26 SEPT 2026History
When did Galileo see the moons of Jupiter? 7 January 1610
On the evening of 7 January 1610, in Padua, Galileo pointed his telescope at Jupiter and saw three small stars in a line beside it. Nobody recorded the minute — but we know the evening.
Short answer: Galileo Galilei first saw the moons of Jupiter on the evening of 7 January 1610, from Padua. He described it in *Sidereus Nuncius* (The Starry Messenger), published that March. He did not record the exact minute; he wrote that it was in the first hour of the night.
Three stars in a line
Galileo had built his own telescope, far better than the spyglasses then sold in Venice. Looking at Jupiter, he saw three small points of light near the planet, two on one side and one on the other, arranged in a straight line. He took them for fixed stars.
The next nights
On 8 January they had moved. Over the following week he watched them change position and saw a fourth. By 15 January he concluded they were not stars at all but bodies circling Jupiter. It was the first direct evidence that not everything in the sky goes around the Earth.
Why there is no exact minute
In 1610 Italy counted hours from sunset, and clocks were not accurate to the minute. Galileo's notes give the night and the order of the objects, not a time in hours and minutes. Any minute attached to this event is an estimate, and honest sources say so.
Sources
See the museum for how we record events whose minute can only be estimated.