# The apparant size of the sun and moon

Has it ever occured to anyone else how fascinating it is that the sun and the moon appear to be the same size from the one vantange point in space that happens to be occupied by the one known creature capable of noticing?

In a solar eclipse, the moon can completely and perfectly cover the sun. When it does, we enjoy it, but wouldn’t a more likey scenario be that they appeared to be of different sizes?

Maybe it’s that scene with Luke Skywalker staring at two – what, moons? Planets? – in the first Star Wars film that got me thinking about this.

Two lights, one for night, one for day, and for human beings, they appear to be equally sized (though in fact greatly different, with one borrowing light from the other).

Imagine you are standing on a large, completely flat desert, which streches as far as the eye can see.

On it are two parallel railroad tracks, set several kilometers/miles apart, but eventually appearing to converge near the horizon.

On the left track is a big cube on wheels. The cube is as big as a two-story house.

On the right track is an amazingly humongous cube, as big on all sides as the Matterhorn, also on wheels.

If both cubes could be pushed with great force, and both stopped at a random distance, what are the odds that from your point of view (somewhere between the two tracks) the two cubes would appear, in the distance, to be the same size?

And if randomly pushed and they did appear equal, how many of you would you shrug and say, “…Coincidence.” – ?

P.S. You can sidetrack this by saying, "Yes, but sometimes the eclipse shows them at apparently different sizes, making a ring of light or even a “diamond ring” effect. This is of course true, but it misses the point of the question: What are the odds that they randomly appear so similar in size?

And to clarify: My question is really, does this (a) blow your mind, (b) make you shrug it off and say, “Coincidence,” or ©, other
– and no matter which answer, why?

I certainly don’t just shrug it off. I’ve often wondered whether there is a physical reason for it to be the case, but clearly not enough to research it, since I don’t know the answer.

I suspect it’s just a coincidence, but I would rather there be a better explanation.

A billion years ago the moon appeared bigger than the sun, and a billion years from now it will appear smaller. The coincidence isn’t that they look the same size, but that you’re alive when they do. Of course, beings alive when the sizes look different don’t wonder if the coincidence has any meaning.

I did wonder about that for a while, so I looked it up. All the experts say it’s a coincidence, and of course I can’t see how it could be otherwise, so now I just shrug.

If it’s not a coincidence, what meaning could it possibly have?

Also, this doesn’t really make sense. The moon is often visible during the day, and not always visible at night.

Furthermore, even if the moon is some sort of counterpart-light to the sun, as a light source, it doesn’t look the same shape as the sun, most of the time.

Holy shit! I never noticed that before!

OK, here’s something really mind-blowing. I have a quarter in my pocket. If I held it out at arm’s length, would it fully cover up the Moon?

I have a nickel in my pocket…

I have a cent in my pocket…

Dime?

Aspirin?

Isaac Asimov addressed this in one of his many many science essays, marveling at this strange and highly unlikely coincidence which we pretty much take for granted.

Another Moon fact that blows my mind is that its rotation equals its orbit around the Earth, which is why we only ever see the one side. I’m sure there’s a good explanation for why this occurs, but it still baffled me when I first learned it.

The moon is tidally locked with the Earth because of the massive influence our gravity has on it. It’s not a rare occurrence - most big moons are tidally locked with their planets. In fact Pluto and it’s moon Charon are similar enough in size that they’re both tidally locked with each other.

http://en.wikipedia.org/wiki/Tidal_locking

Isn’t Mercury tidally locked as well?

Really not a coincidence, and not in the least bit rare.

Doesn’t have to be a coincidence or rare to be interesting.

No.
For many years it was so believed, and this made it into a LOT of science fiction about the planet, including works by Asimov and Weinbaum. But:

http://en.wikipedia.org/wiki/Mercury_(planet)

Thank you. Ignorance fought!

The same angular subtense of the sun and the moon seen from earth is a wonderful coincidence, giving us eclipses, but I see no reason to think it is any more than a coincidence.
As pointed out, the coincidence didn’t hold in the past, and won’t in the future. In much the same way, it’s remarkable that the Earth’s rotational axis points towards the star Polaris, allowing us to use it as the North Star. But it didn’t in the past – the relatively recent past, compared to the time scale of the changing of the earth-moon distance. It wasn’t the North Star only a few thousand years ago, in the time of the first Egyptian, Sumerian, Indian, or Chinese civilizations. And they could certainly have used such a useful guide.

Moreover, there’s no corresponding South Polar Star, nor has there ever been, as far as I’m aware. Not one that was significantly bright, anyway.

Well, it could only mean that the two objects acheiving the same apparent size CAUSED (or at least contributed to) the development of “consciousness” (self-awareness? intelligence?) in Homo sapiens.

How one could cause the other, I’ll leave that to freshmen stoners at the University of Chicago and other such institutions.

By the way, OP – your username is one of my favorites!

I’m pretty sure it’s occurred to young-earth creationists.

Thought so.

By 3000 AD, in fact, Polaris will no longer be the North Star. Many thousands of years after that, it will be again.