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In astronomy, an eclipse happens when one object passes in front of another, obscuring it. The general term for this is event is called a transit, but we use the term eclipse when talking about how the Moon can pass in front of the Sun, or the Earth can pass in front of the Moon.
When your vantage point on Earth, the Moon and the Sun appear to be approximately the same size. Since the Moon is much closer, it can block the light from the Sun, casting a shadow on the surface of the Earth. If you’re standing within that shadow, the Sun and the Moon appear to line up perfectly, so that the Moon is completely darkened.
At that point, when the Sun is fully blocked by the Moon, astronomers call that a total eclipse – the point of maximum eclipse. After that, the Moon continues to move past the Sun, obscuring smaller and smaller amounts.
During an eclipse of the Sun, on a thin path on the surface of the Earth is actually able to experience a total eclipse – this is called the path of totality. People on either side of that path see a partial eclipse, where the Sun is only partly obscured by the Moon at the maximum point of eclipse.
A total eclipse of the Moon is a different story. In this situation, the entire Moon passes into the Earth’s shadow, darkening it fully. Observers across all of planet Earth see this darkening of the Moon – it doesn’t matter where you live. We see a partial lunar eclipse when the shadow of the Earth doesn’t fully cover the Moon, so only part of the Moon is darkened.
Since the Moon orbits the Earth, you would expect to see an eclipse of the Sun once a lunar month, and an eclipse of the Moon once a lunar month. But the Moon’s orbit isn’t lined up with the Sun, it’s tilted by a few degrees. So the three objects really only line up a few times each year. It’s possible for a total of 7 solar and lunar eclipses every year, but that only happens a few times every century.
So if you ever want to see a total eclipse, you need to find out where on Earth the next path of totality is going to be, and then you need to book a vacation to go there. Get to the right spot at the right time, and you should be see the view of a lifetime.
We have written many articles about the eclipse for Universe Today. Here’s an article about the Lunar Eclipse, and here’s an article about the Annular Eclipse.
We’ve also recorded related episodes of Astronomy Cast about Eclipses. Listen here, Episode 160: Eclipses.