How big is the Moon?

Speed "how big is the moon?"
Mom "I don't know but I'll bet if you asked Daddy he would know exactly right off the top of his head."

I did, actually, though not perfectly exactly! The diameter of the Moon is a little under 3500 kilometers, or about one quarter of the diameter of the Earth! (Exactly 3476 km, but I had to look that up! The Earth is 12756 km wide at the Equator, 12715 km from Pole to Pole.)

That's really huge for a moon in comparison to its planet, none of the other planets in our Solar system are even close to being that big compared to their planets. Some astronomers even say that instead of calling it a moon we should call it a "companion planet" but the other astronomers say they're already in enough trouble for changing Pluto from being called a planet to being called a dwarf planet! :D

(Cool Moon image courtesy of Flickr user Hernan Hernandez.)

Because the Moon is so big and so close, its gravity pulls pretty strongly on the Earth. Everything on the Moon side of the Earth really wants to go toward the Moon, but the only part that's really free to move is the water, so that makes all the oceans bulge toward the moon. That's where the tides come from - well, most of them, the Sun also produces some water bulges, but they're not as big. That's because even though the Sun is much much bigger than the Moon, it's a lot further away. Good thing too, it's already hot enough here in the Summer! If the Sun was as close as the Moon we'd be completely melted.

Speaking of the weather, having such a big Moon turns out to be very helpful there too. You know how the Earth spins on its axis, giving us night and day, but did you ever wonder what would happen if it also rolled over top to bottom? It would make the days and seasons go totally crazy!

Remember, the seasons are caused by the tilt of the Earth with respect to the Sun; in our summer the half of the Earth we live on is tilted more toward the Sun, so we get more sunlight; in Australia the seasons are the opposite way around. It's not because the Sun is closer - in fact, the Earth is closest to the Sun in January and farthest away in July, which is the opposite of what you would expect. An Australian kid might have guessed right but for the wrong reasons.

You can see, though, that if the angle of the Earth's axis was always changing because it was tumbling through its axis, the seasons would not happen at the same time every year! Summer might creep around into January and February, then swing back again - you would probably need a computer just to keep track of what months were going to be cold this year. Imagine being a farmer and trying to decide when you should plant your crop! Even the length of the days would change much more than they do now, and not in a regular pattern like we have now with short Winter days and long Summer ones. Daylight Savings Time would change all the time (or we could just get rid of it, which I think is a good idea anyway!).

Luckily for us, because the Moon is constantly pulling on the side of the Earth, it keeps it from wobbling off its axis. The Earth and Moon together act more like a Frisbee than a ball, if you like - or like a pair of bolos, even better. One interesting side effect of the two always being pulled together is that the Moon is tidally locked - it always shows the same side to the Earth, rotating on its own axis in exactly the same time it takes to orbit the Earth. That's why we never see the other side from here on Earth.

Some astronomers are trying to get the money to build a big telescope on the dark side of the Moon for that reason, so that they can get away from all the lights and radio pollution made by people on the Earth. I think that's a really neat idea and I hope they find the money, don't you?

Finally, there's one more reason to be glad we have such a big Moon - it just looks COOL!!! :)

A More Interesting Answer

Speed asked his mother the following:
> Speed "why are there shadows?"
> Mom "because you block the light"
> Speed "a more interesting answer?"

At that point she punted to me. Science to the rescue!

Ah, but shadows aren't always because you're blocking light from a reflecting surface, although that's the simplest way of making them!

You could also be refracting the light away with a lens, for example, because light gets bent through an angle anytime it goes from something less dense (like air) to more dense (like glass or water), or the other way around. When you make a bright spot with a magnifying glass there's always a shadow all around it - try it and see. That's also why you see moving shadows on the bottom of swimming pools, or over hot surfaces where streams of hot air are mixing with cooler air (hot air is less dense than cool air). The light's being deflected away from one spot and piling up in another, making shadows and bright spots - anti-shadows!

If you shine a light past something really, really massive like a planet or a black hole, the gravity can also bend the light, so as well as making a shadow exactly behind the massive object (by blocking the light), you're also making a different kind of shadow all around it by bending the light's path - and an anti-shadow directly behind it! Like this:

Black hole lensing (Picture courtesy of the Wikipedia entry on gravitational lensing.)










Also, because light acts as a wave, you can even make shadows by putting two light beams together out of phase! To do this in a way that you can see you need to use light of exactly the same color and with all the light traveling in the same direction, which is called coherent light - that's the kind of light that comes out of a laser. If the light wave from one laser beam is getting highest just as the other one is getting lowest, you get a shadow where they cancel out, even though nothing is blocking the light!
The easiest way to get two beams of coherent light is to split the beam from one laser using slits in a screen. Here's a simulation.

So there are lots of interesting ways shadows can be made, besides blocking the light!