Solar Eclipse

Prerequisite: none

When the moon blocks out the sun. Not so simple.

Consider:

1) Size and distance
2) Elliptical orbit, suborbit, and perturbation
3) Gravitation and drift

And miraculously enough, solar eclipses still manage to happen roughly every eighteen months. Not in a big blot kind of way either, but a close fit. It is a curious phenomenon.

Details here:
https://www.youtube.com/watch?v=ieUvzy6rnnw

And Matt Parker is totally hilarious. He really knows how to unbox calculators.

To sum up the math involved:

223 * 29.53059 days (regular orbit)
239 * 27.55455 days (between sun and earth)
242 * 27.21222 days (level with sun and earth)

≈ 18 years 11 days 8 hours (lowest common multiple)

Meaning that the moon does:

223 regular orbits, 29.53059 days each
239 between-sun-earth orbits, 27.55455 days each
242 sun-earth-level orbits, 27.21222 days each

to return to the position at solar eclipse roughly every 18 years.

Too good to be true! But it is true.

Or you could just believe that the moon is a boring chunk of rock that occasionally blots out the sun. But see here, how else would anyone notice this without a solar eclipse? The sun's outer atmosphere, the corona, is usually too faint to see. Solar eclipses blot out the excess sunlight, yet leaving the atmosphere exposed, so that the corona is visible. The moon is at just the right size and distance.

But really, humans went crazy about making calendars. While most farmers had enough instinct to sense the seasons, humans just had to formulate the perfect calendar to sync up with the years. Rituals? I do not buy that story. Stonehenge was way too much hassle for merely a solstice marker, unless there was something particularly profound about the phenomenon. You would think that people had better things to do than obsessing over dates.. and recording solar eclipses.