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Track the Moons of Jupiter

The honor of idea #1 goes to one of my favorite astronomers: Galileo.

In 1609/1610 he turned one of the earliest refracting telescopes toward Jupiter and observed a few nearby “stars” that seemed to travel along with the planet. In his notebook, he logged the positions of these four satellites relative to Jupiter over the course of several years.

Galileo's Notebooks

One of the earliest entries (before he observed the fourth moon) reads:

“January 11, 1610. It was in this guise and the star nearest to Jupiter was half as large as the other, and most close to the other, whereas on the other evenings the said three stars all appeared to be of the same size and equidistant one from another: from which it appears that there are around Jupiter three wandering stars up to this time invisible to one and all.”

(trans. Charles J. Donovan)

The four “Galilean Moons” were later named Callisto, Ganymede, Io and Europa. We now know that Jupiter has (at least!) sixty-seven moons; the four that Galileo observed are the biggest, and are fairly easy to find if you can get your hands on a small telescope (or some good binoculars). Back when Galileo made his notes, he was documenting the first known example of something orbiting a solar system object other than the Earth (our Moon) or the Sun (the Earth, though this was somewhat controversial at the time). This was one of the first big leaps in our cosmic perspective: Hey, there are other planets with their own moons…not everything revolves around us! A pretty cool use of a notebook, don’t you think?

Jupiter-moons

Source of quote: http://www.dioi.org/galileo/galileo.htm

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