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| Space on May 09, 2013

Why aren’t there eclipses at every full and new moon?

There’s not an eclipse at every new and full moon, but there are from four to seven eclipses every year. Some are lunar, some are solar, some are total, and some are partial.

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.

Total lunar eclipse for the Americas on night of April 14-15

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 the ecliptic – Earth’s orbital plane – 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. Twice a month the moon intersects the ecliptic – Earth’s orbital plane – at points called nodes. If the moon is going from south to north in its orbit, it’s called an ascending node. If the moon is going form north to south, it’s a descending node. If the full moon or new moon is appreciably close to one of these nodes, then an eclipse is not only possible – but inevitable.

The video below explains an eclipse when the new moon and full moon are aligned with the lunar nodes.

Relative to the moon’s phases, however, the nodes move about 30o westward (clockwise) each month. Hence, the new moon and full moon won’t realign with the nodes again for nearly another six months.

There might be some unfamiliar words in this video, including ecliptic and node. The ecliptic is the plane of Earth’s orbit around the sun. The moon’s oribt is inclined to the plane of the ecliptic. The nodes are the two points every month where the moon’s orbit and the ecliptic intersect.

Even though the moon’s orbit is inclined to that of Earth – and even though there’s not an eclipse with every full and new moon – there are more eclipses than you might think. There are from four to seven eclipses every year. Some are lunar, some are solar, some are total, and some are partial. All are marvelous to behold – a reminder that we live on a planet – a chance to experience falling in line with great worlds in space!

Photo credit: pizzodisevo