Everything you need to know about the 2017 total solar eclipse

August 21 is a big day in the American celestial calendar. For the first time in 23 years, the entire continental United States will have the opportunity to see the sun disappear for a few minutes — in some places, completely — behind the moon. It’s called a solar eclipse, and it’s something that’s pretty rare around here.

What makes the August eclipse all the more special is it’s a total eclipse, which blocks out the sun more completely than a more common annular eclipse. Such a thing hasn’t been viewable from American soil in 38 years! So why should you be excited about this event? Read on and we’ll explain.

Why do solar eclipses happen?

Solar eclipses occur when the moon passes in front of the Sun, blocking out its light. While the moon is only a minuscule fraction of the size of the Sun, the Sun is 94 million miles away from us, and this allows the moon to block out the Sun’s rays across a small fraction of the Earth’s surface.

You might be wondering why solar eclipses are so rare. This is due to two factors. First, the moon must be in the “new moon” phase, which means it is between the Earth and the Sun. Because of this, we see the “dark side of the moon,” and the moon appears invisible. But with new moon phases happening every 29.5 days or so, why aren’t eclipses happening just as frequently? You can thank the tilt of the moon’s orbit with respect to Earth for that.

There’s about a five degree difference at play, which also explains why the moon’s position in the sky seems to change daily (although this also has to do with Earth’s seasonal orbital wobbles, too, which explains why we have seasons). This means that during most new moons, the Moon’s shadow either falls above or below the Earth’s surface.

But generally, two times a year these two orbits line up just right, and an eclipse occurs. If you’re lucky enough to see it, it’s a beautiful sight.