A solar eclipse happens when the moon’s shadow falls somewhere on the surface of Earth.
And a lunar eclipse is the opposite -- when the earth’s shadow falls across the moon.
Total eclipses occur within the darkest part of the shadow -- the umbra. In the lighter
part of the shadow, the penumbra, we get partial eclipses.
So that’s the short explanation. But you might wonder why we don’t get eclipses every
month, given that the moon orbits earth every 29 days or so.
The first reason is that the moon’s orbit path is not in line with earth's orbit around the sun --
it’s tilted 5 degrees. That doesn’t seem like much but keep in
mind that the model we’re showing to you is way off. If the earth and moon are this
size, the distance between them should be around 10 feet.
At this distance, 5 degrees is enough to keep the moon’s shadow off of Earth and the earth's
shadow off the moon most of the time. So why do we ever get eclipses? Because there
are two points where the moon’s orbit crosses the sun’s plane, called nodes. And as the
earth moves along its orbit, those points line up with the sun about twice a year.
If the moon is between the sun and earth at that time, we get a solar eclipse. If it’s
behind earth at that time, we get a lunar eclipse.
In a total solar eclipse, the moon completely covers the sun from the vantage point of some
place on earth. This is possible because by coincidence, the sun and the moon appear to
be the same size in our sky. While the sun is 400 times bigger than the
moon, it’s also 400 times farther away. The moon has an elliptical orbit though, so
its size varies about 12% throughout a month. At its closest distance, we can get total
solar eclipses, but when it is farthest away, we get annular solar eclipses, where a ring
of sunlight remains around the moon. In the far future, earth will only get annular
solar eclipses. We know that because Buzz Aldrin and Neil Armstrong left mirrors on
the moon in 1969. Astronomers bounce lasers off those mirrors to measure the moon’s
distance. And that’s how they found out that the moon is moving away from us by
about 1.5 inches every year. So at some point in the future -- we're talking like 600 million
years or so, whatever creatures live here will witness Earth’s very last total solar eclipse.
The totality lasts no longer than 7.5 minutes and although Earth gets an eclipse like this
every 18 months on average, each one is only viewable by less than half a percent of Earth’s
surface. Eclipse chasers will travel all over the world to put themselves in the path of that shadow.
You’re much more likely to see a total lunar
eclipse in your lifetime. They can last an hour and 40 minutes and they’re viewable
for anyone on the night side of Earth. During a total lunar eclipse, the moon often
turns red. If we didn’t have an atmosphere, the moon would go completely dark, but the gas
molecules in our air scatter the shorter bluer wavelengths of light, while the longer, redder
wavelengths pass through to the moon.
Or to put it another way, during a total lunar
eclipse, the sun projects all of the world’s sunsets and all of the world’s sunrises
onto the moon at the same time.