There’s about to be a penumbral lunar eclipse. On November 30, 2020 at 09:42 Universal Time (04:42 EDT and 01:42 PDT) a full “Beaver Moon” will move into Earth’s shadow in space.

Don’t expect fireworks. Or a “Blood Moon.” Or anything particularly interesting, in fact, save for a strange-looking drop in the Moon’s brightness as 83% of our satellite moves into shadow.

So why get excited about this “Beaver Moon Eclipse?”

A lunar eclipse never comes alone; a solar eclipse always occurs about two weeks before or after a lunar eclipse.

A solar eclipse happens when the Moon’s shadow falls on the Earth, while a lunar eclipse happens when the Earth’s shadow falls on the Moon.

The “Beaver Moon Eclipse” will this year cause a solar eclipse, and it will be the best kind—a total solar eclipse. From a “path of totality” through Chile and Argentina on December 14, 2020, observers will experience 2 minutes 9 seconds of a sudden, dramatic twilight in the day.

But why will that happen because of the “Beaver Moon?” What’s the unusual thing happening that’s causing a lunar eclipse and then a solar eclipse? Why doesn’t an eclipse occur every month?

Here’s everything you need to know about the grand celestial mechanics at work that causes eclipses to be occasional, dramatic events—but always come in groups of two or three:

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Why do lunar eclipses occur?

Eclipses occur when the Sun, the Moon and the Earth are aligned. During a lunar eclipse the Earth is between the Sun and a full Moon, and the Moon moves into Earth’s shadow in space.

Everyone on the night-side of Earth sees the Moon dip in brightness (as with this weekend’s penumbral lunar eclipse) or turn partially or totally red (a total lunar eclipse). The latter is caused by the same physics as causes a sunset; the only light that reaches the Moon’s surface is first filtered by Earth’s atmosphere, which scatters blue light.

Why do solar eclipses occur?

During a solar eclipse a New Moon moves between the Sun and the Earth, sending a narrow shadow flashing across Earth’s surface. For anyone in the “path of totality” there’s a total eclipse of the Sun during which observers can remove their solar eclipses glasses and stare at the Sun’s bright, white outer atmosphere called its corona.

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There’s a total solar eclipse on December 14, 2020 in South America, another on December 4, 2021 in Antarctica, and after a quiet 2020, another in Western Australia on April 20, 2023.

Eclipses of the Sun only happen because the Moon is just the right distance from Earth to sometimes cover 100% of the Sun. By lucky chance the Moon is roughly 400 times smaller than the Sun, but 400 times closer to Earth. That’s why the Sun and the Moon look the same size in our sky.

It’s not quite a perfect match-up, however. The Moon actually orbits Earth in an ellipse, so each month it reaches a point that’s farthest from Earth (apogee) and a point when it’s closest to Earth (perigee). If the Moon is close to apogee during an “eclipse season”—as it was in December 2019 and in June 2020—the Moon doesn’t quite cover the Sun.

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The result is an annular solar eclipse, colloquially known as a “Ring of Fire.” These are actually little more than beautiful partial eclipses; you can’t take your eclipse safety glasses off.

There’s an annular eclipse in Canada on June 10, 2021, and another visible from a path from Oregon through New Mexico on October 14, 2023.

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Why don’t eclipses happen every month?

The Moon’s 29.3 day orbit of Earth is tilted with respect to the plane the Earth orbits the Sun—the ecliptic. The Moon’s apparent path through our sky is similar, but not the same as, the Sun’s apparent path through our sky.

The difference is about a 5º tilt, which means that while most New Moons and Full Moons occur below or above Earth, the Moon’s orbital path always intersects the ecliptic twice every month. These two points are called lunar nodes. It’s only when the Moon is near those two nodes during New Moon or full Moon that solar and lunar eclipses, respectively, can occur.

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What is an eclipse season?

When there’s a New Moon or a full Moon while the Moon is close to a lunar node, an eclipse occurs. If it hits one node, then the celestial mechanics are such that it’s in exactly the right place that it will inevitably hit the other lunar node at the following New Moon of full Moon, thus causing a second eclipse.

An eclipse season begins when the Moon does just that, and though two eclipses—one solar one lunar—is typically all you get during an eclipse season, it’s not unusual for three eclipses to occur during an eclipse season.

Disclaimer: I am the editor of

Wishing you clear skies and wide eyes.