Definition
A total solar eclipse is an astronomical event in which the Moon passes exactly between the Sun and the Earth, in a straight line, and blocks the entire Sun. With the sunlight fully blocked, the daytime sky goes dark as night and the stars come out.
How can something so small cover something so huge?
Hold your thumb right up to your eye and you can hide a giant mountain far away. How big something looks depends on how far away it is, not just on its real size.
In reality, the Sun is about 400 times wider than the Moon. But here's the amazing part: the Sun is also about 400 times farther from Earth than the Moon is.
That's why the Sun and the Moon have almost the same apparent size in our sky. When the Moon is relatively close to Earth, it can cover the Sun completely, with no gaps.
The Moon's orbit is an oval, though, so its distance from Earth changes. If an eclipse happens when the Moon is far away, it looks a little smaller than the Sun, and a bright ring of the Sun's edge stays visible. That's called an annular eclipse, and these happen about as often as total eclipses.
What happens in the split second day turns into night
The moment the Moon swallows the Sun, bright daylight quickly fades into eerie dusk. A sunset-like glow spreads around the entire 360-degree horizon. The temperature drops by several degrees Celsius, and a cool breeze starts to blow.
Startled by the sudden dark, birds think night has come and head back to their nests. Daytime insects go quiet, and night singers like crickets sometimes start chirping. Bright planets and stars that you can't normally see in the day twinkle overhead, creating an otherworldly scene.
At this moment, the white corona, the Sun's outermost layer of atmosphere, which is normally too bright to see, appears against the dark sky like a soft, glowing crown.
Just before and after the Moon fully covers the Sun, a single bright beam of sunlight slips through the valleys on the Moon's rugged surface. It sparkles like a gem on a ring, a sight known as the diamond ring effect.
A little more precisely
The Moon circles the Earth about once a month, passing between the Sun and the Earth. So you might expect a total solar eclipse every month. In reality, they're very rare.
The reason is that the Moon's orbit is tilted about 5 degrees relative to the plane of Earth's orbit around the Sun.
In most months, the Moon's shadow passes above or below the Earth, off into empty space. A solar eclipse can only happen when the new moon falls near one of the two points where the orbits cross, so the shadow actually reaches Earth. And it's a total eclipse only if the Moon is close enough to cover the whole Sun.
On top of that, the dark inner shadow (umbra) that touches Earth is usually only about 100 to 200 km (60 to 120 miles) wide. Somewhere on Earth, a total eclipse happens about once every 1 to 2 years. But for any one particular spot, it happens only about once every 375 years on average (according to a calculation by astronomer Jean Meeus). That's how rare it is.
Common misconceptions
During a total solar eclipse, the Sun gives off special, dangerous rays.
The light and energy the Sun gives off during a total eclipse are exactly the same as usual. The danger comes from looking straight at the Sun during the partial phases, when part of the Sun is still showing, or right when totality ends and the light returns. That same intense sunlight can damage your retina.
Where you meet it
The 2017 "Great American Eclipse" is a classic example. Its path of totality crossed the U.S. from the West Coast to the East Coast, and millions of people traveled to the path to watch it.
During the 1919 total solar eclipse, astronomers measured how much the light from stars near the Sun was bent by the Sun's gravity. The results matched the prediction of Einstein's general theory of relativity, and the observation helped the theory become widely accepted.
In one sentence
The Sun and Moon happen to have just the right size and distance ratio, so the Moon can fully cover the Sun and reveal the glowing corona.