Why is the moon (as it appears) exactly the same size as the sun?

Question

The moon appears to be exactly the same size as the sun--when there is an eclipse of the sun you can see this. It seems unlikely that this is by chance--is there some natural law to explain it?

Answer 1

Of course it's because their distances are proportional to their diameters. However, there is an underlying reason why. If it were not so, earth would not have evolved life intelligent enough to ask such questions. Little green men on some distant planet would, instead, be pondering how lucky *they* were to have a moon subtending about he same angle as their sun.

Two conditions for highly evolved life to occur on any planet are that both the sun and moon have a "significant" effect on the planet - but not too significant. If the sun is too small or too big in the sky, the planet will freeze or boil. If the moon is too small or too big in the sky, the planet's axis would be unstable (preventing stable climate) or the moon would have been ripped from the planet too recently for intelligent life to have evolved since (the moon was ripped from the earth by a planetary collision). In both cases, the solid angle is about the same. Now, I admit that it probably doesn't have to be quite as close as it actually is (there is a little luck there), but they would have to be the same within a factor of two or so.

I'm not, of course, implying any causal relation - even in the sense of a common cause. So, technically, it is still just pure luck. But so is having vast liquid oceans, not too many asteroids in the system, and being the right distance from the sun. Such "coincidences" are merely indicative of how rare planets are that support life capable of pondering such issues.

Incidentally, the above argument is based on "anthropic reasoning". That is, if the conditions for intelligent life are extremely rare, one should be able to identify what those rare conditions are. That implies that many such rare coincidences, such as the subject one, are instead expected, provided they are necessary for intelligent life. Other examples abound.

Answer 2

It is chance that the Moon is both 400 times smaller than the Sun and 400 times closer. But it is not just a coincidence of distance and size. It is also a coincidence of time because the Moon's orbit is slowly spiraling out from Earth due to tidal forcing, at about an inch a year. In the distant past it was much closer and so covered the Sun with plenty of room to space and in the distant future it will be too far away to completely cover the Sun at all. SO if we did not live at this time we could not see those nice eclipses where the Sun is just barely covered, showing all that interesting detail on all sides!

Answer 3

During any TOTAL solar eclipse, the sun and the moon APPEAR to be the same size. They were NEVER actually physically the same size. The moon is about 1/6 the size of the earth, and the diameter of the Sun is about 109 times larger than the earth (but when you talk about VOLUME, the Sun is more than 1 million times MORE VOLUME than the earth)

Answer 4

You have in your mind the total eclipse of the Sun case only.

There is actually another type of solar eclipse (it's called annular) where the Moon is not blocking completely the Sun. Instead, the Sun can be seen as a ring around the Moon, so in that case it never becomes really dark on Earth.

Anyway, the total eclipse case you're referring to is actually coincidental. As the Moon is moving slowly away from us, there will be a day (in millions or billions of years) that we won't have total eclipses of the Sun any more (we'll only have annular).

Answer 5

It is pure happenstance. In fact, it is not always the same size as the sun. The moon's orbit is elliptical, so sometimes it is closer to earth and other times further away. When an eclipse happens when the moon is further away, you get an annular eclipse of the sun. There is a bright ring around the moon.

HTH

Charles

Answer 6

Oh it's very simple. The sun is many times bigger than the moon, however the moon is much closer to the earth.

The moon orbits the earth at about 238,855 miles or 384,400 km. However, the sun is at a distance of 92,955,820 miles or 149,597,890 km from the earth. Thus, they appear the same size because of the great difference in distances from us.

Hope this helps. :)

Answer 7

It actually is by chance. The sun is about 400 times the diameter of the moon, and the sun is also 400 times farther away from us than the moon. Net result? They are equal in size.

Answer 8

the moon is much smaller then the sun, but the sun is muhc farther away. They are not exactly the same size though, in an eclipse you can usually still see rays of sun. look it up on google images

Answer 9

Because the moon is closer to Earth than the sun, it will look the same size since the sun, although larger, is farther away.

Answer 10

Well the moon is smaller then the sun but because it is closer it just looks bigger than the sun. But why they look about the same size it is just a coincidence.