What did Galileo discover on 7 January 1610?

Three of Jupiter's four largest moons, initially mistaken for fixed stars. He identified the fourth days later and recognised they orbit Jupiter.

It showed objects orbiting something other than the Earth, undermining the assumption that everything in the heavens revolves around us.

Where was he observing from?

Padua, in the Republic of Venice, using a telescope he had built and improved himself.

January 7, 1610

1610-01-07 19:00 UTC

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Galileo sees moons around Jupiter

Through a homemade telescope, Galileo notes three small stars beside Jupiter that move with it.

Source ↗

What happened in this minute

On the evening of 7 January 1610, Galileo recorded three faint points in a line beside Jupiter. Over the following nights they changed position and a fourth appeared, and he concluded they were orbiting the planet.

This was the first direct evidence that not everything in the sky circles the Earth. Published weeks later in Sidereus Nuncius, it moved the Copernican argument from calculation to observation.

The evening timestamp follows his observation log; the precise minute is not recorded.

Questions and answers

What did Galileo discover on 7 January 1610?: Three of Jupiter's four largest moons, initially mistaken for fixed stars. He identified the fourth days later and recognised they orbit Jupiter.
Why did it matter?: It showed objects orbiting something other than the Earth, undermining the assumption that everything in the heavens revolves around us.
Where was he observing from?: Padua, in the Republic of Venice, using a telescope he had built and improved himself.