From the October 2010 issue

What causes the variety of totality length from one solar eclipse to the next? Why does the length of totality vary along the path of an eclipse?

John Carlson, Colon, Michigan
By | Published: October 20, 2010
View of a solar eclipse. 21 June 2001
Aadil Desai

First, the Moon’s distance from Earth varies over its orbit. Second, Earth’s elliptical orbit causes the Sun’s distance and apparent diameter to vary by about 3 percent from perihelion (closest approach) to aphelion (farthest approach).  Near the sunrise and sunset terminators, the Moon’s shadow hits Earth’s surface at a steep angle. This geometry causes the shadow to move quickly with respect to the surface, which shortens totality’s duration.