It’s eclipse season! There was a partial solar eclipse a couple of weeks ago. And on Monday night, we’ll have a total lunar eclipse. That one will be visible across all of the United States.

It’s not a coincidence that the eclipses are taking place two weeks apart. In fact, eclipses always come in pairs — unless they come in threes — and always two weeks apart.

That’s because eclipses require precise alignments of Earth, Sun, and the new or full Moon. But the Moon’s orbit around Earth is tilted a bit compared to Earth’s orbit around the Sun. So most months, when the new or full Moon crosses Earth’s orbit it’s either above or below the point where it would create an eclipse.

When the Moon crosses Earth’s orbit at just the right time, though, it brings Moon, Earth, and Sun into alignment. In fact, the alignment happens twice, about two weeks apart — an eclipse season. One alignment creates a solar eclipse, while the other creates a lunar eclipse. They can happen in either order. And the cycle repeats roughly six months later.

The total length of an eclipse season is about five weeks. So if there’s an eclipse early in that period, there can be a third eclipse at its end — usually two poor eclipses flanking one good one. And if the year’s first eclipse season begins in early January, there can be three of them during the year — producing as many as seven eclipses — which will happen in 2038.

More about the upcoming eclipse tomorrow.

Script by Damond Benningfield

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