Galileo Spots Jupiter's Moons
Next: January 7, 2027
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The story
Where it comes from
This Story marks the anniversary of 7 January 1610, when the Italian astronomer Galileo Galilei turned his improved, 20-power homemade telescope on Jupiter and saw three small points of light beside it. He took them at first for distant stars, but night after night they moved along with the planet and changed places with one another, and a fourth appeared. By 15 January he had worked it out: they were moons circling Jupiter, strong evidence that not everything in the heavens revolves around the Earth.
How it's celebrated
There is no formal observance, but on a clear night even a pair of steady binoculars can show Jupiter's four largest moons as tiny dots in a line, just as Galileo saw them. Sketch where they sit, look again the next night and watch them move. NASA and many observatories share free guides to finding Jupiter in the evening sky.
Did you know
- Galileo named the moons the 'Medicean Stars' after his patrons, the Medici, but in his own notes he simply numbered them I to IV.
- Johannes Kepler suggested the names Io, Europa, Ganymede and Callisto, but they did not catch on for more than 200 years.
- Galileo announced the discovery in Sidereus Nuncius, 'The Starry Messenger', printed in Venice in March 1610, and it made him famous.
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