The Italian physicist Galileo Galilei was not the inventor of the 
telescope, nor was he the first to observe the heavens through a 
telescope.  He was, however, the first scientist to make careful,  
systematic observations of the heavens with a telescope and to 
publish the results of those observations in a small booklet called 
Siderius Nuncius ("The Starry Messenger").  In this book, published 
in 1610,  Galileo described the phases of Venus and argued that they 
proved that the planet orbited the Sun instead of the Earth.  He saw 
mountains on the Moon, proof that it was a real planet like the 
Earth, rather than an
ethereal heavenly ball.  And he was able to make out the many, many 
stars which make up the Milky Way, stars which are too faint and 
distant to be seen individually from the Earth, but whose light blends 
to produce the white band of light visible today only from very dark 
sites.

The most famous of Galileo's discoveries with his first telescope,  
however, were the four satellites of Jupiter which today we call the 
Galilean satellites.  (He called them the Medician Stars, in hopes of 
financial support from the powerful Medici family, but they have 
rightly become known by his name).  Night after night he observed 
them and sketched the changing alignments he saw.  [FIGURE]  
Gradually he realized that there was a pattern to the motions, and 
that the four small "stars" were actually moving around Jupiter.  
This was to become one of the most conclusive proofs that the Earth 
was not at the center of the Solar System, as was commonly believed 
in Galileo's time.  After all, if these four stars could travel around 
something other than the Earth,  could not other planets as well?

In this exercise you will observe Jupiter with the MicroObservatory 
at least once a day for up to three weeks.  An image archive is 
provided in case you have trouble with the weather.  A number of 
ways can be used to find which moon is which, and these are 
discussed in "Doing the Activity".