Why isn’t there a lunar eclipse every month?

A full moon happens when the Sun, Earth and Moon line up just right and the full illuminated disk of the lunar surface is visible from Earth.  But the Sun must be behind the Earth in order to fully illuminate the moon, why isn’t the Earth casting a shadow?  It is but not necessarily on the moon.  The moon’s orbit is slightly tilted so the Earth’s shadow usually is cast above or below the moon. About every 6 months and 6 days, the orbits line up in a way that some or all of the Earths shadow is cast on the lunar surface. Each of these eclipses cycle through 1-4 total eclipses, followed by 1 or more partial or penumbral eclipses.  That’s just the way the cycle works based on the orbits of the Moon and Earth and where they line up with the Sun.

The next lunar eclipse will occur on June 15, will be total, and will be visible from South America to Australia.  The next lunar eclipse with any visibility from North America is a penumberal eclipse on May 25, 2013. The next total eclipse ith any visibility from North America will occur on October 8, 2014. Totality will be visible from the western United States and Canada. North American viewers further east will miss totality as the moon sets and the sun rises.


June 15, 2011