Syzygy and Selenelion

Since the recent lunar mission by Japan has been in the news lately, let’s revisit a space related issue with two words every stargazer should know—syzygy (pronounced sih-ZIH-jee) and selenelion.

Syzygy is when the sun, Earth, and the moon align. This happens twice a month during the full moon and the new moon. Syzygy can also occur when the sun, Earth, and another planet line up. During a new moon syzygy, the sun and the moon lie on the same side of Earth. We know this as a conjunction.

The full moon syzygy happens when Earth comes between the sun and the moon. People know this as opposition. And a third example of syzygy is during a total lunar eclipse, when the full moon is on the opposite side of the earth from the sun.

A fourth example of syzygy happens during an annular eclipse when the moon aligns between Earth and the sun. It leaves a ring of the sun’s light still visible around the moon. This alignment usually occurs every year or two, but it is only visible across a small area on Earth. This is because the moon is so much smaller than the sun.

A solar eclipse is the fifth example of syzygy. This happens when the moon orbits Earth and it moves between Earth and the sun in its passage. This is called a solar eclipse, and the moon appears as a dark shadow on just a small part of the sun’s gigantic surface.

Since both the sun and the moon orbit Earth, why don’t eclipses occur every single month at the full and new moons?

If Earth and the moon orbited on the same plane around the sun, we would indeed have a total solar eclipse—and a total lunar eclipse—every month. But the moon’s orbit is inclined to Earth’s orbit by about five degrees. So, no total eclipses every month.

Now what is a selenelion, which is sometimes called a selenehelion? First, it helps to understand what the word means. It is a French word that comes from the ancient Greek words selēnē, moon and hēlios sun. This is an uncommon eclipse phenomenon where you can see the sun near the horizon and the eclipsed moon near the opposite horizon at the same time.

Wouldn’t that be a cool thing to see?

You can now impress your friends with your stargazing knowledge. Look how much you have learned in a short time about eclipses and planet alignments. Plus, you now have two interesting words you can use that most adults have never heard before. Have fun using them and impressing the people around you. Who knows, maybe you can interest them into learning more fun facts about the world around them.

AIME