NASA: The moon's role in a solar eclipse

(NASA) -- While the sun is the main focus of a solar eclipse, our moon plays the most crucial role in creating this unique event.

This video explains what happens during a total solar eclipse and a partial eclipse and how often they both occur.

The video also explains how a solar eclipse differs from a lunar eclipse.

In addition, the video examines how the two parts of the moon’s shadow, the umbra and penumbra, affect how we see an eclipse on the Earth, and illustrates the surprising true shape of the umbra.

The video concludes by highlighting how data from NASA’s Lunar Reconnaissance Orbiter has helped us better map a solar eclipse’s path of totality. Visualizations included in this piece showcase the August 21, 2017 total solar eclipse happening in the United States.  

Video narration: [VIDEO COURTESY NASA]

A solar eclipse occurs when the moon moves between the Earth and the sun, blocks the sun’s light, and casts a shadow on the Earth.

When the moon completely covers the bright disk of the sun, that’s a total solar eclipse, and it happens roughly every year and a half somewhere on Earth.

A partial solar eclipse is more common, happening at least twice a year.

A lunar eclipse, on the other hand, is where the moon moves behind the Earth, so it’s now the Earth blocking the sun’s light on the moon, creating a shadow on it with a red tint.

The easy way to remember the difference is to remember what gets darker. With a solar eclipse, the sun gets darker, and during a lunar eclipse, the moon does.

A solar eclipse is a rare event that not many get to see, because the moon’s shadow is relatively small. This limits the locations on Earth that get to see it.

You have to be on the sunny side of the planet, and you have to be in the path of the moon’s shadow.

So, if you find your area in the path of totality one year, you’ve hit the jackpot, because on average, that same spot on Earth will only get to see a solar eclipse every three hundred seventy-five years!

During a solar eclipse, the moon casts a shadow with two parts: the umbra and penumbra.

The moon’s umbra is the part of the moon’s shadow where the entire sun is blocked by the moon.

In space, it’s a cone extending some 400,000 kilometers behind the moon. It’s when the small end of this cone hits the Earth that we experience a total solar eclipse.

Most eclipse maps depicting the path will show you a circle representing the shadow of the moon, but in actuality, the true shape of the umbra is more like an irregular polygon with slightly curved edges. This is because the moon isn’t a perfect circle – it has mountains and valleys on its surface, which all affect the passing sunlight and subsequent shape of its shadow.

And scientists now have a greater understanding of the shape of the moon’s surface, thanks to NASA’s Lunar Reconnaissance Orbiter.

The unprecedented level of detail from the topographic maps, photos, and other datasets has allowed us to more precisely pinpoint the regions on Earth falling within the path of totality of a solar eclipse.

So if you get the chance to witness a solar eclipse, always remember that our little moon plays a role that's quite large.