Galileo and Jupiter’s moons: another centre of motion

On January 7, 1610, Galileo examined Jupiter through a telescope he had improved and saw three small points beside the planet. They lay almost in a straight line. At first he treated them as stars and drew their positions relative to Jupiter.

The arrangement changed the next night. He began repeatedly recording this small field, comparing the number, spacing and direction of the points from night to night.

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The objects appeared east or west of Jupiter but remained close to it, accompanying the planet against the background stars. Further observations revealed four bodies. Their recurring changes could be explained by motion around Jupiter.

Sidereus Nuncius preserved diagrams of observations from January into early March. Points, symbols and separations turned fleeting telescopic views into records that could be compared.

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Galileo published Sidereus Nuncius in March 1610. Alongside Jupiter’s moons, it described uneven terrain on the Moon and resolved much of the Milky Way’s diffuse light into stars. He named the four bodies the Medicean Stars and dedicated the book to the ruler of Tuscany.

Its diagrams showed other observers where to point a telescope and what to seek. Instrument and printed book carried the observations beyond Galileo’s own workshop.

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Jupiter’s moons supplied an orbital centre that could be observed repeatedly: several bodies revolved around Jupiter while Jupiter itself moved across the sky. Galileo used this example to answer objections to Earth carrying the Moon around the Sun, supporting the Copernican system.

The four large satellites became known as Io, Europa, Ganymede and Callisto. Their eclipses later served for timing and investigating light propagation, while spacecraft revealed their individual geological environments.

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