The Italian astronomer Giovanni Domenico Cassini had pioneered the use of the eclipses of the Galilean moons for longitude measurements, and published tables predicting when eclipses would be visible from a given location.
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Cassini noted that the other three Galilean moons did not seem to show the same effect as seen for Io, and that there were other irregularities which could not be explained by Rømer's theory.
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Rømer appears to be collecting data on eclipses of the Galilean moons in the form of an aide-mémoire, possibly as he was preparing to return to Denmark in 1681.
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The orbit of Io is also slightly irregular because of orbital resonance with Europa and Ganymede, two of the other Galilean moons of Jupiter, but this would not be fully explained for another century.
Galilean moons | Shepherd Moons | The Maker of Moons (short story) | The Maker of Moons | "The Maker of Moons" | Moons of Mars | Galilean invariance | World of the Three Moons | Moons of Jupiter | Moons and Mushrooms | Many Moons | Jupiter's moons | Jan Moons | Galilean | four of Jupiter's moons | Eight Moons |
It is meant to be an inexpensive means by which millions of people can view the same things seen by Galileo Galilei, such as the craters of Earth's Moon, four of Jupiter's moons, and the Pleiades.