. The exact rate of the Moon's movement away from Earth has varied a lot over time. It depends both on the distance between the Earth and the Moon, and the exact shape of the Earth. The details of continents and oceans moving around on Earth actually change the rate, which make it a very hard thing to estimate. The rate is currently slowing down slightly, and it is estimated that in about 15 billion years the Moon's orbit will stop increasing in size. |