17 June 2019

I went to bed too early and couldn’t sleep, so eventually got up to read a book in the study. A red full moon hung low in the sky above my friend’s farm. A ‘strawberry moon’, apparently. Not on account of its colour, but on account of its being strawberry season.

Slightly above and to the right of the moon, a planet shone like a spotlight. I checked the astronomy app on my phone: as I’d guessed, Jupiter. It really was astonishingly bright. So I opened the study window and trained my crappy binoculars on it, using the swivel chair as a totally ineffective tripod. The image darted about, but I eventually managed to steady the binoculars sufficiently to make out three small pinpricks of light close to the planet, at about eight o’clock from it. After a while, I spotted a fourth pinprick on the opposite side, slightly farther out, at about two o’clock. I was looking at the Galilean Moons, Io, Europa, Ganymede, and Callisto, first spotted through a telescope less powerful than my binoculars by Galileo Galilei on 7th January 1610 (and second spotted by German astronomer Simon Marius just one night later).

Contrary to popular myth, the observation of four satellites orbiting another planet did not immediately prove the newfangled Copernican heliocentric system to be correct. There were numerous other rival models that allowed for orbits around other heavenly bodies. But Galileo and Marius’s independent observations did disprove the ancient and already somewhat discredited Ptolemaic system, which held that everything in the universe revolved around the Earth.

An entire world system refuted by some simple observations you might make in the comfort of your own study. How cool is science?

(I never did get round to the book.)

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