Eclipses

I liked Neil deGrasse Tyson a couple decades ago, but this is an astronomically bad take:  He had tweeted "Lunar eclipses are so un-spectacular that if nobody told you what was happening to the Moon you’d probably not notice at all.   Just sayin’." 

Folks noticed and learned a lot from lunar eclipses millenia ago including gauging rough ideas of the relative sizes of the Earth, Sun, and Moon.

In China both Lunar and Solar eclipse records date back to at least 770 BCE.

In Greece, c. 500 BCE, Paramendes of Ela and Anaxagoras of Clazomenae understood the Moon to shine by reflected sunlight.  

Both Paramendes and his contemporary Pythagoras understood the Earth to be spherical.

Anaxagoras ~480 BCE also understood both that solar eclipses came from the Moon's shadow cast on the Earth and that lunar eclipse came from the Earth's shadow cast on the Sun.

c. 250 BCE, Aristarchus of Samos worked out how to determine the relative sizes of and distances to the Sun, Moon, and Earth using lunar phases, lunar and solar eclipses and geometry. 

Solar eclipses work because the Sun and the Moon subtend approximately the same half a degree angle as seen from the Earth.

The Moon must be smaller than the Sun because looks about the same angle wide and comes in between us and the Sun.

When the Moon is in its 1st and 3rd quarter phases, so that half of the near side to the Earth is lit, the Earth-Moon-Sun triangle is a right triangle with the Moon at the 90 degree angle.

So Aristarchus measured the angle between the Sun and Moon when the Moon was in its quarter phase and using trigonometry - or ratios of lengths of triangle sides for given angles, determined how many times further from the Moon, the Sun was from us.

Since the Sun and Moon take up the same angle as seen from Earth, their diameters form the base of similar isoceles triangles.  The ratio of the distances to each is the same as the ratio of sizes of each.

With a Lunar eclipse, you can judge how many diameters of the Moon, it moves relative to the stars as it passes through Earth's shadow.  

Again, using geometry, with that distance across Earth's shadow as the base of a triangle, Aristarchus estimated the relative sizes of the Earth and Moon.  ~2250 years ago.

And it wasn't long after before we had a good estimate of the size of the Earth in an absolute sense, and thus the sizes of all three objects.  Eratosthenes worked that out with the angles of shadows cast on the Summer Solstice in Alexandria, and Syene to the south.

But I'll leave it to Carl Sagan to talk about Eratosthenes' measurements and calculations from 2200 years ago.

Still, lunar eclipses were a key to understanding the sizes and distances in the solar system.   

The scale that it for the size of the Sun  that it suggested to Aristarchus made him believe in heliocentricm when few others would.  He was later cited by Copernicus when presenting his heliocentric theory 1700 years later.

And that's pretty spectacular