Galileo's First Jupiter Observations (3 of 4)
Io is disappearing behind Jupiter.
February 2, at the seventh hour
February 3, at the seventh hour
February 4, at the second hour
February 4, at the seventh hour
Galileo doesn't give a time for his observation on the 7th. For the half dozen other observation reports that are missing a time, I've assumed a time one hour after sunset. For this one, a time later in the evening is more consistent with the absence of Io from his sketch.
February 8, at the first hour
I was of two minds whether the one closest to Jupiter was
only one, or two little stars, for it seemed now and then that there was another
star near it, toward the east, extremely small, and separated from it by only 10
February 9, at 30 minutes
February 10, at 1 hour 30 minutes
Both Io and its shadow are transiting Jupiter.
February 11, at the first hour
February 11, at the third hour
February 11, at the fifth hour plus a half
Throughout the night of the 11th, Galileo watches Io emerge to the east of Jupiter.
February 12, at 40 minutes
This observation on the 12th is the only time Galileo sketches a moon that isn't there. Both Europa and Io are moving from east to west (toward the right in the figure). They pass in front of Jupiter before sunset and remain to the west of it all night. There are no other Jovian moons within reach of Galileo's telescope (the first non-Galilean moon, Amalthea, was discovered in 1892 by E.E. Barnard using the Lick Observatory's 36-inch refractor), and no stars brighter than magnitude 9.5 are within 10 arcminutes of Jupiter (a circle almost as big as the width of this diagram).
Galileo may be seeing an asteroid. Unfortunately, JPL's search for small bodies doesn't support dates prior to 1800, in part because of the difficulty of calculating accurate orbits for these objects further into the past.
So we don't yet know what Galileo sees here, and he writes that
fourth hour the little star that was close to Jupiter to the east no longer
February 13, at 30 minutes