If you have a clear sky, don’t forget to look overhead tonight! And go get your binoculars or small telescope…
After an overnight flight and a train that brought me to Florence (Firenze), Italy, where I’ll be teaching this week, I decided to fight off sleep by taking a walk down into the city and wandering around for a while. It was a beautiful evening, with deep blue twilight. And it wasn’t long before the planet Jupiter, and then the full Moon, rose above the buildings and high into the sky. I caught a photo of them, between the Duomo (cathedral) and its campanile (bell tower). Jupiter is the little white dot directly above the moon, at the top of the second set of windows on the campanile.
I then pulled out my binoculars, which aren’t quite as powerful as Galileo’s telescope was 400 years ago, but are still enough to reveal what Galileo discovered. Just as Galileo (and his competitor Thomas Herriot) did in 1609, I could see all sorts of structure to the Moon’s surface, including what we now know are basalt plains, and hints of impact craters. [Admittedly, impact craters and mountains are actually easiest to see when the Moon isn't full, because then the shadows that mountains cast are longer.]
And looking at Jupiter, which is relatively close to Earth right now, I could easily see that it was a disk, not a dot like a star, and that there are three dim dots, sitting in a line that passes through the planet. These are three of Jupiter’s four large moons: Callisto, Ganymede, Europa and Io. [The fourth one might well be visible if you're lucky and have good eyes and good timing.] If you watch them day by day, they will change position, a fact that Galileo used to guess they were moons orbiting Jupiter.
So if you have good weather, tonight’s a great opportunity for some simple but very satisfying astronomy. Don’t miss the naked-eye view that’s on offer right now or in a few hours, depending on where you reside. And if you’ve got binoculars handy, you can relive Galileo’s remarkable discoveries about the Moon and Jupiter, and contemplate how the first telescopes forever changed the way humans envisioned their cosmos.