NOVA

A total solar eclipse will cross the United States and people are really excited. That's because many Americans have never seen a total solar eclipse. But why is that? After all, you only need three things to make a solar eclipse, the Sun, the Earth, and the Moon in between.

The Moon passes between the Earth and the Sun every 27 days. So why don't we get an eclipse every month? Well, solar eclipses only happen when the Moon's shadow falls on Earth. And most months that shadow misses us.

It's all due to the science of orbital mechanics. See the Earth revolves around the Sun in this plane. The Moon revolves around the Earth in this plane, which is five degrees tilted. The Moon's shadow only lands on us when the Moon and the Earth's planes line up, making the Sun, Earth, and Moon all align.

Eclipses happen every year, but you still might not get a total solar eclipse. Most commonly, a partial solar eclipse occurs where the Moon covers only part of the Sun. And about one third of solar eclipses are annular. They happen when the Moon's orbit takes it slightly farther away from the Earth.

The outer edges of the Sun are exposed producing a glowing ring around the Moon. Total solar eclipses only happened when the Moon is close enough to the Earth to block out the entire Sun. That means around every 18 months there's a total solar eclipse somewhere on Earth. Luckily for us a total solar eclipse is happening across the United States from Oregon to South Carolina, along this path.

It's the first one in 99 years to go all the way across America. So hope for clear skies, and don't forget your ISO approved, eclipse glasses.