Why aren’t there eclipses at every full and new moon?Posted: December 7, 2011
Eathysky.org always has good explanations.
A lunar eclipse happens when the Earth, sun and moon align in space, with Earth in the middle. At such times, Earth’s shadow falls on the full moon, causing a lunar eclipse.
A solar eclipse happens at the opposite phase of the moon – new moon – when the moon passes between the sun and Earth. Why aren’t there eclipses at every full and new moon?
The moon takes about a month to orbit around the Earth. If the moon orbited in the same plane as Earth’s orbit, we would have two eclipses every month. There’d be an eclipse of the moon at every full moon. And, two weeks later, there’d be an eclipse of the sun at new moon for a total of at least 24 eclipses every year.
But the moon’s orbit is inclined to Earth’s orbit by about 5 degrees. That’s why there’s not an eclipse every month.