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Why no eclipse every full and new moon?

This year, in 2016, there are 12 full moons and 13 new moons, but only four eclipses – two solar and two lunar.

Total lunar eclipse composite image by Fred Espenak

Total lunar eclipse composite image by Fred Espenak

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?

Supermoon total solar eclipse March 8-9

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 from 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.

Solar and lunar eclipses always come in pairs, with one following the other in a period of one fortnight (approximately two weeks). For example, the descending node solar eclipse on March 9, 2016, will be followed by an ascending node penumbral lunar eclipse on March 23.

Is it possible to have three eclipses in one month?

The video below explains why a pair of eclipses happens when the new moon and full moon are closely aligned with the lunar nodes.

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 orbit is inclined to the plane of the ecliptic. The nodes are the two points where the moon’s orbit and the ecliptic intersect.

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

In the year 2016, the new moon and full moon align with the lunar nodes to present a pair of eclipses in March, and a pair of eclipses in September. In September, an ascending node annular solar eclipse on September 1 is followed by a descending node penumbral lunar eclipse on September 16.

Dates of solar and lunar eclipses in 2016

Once again, the nodes move about 30o westward (clockwise) along the ecliptic each month. Therefore, a waning crescent moon crosses the moon’s descending node on April 5, 2016, to carry the moon to the south of the ecliptic as it turns new on April 7, 2016. Likewise, a waxing gibbous moon crosses its ascending node on April 18, 2016, to bring the moon to the north of the ecliptic when it turns full on April 22, 2016.

Node passages of the moon: 2001 to 2100

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

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