During a solar eclipse, the Moon casts a dark shadow, called the umbra, onto the Earth’s surface, as well as a lighter shadow called the penumbra. The umbra can be about 170 miles (274 kilometers) in diameter, darkening the sky and cooling the air just as if the Sun were setting. Temperatures have been known to drop many degrees during a solar eclipse, especially during a total eclipse. For example, in Baja, California, the temperature dropped from 90°F to 74°F (32°C to 23°C) on July 11, 1991, because of an eclipse. The effect, however, is short lived, as total eclipses rarely last more than seven minutes, and specific locations on the planet only experience them once every four centuries or so.