Image by Proggie
Full moon through telescope
Our Moon is huge in proportion to the size of the Earth. No other moon in the Solar System is proportionately as large as its parent planet. (That wasn't always the case until Pluto was downgraded!) It's about a quarter the size of the Earth and about a quarter million miles away. The surface area is roughly 10 billion acres, or about the size of North and South America combined.
The Moon appears white to us, but only because the sky around it is so dark. Actually, the Moon has the color and reflectivity of asphalt. It is slightly egg-shaped, with its center of mass somewhat displaced from its geographical center. Because of that, Earth's gravity locks it into position so the same side always faces the Earth. It also causes the Moon to rock slightly from side to side - what we call "libration". That, along with its egg shape, allows us to see about 59% of the lunar surface, but not all at the same time.
What we cannot see is often called the "dark side of the Moon". It isn't dark at all. It gets just as much sunlight as our side. It's only dark in the sense that we never see it from Earth. The astronauts have seen it, mapped it, and photographed it. I guess I should say "cosmonauts" because the Russians got there first, and in a naming frenzy, gave all the features they found Russian names. The backside is dominated by more and larger craters, suggesting the Moon has saved Earth from large inbound objects many times. That's one advantage to having a large moon.
The Moon is vitally important to life on Earth. It keeps us spinning on an even keel, ensuring our axis doesn't move very much in relation to our orbit, thus regulating our seasons. In comparison, Mars, which has two tiny moons, gyrates wildly over the course of millions of years, subjecting the poles to unrelenting sunlight or darkness from time to time.
The Moon also drives our tides, which is vital to aquatic life, and in turn everything else up the food chain. People often ask why the tides follow the Moon and not the Sun. They guess, incorrectly, that the Moon's gravity is stronger than the Sun's, because the Moon is closer. If that were the case, the Earth would orbit the Moon and not the Sun. The Sun's gravity is about 3 and a half times greater than the Moon's. It's the difference in the Moon's gravity from one side of the Earth to the other that causes tides. Gravity follows the "inverse square law", meaning it decreases in proportion to the square of the distance. I don't want to turn this into an algebra class, so let's just say the strength falls off dramatically when the distance is small, and tapers off slowly when the objects are farther apart. Since the Sun is much farther away it exerts virtually the same pull on the far side of the Earth as it does the near side. Because the Moon is closer to Earth, its gravity is stronger on the near side of the Earth than the far side. And that large difference is what causes the seas to migrate toward the far and near sides of the Earth. That same tidal force also distorts the solid portion of the Earth - ever so slightly - as it spins within the uneven gravity field.
Where did the Moon come from? Classical astronomy has proposed three theories:
1. That the Earth and Moon both formed at the same time from the same primordial debris cloud. But, if that were true, they would have a similar composition, and they don't. The Earth has a large molten iron core...the Moon's core is very small, if it even has one at all. The Moon is mostly made of less-dense "crust-type" material.
Which brings us to Theory 2: That the moon material was "slung off" by centrifugal force while the Earth was still in a liquid/gaseous state. This would account for the Moon's lesser density. But it also requires a rotational speed of about 2 hours per rotation. Knowing Earth's current rotational speed of 24 hrs/rotation and the time that has elapsed since then, it's impossible the proto-Earth was spinning anywhere near fast enough for that to happen.
The Third theory says the Moon was formed as a mini-planet in its own orbit around the Sun, but was captured by Earth's gravity and assumed orbit around the earth. While that's possible in theory, too many variables would have to align for it to be plausible. It is overwhelmingly more likely the Moon would have either been pulled in for a spiraling impact with Earth, or zoomed on by after having its path altered to a varying degree. Also, if a capture had occurred, the Moon would be slowly drawing closer to Earth, headed for an eventual impact. It isn't. In fact, the Moon is receding at about 1 and a half inches per year.
Together with the 3 classical theories damaged, 1 astronomer said "The sole realistic conclusion left is that the Moon just does not exist." Consequently along came the outrageous principle a Mars-sized
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