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The Day the Moon ate Saturn

30 Aug 2019, 13:00 UTC
The Day the Moon ate Saturn
(200 words excerpt, click title or image to see full post)

The solar system is flat! Kinda.

Most of the planets orbit in the same plane; if you took a rocket and went, oh, 10 billion kilometers away in the right direction you'd see all the planets' orbits are nearly aligned, flat. If you sat there and waited, they'd look like they were in a line, moving back and forth as they orbited the Sun.

Except… that alignment isn't exact. They each have their own orbital inclination, or tilt. We say the Earth's is 0 by definition (it's also the same as the Sun's path around the sky every year, since that's literally a refection of Earth's orbital motion; we call that line in the sky the ecliptic), but the others tilt by a few degrees from that.

Our Moon's orbit is tilted with respect to the ecliptic, too, by about 5 degrees. That's not much, but the Moon is about 0.5° in size on the sky. Its orbital tilt is 10 times its apparent size! So even though a star might be exactly on the ecliptic in the sky, the Moon might only rarely pass directly in front of it. When it happens it's called an occultation.

It's worse for ...

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