Columbus knew that lunar eclipses don't go on forever. Astronomers say none can last more than 3 hours and 40 minutes. Totality cannot run more than 1 hour and 40 minutes.

The period of totality for the May 15-16 eclipse will be 53 minutes.

Another total lunar eclipse occurs later this year, on Nov. 9; totality will last just 24 minutes. An eclipse on May 4, 2004 will have a period of totality of 1 hour and 16 minutes.

Why the difference? Longer stretches of totality mean the Moon is traveling smack dab through the middle of Earth's shadow. Shorter total eclipses occur when the Moon is nearer the top or the bottom of the shadow.

Next: Eclipse speed limits