Summary: Atmospheric Moons Galileo Would Have Loved
S. K. Atreya, A chapter in Galileo's Medicean Moons - Their Impact on
400 Years of Discovery (C. Barbieri et al., eds.), Cambridge University
Press, 2010, in press.
1. Historical Perspective
The night of January 8, 1610 was Galileo Galilei's eureka! moment. On this night,
Galileo realized that the objects he had observed in close proximity to Jupiter the pre-
vious night with his "occhiale" (telescope) were not fixed stars, but were actually the
moons of Jupiter, for they had moved and their configuration around Jupiter had changed
substantially. With observations during the course of next seven days, Galileo confirmed
without any doubt the presence of four moons orbiting Jupiter. Galileo named them the
"Medicean Moons," after the Medicis of Florence, and now known as the Galilean Moons.
On March 13, 1610, two months after the observations, Galileo published his findings in
Sidereus Nuncias that is generally translated as Starry Messenger, or sometimes Starry
"Message", apparently referring to Galileo's response to Father Orazio Grassi who had
accused Galileo of pretending to be a herald from heaven after seeing the title of the
book. With the observations of the moons of Jupiter, Galileo had placed the Coperni-
can hypothesis of heliocentric system on a firm footing. It marked arguably the most
monumental turning point for humanity.
With his telescope Galileo went on to reveal a multitude of other mysteries of the