What Causes Lunar Eclipses?

If you pay attention to the phases of the moon and to the shapes and motions during an eclipse, it is easy to see for yourself.

For a few days every month, the nighttime is illuminated by the glow of a helpful full moon. It rises in the east just as the sun sets in the west, it shines down on the world from high in the sky throughout the night, and it sets in the west just as the sun rises in the east again. But on special and rare nights, this helpful ruler of the night is overshadowed or eclipsed by something that steals its light, and turns it a dark red or orange color. It is as if the gods have become angry and replaced the normal helpful moonshine with a dull red glare over the world.

If these events are due to the whims of the gods, then we probably shouldn't be able to predict them. But people have been able to predict eclipses since the time of the Ancient Greeks, roughly 2500 years ago. The Ancient Greeks were not only able to say ahead of time when some eclipses would happen, but they were also able to explain in a non-mythological way for the first time in history why eclipses happen. They gave us a scientific explanation. And they only needed clues than anybody can see with their eyes. You have the same clues. Can you figure it out?

The clues we need come from paying attention to the phases of the moon, and to the exact shapes and motions of things when eclipses happen. Children in science class should gather and study these clues in detail. For our purposes here, I'll summarize.

Gathering the Clues

If you ever notice a sunset decorated by a beautiful sliver of a crescent moon, point to the moon with one arm, and point to the sun just below the horizon with the other arm. How far apart are your arms? Maybe one-third of a right angle? What about tomorrow? Will the moon be in the same place tomorrow evening? If you watch what happens to the moon every evening for the next week or two, you will see it gradually creep towards the east each night. It will get farther and farther away from the sunset, and it will grow thicker and brighter each day. After a week and a half or two weeks (depending on when you started), it will finally be a full moon, rising in the east just as the sun sets in the west. It is as far from the sun as it can be in the sky, and it is full. Now, if you get up at sunrise and look for the moon in the morning sky, you will see the same thing happen in reverse. The moon will approach the sunrise from the far side of the sky, growing thinner and fainter as it goes from west to east towards the sun. Finally, about two weeks after the full moon, the moon will disappear somewhere into the glare surrounding the sun, shortly to be reborn as a “new moon.”

The moon alternates every two weeks between a full moon and a new moon. It does this by steadily circling around us once a month, from west to east in the sky. Each time it passes the sun it becomes a new moon, and each time it passes through the far side of the sky, away from the sun, it becomes a full moon. (Whenever we watch the moon change positions in the sky night after night, we are seeing this circling motion. Sometimes the moon will pass by a planet or a bright star, and we can see this motion in space almost in real time. We can see the moon move from one side of the star to the other side over the course of an hour or two. But the motion is always from west to east, and it is always fairly steady, at the rate of one full circle each month, or about a quarter-turn per week.)

Here is your first and most important clue to the cause of lunar eclipses: they only happen to a full moon, when the moon is on the far side of the sky from the sun. We never see something dark move temporarily across a quarter moon or a gibbous moon, only across a full moon. Lunar eclipses only happen at the time of a full moon. And when the moon and sun are on opposite sides of us in the sky, doesn't this mean that they must be on opposite sides of us in space, as well?

Here is your second clue: during an eclipse, we can still see the moon (usually). It is not totally blocked from view, it is merely darker, as if a cloud or a shadow is passing over it. If a large dark object in space were passing between us and the moon, we wouldn't be able to see the moon at all.

Those two clues are probably enough to tell you what is causing the moon to darken. But here is another clue: Which way and how fast does the shadow move? The shadow always starts on the eastern side of the moon in the sky, and passes across the moon from the east towards the west, as if the shadow is moving across the sky from east to west, just like the moon, but it is moving a little faster than the moon and it overtakes the moon as they both race across the sky from east to west. But doesn't the sun move from east to west, a little faster than the moon? If you could measure precisely, you would find that the shadow is always exactly on the opposite side of us from the sun. If you could see the sun below the horizon, and point to it with one arm, and point to the shadow with the other arm, they would make a straight line. And the full moon falls backwards through this shadow, from the shadow's westward side towards the east, in just the same way that the new moon passed the sun from west to east two weeks earlier, and will again two weeks later.

So what causes lunar eclipses? Let's list the clues:

Now, have you figured out what that mysterious dark circle is? If it is a shadow, what is casting the shadow? How do you know?

The Cause of Lunar Eclipses

If the moon is over there, with a shadow landing on it, and the sun is over there, on the opposite side of us, what's in the middle that could be casting the shadow? We are! If we only see a shadow on the moon when the moon is on the opposite side of us from the sun, then it makes perfect sense to think that it's our shadow. We see the shadow of our own home crossing across the moon when we see a lunar eclipse.

But wait a minute. If the Earth has a shadow, if it can create a “hole” in the sunlight in outer space, then Earth's shadow must always be there. Earth's shadow must be up there in every night sky, pointing out there into space away from the sun. We just never get to see it because there is nothing for it to land on. If we could see it in the night sky, it would do exactly what the sun does during the day, but on exactly the opposite schedule. It would rise in the east just as the sun sets in the west, it would be high in the sky in the middle of the night, and it would set in the west just as the sun rises in the east. It would rule the nighttime sky, exactly as the sun rules the daytime sky. But isn't that exactly what the full moon does? If the moon goes on the opposite side of us from the sun every month, why doesn't it pass through the shadow every month? Why do we usually have “normal” full moons, and we only rarely have lunar eclipses?

And another thing. If the moon is passing through our shadow, why does it glow red? A good scientist doesn't immediately assume that his first explanation must be the correct one. There are always details to be ironed out. Before we are so sure we are right about the cause of lunar eclipses, we should be able to explain why we don't have them every month, and why our shadow appears to be colored. Are there any more questions we should answer? Can you think of any puzzles we haven't solved yet? Do you have any more questions about lunar eclipses?

Things We Can Learn From Lunar Eclipses

If the Earth's shadow is indeed the true cause of lunar eclipses, then lunar eclipses give us a special opportunity. Most of the time, our shadow goes outward into space like a tail, with nothing to land on, so we never get to see it. But during a lunar eclipse, the moon gives us the chance to see the shadow for real. There is only one thing in outer space close enough for our shadow to land on it (and big enough for us to see the shape of it) and that is the moon. Eclipses of the moon give us the rare opportunity to see our own shadow, and to study it.

What shape is the shadow? It is too big to fit entirely on the moon, so we only get to see pieces of it. But we can see different pieces of it at different times, and the edge always has the same shape. What shape? The edges are always curved, as if they are portions of a circle. During a lunar eclipse, we get to see the shape of our own world by seeing the shape of our shadow.

How big is the shadow? It seems to be a few times larger than the moon. Shadows aren't always the same size as the things that cast them. But unless there is a huge difference, we can say that since the Earth's shadow is a few times larger than the moon, the Earth itself is probably a few times larger than the moon as well. During a lunar eclipse, we get to see with our own eyes that the Earth is round, and a few times larger than the moon.