400 years ago today, Galileo got the bright idea of taking his newly made telescope and pointing it at Jupiter. He saw some new stars, strangely in a straight line with Jupiter. Over the next couple of nights he kept looking, saw a fourth star, and then realized that the “stars” were moving with Jupiter, but not exactly. From his Starry Messenger:
On the seventh day of January in this present year 1610, at the first hour of night, when I was viewing the heavenly bodies with a telescope, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself, I perceived (as I had not before, on account of the weakness of my previous instrument) that beside the planet there were three starlets, small indeed, but very bright. Though I believed them to be among the host of fixed stars, they aroused my curiosity somewhat by appearing to lie in an exact straight line parallel to the ecliptic, and by their being more splendid than others of their size. . . . There were two stars on the eastern side and one to the west. The most easterly star and the western one appeared larger than the other. I paid no attention to the distances between them and Jupiter, for at the outset I thought them to be fixed stars, as I have said. But returning to the same investigation on January eight – led by what, I do not know – I found a very different arrangement. The three starlets were now all to the west of Jupiter, closer together, and at equal intervals from one another . . . .
On the tenth of January . . . there were but two of them, both easterly, the third (as I supposed) being hidden behind Jupiter . . . . There was no way in which such alterations could be attributed to Jupiter’s motion, yet being certain that these were still the same stars I had observed . . . my perplexity was now transformed into amazement. I was sure that the apparent changes belonged not to Jupiter but to the observed stars, and I resolved to pursue this investigation with greater care and attention . . . .
You just know that what he really said to himself here was “Holy …”
I had now decided beyond all question that there existed in the heavens three stars wandering about Jupiter as do Venus and Mercury about the sun, and this became plainer than daylight from observations on similar occasions which followed. Nor were there just three such stars; four wanderers complete their revolution about Jupiter . . . .
Here we have a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the sun in the Copernican system, are mightily disturbed to have the moon alone revolve about the earth and accompany it in annual rotation about the sun. Some have believed that this structure of the universe should be rejected as impossible. But now we have not just one planet rotating about another while both run through a greater orbit around the sun; our eyes show us four stars which wander about Jupiter as does the moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve years.