Why Eclipses Don't Occur Monthly

  • By Saad Amer
  • Posted 08.21.17
  • NOVA

Why don't solar eclipses happen every month? After all, you only need three things to make a solar eclipse: the Sun, the Earth and the Moon.

Running Time: 01:47


Saad Amer: A total solar eclipse will cross the US, 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 Âoon passes between the Earth and the Sun once 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 orbits the Earth in this plane, which is 5 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.

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 happen when the moon is close enough to 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 solar eclipse is happening on August 21st 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!



Producer, Host
Saad Amer
Darkside Studios
­Alex Cherney, Footage Farm, Lee Lusted, NASA, NHK

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