Star Talk - Lunar Legends and Astronomy of the Moon

Tonight we focus on the Moon. When I talk about the Moon, the problem isn't finding things to say…it's choosing what I have to leave out. There are so many interesting things about our closest neighbor in space that I could talk til moonset.

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.

With the three classical theories discredited, one astronomer said "The only logical conclusion left is that the Moon simply does not exist." Then along came the outrageous theory that a Mars-sized asteroid collided with the Earth, knocking off a huge chunk and a large cloud of debris from both the Earth and the asteroid. The rest of the asteroid, nicknamed "Orpheus", merged with the Earth. Surprisingly enough, this is the current favorite. It's by no means a slam-dunk certainty, but, so far, it fits the physical evidence better than any of the others.

Meteors constantly bombard the Moon, but with no atmosphere to burn them up, they all impact the surface, leaving it pocked with craters of all sizes. With no wind or rain to disturb them, they remain as they are until a nearby impact disturbs or obliterates them. The footprints left by our astronauts are still there, and likely will be for billions of years. Recent exploration shows the Moon MIGHT have a limited amount of water, locked up as ice in craters near the poles. If true, this would be an incredible boost to the idea of establishing a permanent manned base on the Moon. Looking at our side of the Moon, we see large smooth darker patches. The ancients thought these were water and named them "seas". We now know they are flat lava plains. As the Moon cooled and solidified, it was stressed dramatically by the Earth's tidal force, causing cracks and fissures. Rather then spewing up from volcanoes, molten lava seeped up to the surface thru the cracks, then solidified into huge sheets. You can clearly see the difference in terrain thru a telescope.

Have you noticed the full moon looks bigger on the horizon than it does high overhead? Well, it isn't. This effect is purely an optical illusion caused by comparing the size of the rising moon to earth-based objects on the horizon. If you hold a ruler at arm's length and measure the diameter you will find it the same whether low down or high in the sky.

If you ask any cop, fireman or maternity nurse; they will tell you they are busier when the Moon is full. But the actual statistics show no difference. Oh, they sincerely believe the moon is a factor, but it's all psychological. They remember the busy nights they "blamed" on the moon, and tend to not note the busy nights at other times.

Or, how about the "Harvest Moon" in September? This one actually has some basis. Because our axis is tipped relative to the moon's orbit, in September the Moon is displaced more horizontally each night rather than vertically. That is, it rises at about the same time, but further south for the few days around the full moon. Conversely, at the spring equinox in March, the moon rises much later each night, but follows almost the same path. So, the Harvest moon really does appear to "hang" in the sky to assist farmers in gathering their crops. And THAT's not just their imagination!