January 7, 1610

Galilean moons

Galileo’s first recorded sighting of Jupiter’s four large moons

The famous discovery began as three pinpricks Galileo took for fixed stars — and two of the four moons, Io and Europa, were blurred into one of them.

The first hour of night, 7 January 1610

During the first hour of night at Padua on 7 January 1610, Galileo examined Jupiter through an improved telescope of approximately twenty power and recorded three small but bright points close to the planet.

The three visible points lay almost exactly on a line parallel to the ecliptic: two east of Jupiter and one west.

Three points, four moons

Galileo initially classified the points as fixed stars, although their straight alignment and brightness surprised him. He did not recognise satellites on 7 January.

Modern reconstruction identifies all four large moons as present in his view: Io and Europa were too close together to be resolved separately, so he recorded them as one point.

Six more days to the discovery

When he observed again on 8 January, the changed arrangement made him suspicious. By 11 January he concluded that three bodies moved around Jupiter, and on 13 January he first recorded four distinct bodies at once.

Simon Marius later asserted an independent observation dated 29 December 1609 in the Julian calendar, equivalent to 8 January 1610 Gregorian; the scope and reliability of that priority claim remain debated.

Galileo named the four bodies the Medicean Stars for Cosimo II de' Medici and his family. Io, Europa, Ganymede and Callisto are names associated with Marius's later publication.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes

How this was checked

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What the sources leave uncertain

Galileo used the Gregorian calendar, so 1610-01-07 is not an Old Style/New Style conversion.
The statement that he “observed four” is retrospective: Galileo himself saw and drew only three points that night. Identifying one point as the unresolved combined appearance of Io and Europa depends on modern orbital reconstruction.
Simon Marius later asserted an independent observation dated December 29, 1609 in the Julian calendar, equivalent to January 8, 1610 Gregorian. The scope and reliability of Marius’s priority claim remain debated, but they do not unsettle Galileo’s documented January 7 observation.