ASK TOM: Total lunar eclipse always on a full moon?

Dear Tom,

At the precise moment of a full moon, wouldn’t the sun be directly behind the Earth and the moon directly in front of the Earth, in exactly opposite directions? If so, the Earth’s shadow would eclipse the moon, so why isn’t there a total lunar eclipse every full moon?

– Ray Norton, Gurnee


Dear Ray,

Dan Joyce, astronomer at Triton College’s Cernan Earth and Space Center, confirmed that at the time of lunar fullness, the sun and moon are just about opposite each other, but because the moon’s orbit is tilted 5.2 degrees to the plane of the Earth’s orbit around the sun (the ecliptic), the moon rarely lines up with the Earth’s shadow. When it does, a total lunar eclipse does occur, and it will always be on the full moon. The next two total lunar eclipses will be April 15, 2014, and Oct. 8, 2014.