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Why no eclipse every full and new moon?

This year, in 2017, there are 12 full moons and 12 new moons, but only four eclipses – two lunar and two solar.

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?

Dates of lunar and solar eclipses in 2017

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 ascending node penumbral eclipse on February 10-11, will be followed by a descending node annular solar eclipse on February 26.

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 2017, the new moon and full moon align with the lunar nodes to present a pair of eclipses in February, and a pair of eclipses in August. In August, a descending node partial lunar eclipse on August 7 is followed by an ascending node total solar eclipse on August 21.

Dates of solar and lunar eclipses in 2017

Node passages of the moon: 2001 to 2100

Once again, relative to the moon’s phases, the nodes move about 30o westward (clockwise) along the ecliptic each month. So in March 2017 – the month following the eclipse month of February 2017 – it’ll be a waxing gibbous moon (not a full moon) that crosses the moon’s ascending node on March 11 and a waning crescent moon (not a new moon) that crosses the moon’s descending node on March 25.

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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