Galileo's First Jupiter Observations (1 of 4)
January 7, in the first hour of the night
On the first night that Galileo observes Jupiter, he sees it in the midst of what he believes are three fixed stars forming a striking pattern, a straight line parallel to the ecliptic.
Galileo misses Callisto. At this point he believes he is recording the movement of Jupiter relative to the three stars he saw on the previous night. He is puzzled because Jupiter appears to be moving in the wrong direction.
After being in the middle of it and to the east of it, Jupiter is now west of the star pattern. Unwilling to believe that Jupiter zigzags like this, Galileo concludes that the stars are actually following Jupiter. Note that his telescope isn't capable of separating Europa and Ganymede or Jupiter and Io.
Io, Europa, and both of their shadows are transiting (passing in front of) Jupiter. Galileo's telescope is incapable of seeing this.
January 12, in the first and third hour
One hour after sunset, Io and Callisto are still hidden by Jupiter's glare.
Io emerges two hours later:
In the third hour a third little star, not at all
seen earlier, also began to appear. This almost touched Jupiter on the eastern
side and was very small.
Galileo is able to see all four moons for the first time.
January 15, in the third hour
Two days later, the four moons are well spaced and easily distinguished. Galileo notes at this point that, like planets and unlike stars, the moons do not twinkle. This distinction was well-known at least as far back as Aristotle's On the Heavens, and it supports the idea that the moons are solar system objects and not stars.
January 15, in the seventh hour
Rapidly-moving Io disappears over the course of four hours. Galileo also notices Ganymede and Europa getting closer together.
January 16, in the first hour
Although he sees Europa and Ganymede as a single object, Galileo reports that it is brighter than Callisto.
January 17, 30 minutes after sunset
Galileo writes that the moon east of Jupiter appears twice as large as the one to the west.
January 17, after four hours, that is, around the fifth hour
Four hours later, he is able to resolve Europa and Ganymede into two distinct
objects, writing that
I suspect [they] had earlier been united...
January 18, 20 minutes after sunset
January 19, at the second hour
Galileo thinks he might see a second moon just to the east of Jupiter but is uncertain enough that he doesn't include it in his sketch.
January 19, at the fifth hour
A few hours later, Io emerges from Jupiter's glare.
January 20, at 1 hour 15 minutes
A shadow transit of Ganymede.
January 20, around the sixth hour
January 20, at the seventh hour
Throughout the night of the 20th, Galileo suspects there are three moons to the west of Jupiter, but he is unable to resolve all three of them until seven hours after sunset.