Starry Messenger, by Peter Sis, Frances Foster Books, 1996.
First of all, this book is gorgeous. Amazingly and staggeringly and phenomenally gorgeous.
The story itself is a great introduction to the history of astronomy for early grade school children. There is not a happy ending, because Galileo's story did not have a happy ending, but I like honesty in children's literature.
Overall, I thought this book was VERY well done. I was not surprised to see Frances Foster's name associated with it.