The highly anticipated total solar eclipse on Aug. 21 may be spectacular, but it won't last long.

The duration of totality, as experienced by observers on the ground, tops out at a few precious minutes for all total solar eclipses, and some don't even last that long.

This brevity is rooted in the movement of the dark shadow of the moon known as the umbra, for when you are inside the umbra, the entire solar disk of the sun appears to be covered by the dark disk of the moon. (Don't worry; we'll get to details about the Aug. 21 event. But first, some eclipse basics.) [Where to See the 2017 Total Solar Eclipse, State by State]

The umbral shadow is projected out into space by the moon and is shaped like a long, tapering cone. That shadow cone is about 235,000 miles (378,000 kilometers) long. But the moon's average distance from Earth is about 239,000 miles (385,000 km).

So, in order for the umbra to touch the Earth, it must be closer than the average Earth-moon distance. But even when the moon is at the closest point to Earth in its elliptical orbit, the umbra is but a dark "dot" measuring no more than 170 miles (274 km) across. And many times, the shadow width is considerably smaller than that. During the Aug. 21 total solar eclipse, for example, the shadow will average only about 68 miles (109 km) wide.

The other thing to take under consideration is the speed of the shadow. During its orbit around Earth from west to east, the moon moves at an average speed of 2,288 mph (3,683 km/h). Because the natural satellite is traveling in an elliptical orbit, the moon's speed is not constant; the object moves faster when it's closer to Earth.

The moon's shadow moves at the same speed as the moon itself. So one might think that during a solar eclipse, the speed of the moon's umbra on the Earth would average close to 2,300 mph (3,700 km/h).

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Total Solar Eclipse Summer August 21st 2017 T-Shirt

Total Solar Eclipse August 21st 2017 T-Shirt

Mandela Effect - T-Shirt

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