400 Years Ago, Galileo Discovered Four Jovian Moons

krswan writes "OK, the moons themselves are much older, but on January 7, 1610 Galileo first observed '4 fixed stars' surrounding Jupiter. Observations of their changing positions led Galileo to postulate they were really moons orbiting Jupiter, which became further evidence against Aristotelian Cosmology, which led to problems with the Roman Catholic Church, etc... Jupiter will be low in the southwest (in the Northern Hemisphere) after sunset this evening — nothing else around it is as bright, so you can't miss it. Celebrate by pointing binoculars or a telescope at Jupiter and checking out the moons for yourself."

Comments

Happy Io Discovery Day, /.

[volcanopele]

Definitely a good time to check out Jupiter and the four Galilean moons before conjunction which happens in the next couple of months, so Jupiter would then be too close to the Sun.

A minor quibble with the summary above. On January 7, 1610, Galileo only recorded 3 "fixed stars" next to Jupiter. Two of the Galilean moons, Io and Europa, were too close together for Galileo to separate with his 20x power telescope. He continued to observe three moons at most, either because one or more moons were too close to Jupiter and were lost in the glare of the planet, Callisto was too far from Jupiter and was thus out of his telescope's field-of-view, or two of the moons were too close together, during subsequent nights, until January 13, when he was able to see all four for the first time.

Wikipedia is wrong on one point. True, his first observation of all four moon at once didn't come until January 13 and he didn't realize that there were four and not three until that time, but that doesn't mean that one moon's discovery (in Wikipedia's case, Ganymede) should be attributed to that date. By that point, he had observed all four on multiple occasions, just not all four at once. And to that point he hadn't even come to the conclusion that they were in orbit around Jupiter with their own separate orbits, moving a different speeds, until two days later, let alone ascribe identities to each of the stars he saw, connecting one star he saw with another from a different day, beyond the one to the east, the one to the west, and the one in the middle.