Jane Houston Jones
The end of 1609 and the first months of 1610 mark the beginning of modern astronomy. 400 years ago today, January 7th, Galileo Galilei looked up towards the constellation Orion. He aimed his telescope at an object brighter than any of the surrounding stars - the planet Jupiter.
The view through his telescope startled him. He did not see only one object, but rather, one large world, with four smaller objects nearby.
These four objects are the moons we now call Io, Europa, Ganymede and Callisto.
Galileo wrote in his book Sidereus Nuncius, which was published in 1610 the following words:
“I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter.”
On the 400th anniversary of their first sighting, use a pair of binoculars to spot Galileo’s four tiny moons directly next to the planet. On the evening of January 7 look to the southwest after sunset. Europa and Ganymede will appear to the upper left and Io and Callisto on the lower right of Jupiter.
Letters with translations http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html