Why not an eclipse at every full and new moon? | Astronomy Essentials

Total lunar eclipse composite image by Fred Espenak.

A lunar eclipse happens when the Earth, sun and moon align in space, with Earth between the sun and moon. At such times, Earth’s shadow falls on the full moon, darkening the moon’s face and – at mid-eclipse – sometimes turning it a coppery red.

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?

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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 a minimum of two eclipses every month. There’d be an eclipse of the moon at every full moon. And, one fortnight (approximately 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 five 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 sweeps appreciably close to one of these nodes, then an eclipse is not only possible – but inevitable.

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The plane of the moon’s orbit is inclined at 5 degrees to the ecliptic (Earth’s orbital plane). In this diagram, the ecliptic is portrayed as the sun’s apparent annual path through the constellations of the zodiac. The moon’s orbit intersects the ecliptic at two points called nodes (N1 and N2).

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 lunar eclipse on November 30, 2020, will be followed by a descending node total solar eclipse on December 14, 2020.

Then exactly six lunar months (six full moons) after this ascending node penumbral lunar eclipse on November 30, 2020, there will be a descending node total lunar eclipse on May 26, 2021.

Then exactly six lunar months (six new moons) after the descending node total solar eclipse of December 14, 2020, there will be an ascending node annular solar eclipse on June 10, 2021.

This year, in 2020, the middle of the eclipse season falls on June 20 and December 11. At the middle of an eclipse season, which recurs in periods of about 173 days, the lunar nodes are in exact alignment with the Earth and sun.

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 nodes, the moon’s phases recur about 30 degrees farther eastward (counterclockwise) along the zodiac each month. So the next pair of eclipses won’t be forthcoming for nearly another six calendar months (6 x 30 degrees = 180 degrees), to fall on May 26, 2021, and June 10, 2021.

Node passages of the moon: 2001 to 2100

Phases of the moon: 2001 to 2100

The following new moon and full moon happen again nearly 30 degrees farther eastward as measured by the constellations of the zodiac in about 29.5 days. But the moon returns to its node a good two days earlier than that, or in about 27.2 days. After the eclipses of November 30 and December 14, 2020, it’ll be a waxing gibbous moon (not a full moon) that crosses the moon’s ascending node on December 28, 2020, and a waning crescent moon (not a new moon) that crosses the moon’s descending node on January 10, 2021.

A heliocentric or sun-centered view of eclipses in 2020. Earth-moon orbit shown at new and full moon dates. Sizes of Earth, moon, sun very exaggerated. The plane of the moon’s orbit is in blue, with the dark-blue half to the north of the ecliptic, and the light-blue half to the south of the ecliptic. The line dividing the dark-blue and light-blue sides depicts the line of nodes. There’s an eclipse if the moon is full or new when it is in or near the ecliptic or sun-Earth plane. This year there are 5 eclipses, instead of the most usual 4, because a 3rd eclipse season begins before the end of the year. Illustration via Guy Ottewell.

Even though the moon’s orbit is inclined to that of Earth – and even though there’s not an eclipse with every new and full moon – there are more eclipses than you might think.

There are from four to seven eclipses every year. Some are solar, some are lunar, 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 via pizzodisevo.

Bottom line: There’s not an eclipse at every full moon and every new moon because the moon’s orbit is inclined to Earth’s orbit by about five degrees. Most of the time, the sun, Earth and moon don’t line up precisely enough to cause an eclipse. But sometimes, more often than you might expect, they do!

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