Imagine that you are at home watching TV. You’re watching your favorite show when your little brother walks by and stands in front of the TV. Now, you can’t see the TV anymore, but he eventually keeps walking. A few minutes later, he’s back in the same spot, blocking your view. You notice that after he does this a few times that he’s actually walking around the couch at a certain speed. So you look at the clock, and you notice that he walks in front of the TV once every three minutes. You tell your friend that you predict that there will be another “Brother-TV eclipse” (meaning your brother will block the TV) in three minutes. Your friend is astonished when your prediction is correct! This is similar to how astronomers predict solar and lunar eclipses, except that their calculations are a little more complicated.

I’ll use a solar eclipse as an example. With a solar eclipse, the moon blocks the sun. This can only occur because the sun is about 400 times bigger than the moon but about 400 times farther away! Astronomers can predict when this will happen because after many years of observation, they were able to determine how fast the moon is moving and orbit and how fast the sun is moving in orbit. (You can see how this calculation might be a little harder than your brother calculation since your brother was the only thing that was moving in that case.) To make it a little more difficult, you have to be in the right spot with the right angle to see the solar eclipse.

There a lot of other things to consider when trying to calculate eclipses. For example, the earth and moon and sun are all feeling the pull of gravity from each other and from other planets, which can slightly change their orbit and motion. Astronomers also have to remember that these rotating bodies are not all perfect spheres; the earth, for example, is sort of stretched out. Now, after considering all of these different things, astronomers punch the numbers into a computer program and can use their math to predict the next eclipse!

If you want to do it yourself, some astronomers came up with this equation, which can work all the way until the year 2050:

ΔT is just the year in which the next solar eclipse would occur.

The “t” is based on the year and month you are in. First, you have to figure out “y” below:

Then, you can find “t”, which uses the number that got for “y”, and then you can plug that in.

If you solve and get a value for ΔT (which relates to the earth's rotation), you still aren't done yet! You would have to use this value and plug it into some special software that includes the position, speed, acceleration, and shape of the earth, moon, and sun. Then, you'd have a good guess of the next solar eclipse. ​