From the May 2012 issue

What determines the length of totality during a solar eclipse?

Richard Vaun, Holley, New York
By | Published: May 29, 2012 | Last updated on May 18, 2023
The Moon’s shadow sweeps across Earth’s surface during the August 11, 1999, solar eclipse. The length of totality depends on where on the planet’s surface the shadow falls, how far away the Moon is from Earth, and how far Earth is from the Sun at that time. CNES/Mir 27 Crew
Totality occurs within the Moon’s shadow, as cast by the Sun, falling onto Earth. So, because the Moon moves at different speeds in its orbit around Earth and the planet moves at different speeds in its orbit around the Sun, the shadow sweeps through space at differing rates.

Further, we live on a rotating planet, so how fast Earth’s surface at a particular spot moves through space factors into totality’s length. At the equator, Earth completes one spin about its circumference of 24,901 miles (40,074 kilometers) in 24 hours, making the speed roughly 1,000 mph (1,600 km/h). Near the poles, a point on Earth’s surface moves around the planet’s spin axis much more slowly.

So, the longest totalities occur when the Moon’s shadow crosses Earth near the equator, the Moon is closest to our planet, and Earth is farthest from the Sun. The theoretical maximum is about 7½ minutes, but that won’t happen until the next century, and now the longest totalities are around 6½ minutes.

In 1973, the speed of the shadow across Earth slowed enough that a supersonic Concorde could keep up with it for 74 minutes, although the eclipse was high overhead and holes (with windows) had to be cut into the airplane. — Jay M. Pasachoff, Hopkins Observatory of Williams College, Williamstown, Massachusetts