Unless you go to bed early, you must have noticed that bright object hanging over the eastern hills in the late evening. It is the planet Jupiter, the fifth planet out from the Sun – we live on the third. Being so bright and unmissable, it was discovered so long ago that we have no record of it. However, we do know that when Galileo got hold of a telescope in the 17th Century, he was the first person to point a telescope at Jupiter. What he saw was important in leading him to his conclusion that the Earth and all the other planets orbit the Sun. The official opinion at the time was that everything, with no exceptions, orbits the Earth. If you have binoculars or better still, a small telescope, you can see what Galileo saw, and check his conclusions for yourself. Actually, even with cheap optical equipment, we can get a better view than Galileo did.
On the occasion of the recent anniversary of Galileo’s discoveries, a cheap but remarkably good telescope kit appeared on the market. It can either be set up as a conventional modern telescope, or in a configuration similar to that used by Galileo. Trying this mode of observing shows two things: firstly that telescopes have improved enormously since the 17th Century, and secondly, that Galileo must have been an intensely dedicated observer.
These evenings, Jupiter rises during the evening, and around 10 p.m. you should see Jupiter rising spectacularly in the east. It looks like a very bright star, but if you look carefully you will see that it is not twinkling, which on most evenings is a good indicator that you are looking at a planet.
Through binoculars or a small telescope, Jupiter itself appears as a tan disc, crossed by two or more darker belts. If Jupiter were higher in the sky, more magnification will show the cloud belts more clearly. However, close to the horizon our turbulent atmosphere makes using high magnification a waste of time.
What stands out very clearly is the presence of up to four faint, starlike objects, in line with the planet, like beads on a wire. If you do what Galileo did, and observe every night, sketching the positions of the planet and those “stars”, you will see their positions changing from night to night, with sometimes only two or three of them being visible. Galileo concluded correctly that he was seeing four of Jupiter’s moons, and that they are in orbit round Jupiter, all in the same plane, like marbles rolling around an invisible plate and us seeing the plate from edge-on. Galileo wanted to call them the Medician Stars, probably to get him on the good side of Cosimo Medici, Duke of Tuscany. However, they were officially named Io, Europa, Ganymede and Callisto, and collectively, the “Galiliean Satellites”. Even today, when we know what is going on, interpreting those changing positions as a evidence the bodies are orbiting Jupiter takes quite a bit of work. Imagine how it must have been with Galileo, especially using that crude telescope. We know he had a high opinion of himself as an astronomer. Perhaps he was right.
Saturn is now lost in the sunset glow. Mars is low in the Southwest after sunset. Jupiter rises around 10 p.m. and Venus around 3 a.m. The Moon will reach last quarter on the 7th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.