Why no eclipse every full and new moon?

Golden crescent sun during eclipse (through a filter), with prominences along the edge. — View at EarthSky Community Photos. | David Hoskin in Halifax, Nova Scotia, Canada, captured this photo of the partial annular eclipse on June 10, 2021. He wrote: “I drove out to York Redoubt, a historical site outside of Halifax, to set up to photograph the partial solar eclipse. This hydrogen alpha image was taken at the peak when about 68% of the sun was covered by the moon.” Stunning detail! Thank you, David.

Why do we have eclipses?

A solar eclipse happens at new moon, when the moon passes between the sun and Earth. A lunar eclipse happens at full moon, when the Earth, sun and moon align in space, with Earth between the sun and moon. During a lunar eclipse, Earth’s shadow falls on the full moon, darkening the moon’s face and – at mid-eclipse – sometimes turning it a coppery red. We typically have between four and seven eclipses – some partial, some total, some lunar and some solar, every year. 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 a minimum of two eclipses every month. There’d be an eclipse of the moon at every full moon. And, 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.

Moon’s orbit is inclined

But that doesn’t happen, and the reason is that the moon’s orbit around Earth is inclined to Earth’s orbit around the sun 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 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.

Earth at center with moon's orbit crossing the ecliptic line twice. — The moon’s orbit tips 5 degrees relative to Earth’s. Eclipses only occur when the moon crosses the ecliptic during a full or new moon. Image via Wikimedia Commons (CC BY-SA 4.0).

An eclipse does not come alone

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 partial solar eclipse on April 30, 2022, preceded a descending node total lunar eclipse on May 15-16, 2022.

Then exactly six lunar months (six new moons) after the ascending node annular solar eclipse of April 30, 2022, there will be a descending node partial solar eclipse on October 25, 2022.

And exactly six lunar months (six full moons) after this descending node total lunar eclipse on May 15-16, 2022, there will be an ascending node total lunar eclipse on November 8, 2022.

Eclipse seasons

More often than not, two eclipses – one solar and one lunar – occur in one eclipse season. Eclipse seasons occur approximately 34 to 35 days. Sometimes, though, when the initial eclipse happens sufficiently early in the eclipse season, there can be three eclipses in one eclipse season (two solar and one lunar, or two lunar and one solar). This last happened in July-August 2020 (lunar/solar/lunar), and will next happen in June-July 2029 (solar/lunar/solar).

This year, in 2022, the middle of the eclipse season falls on May 15 and November 4. 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.

Moon phases shift along the zodiac every month

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). Following the April 30 and May 15-16 eclipses, the next pair of eclipses falls on October 25 and November 8, 2022. The first pair of eclipses in 2023 are on May 5-6 (partial lunar eclipse) and November 20 (total solar eclipse).

Node passages of the moon: 2001 to 2100

Phases of the moon: 2001 to 2100

The following new moon and full moon happen 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.

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. All are marvelous to behold. They’re a reminder that we live on a planet and a chance to experience falling in line with great worlds in space!

Bottom line: If the Earth and moon orbited on the same plane around the sun, we’d have a total solar eclipse – and a total lunar eclipse – every month. But we don’t, because the moon’s orbit is inclined to Earth’s orbit by about 5 degrees. In 2022, there are four eclipses: two solar and two lunar.

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