Happy Birthday, Giovanni Domenico Cassini
Born in 162
5 in Italy, Cassini was an astronomer. He discovered four of Saturn’s moons and the dark gap between two of Saturn’s rings. (The gap, pictured left, is named the Cassini Division.) He also co-discovered Jupiter’s Red Spot and (with a colleague) measured the distance to Mars. According to Wikipedia, this was the first time anyone measured “the true dimensions of the solar system.”
After working for years at the Panzano Observatory in Italy, and teaching at the University of Bologna, Cassini moved to Paris, France, and helped set up an observatory there. He was director of the Paris Observatory for the rest of his life and adopted his new country 100%.
Astrology v. Astronomy
For 41 years, Cassini served as astronomer and astrologer to Louis XIV of France.
Astronomy is the study of objects, matter, and energy outside of the Earth’s atmosphere. In other words, astronomy is the study of stars, planets, galaxies, comets—everything in the universe that is in “outer space” rather than on Earth. It’s a science, which means it is based on observation, testable hypotheses, and evidence.
Astrology, on the other hand, is the supposed foretelling of the future based on the supposed influence of the stars and planets on people. You notice I said “supposed.” Twice. I got that word from a regular old dictionary definition of astrology, and it’s there because astrology plain old doesn’t work.
Yet here was Cassini, this brilliant man, being an astrologer as well as an astronomer? Wh-wh-wh-what?
During Cassini’s lifetime, scientific principles were just starting to be developed, and Cassini helped to develop the methods of astronomy. Way back then, astrology and astronomy were not thought to be very different, and there was a lot of crossover. Cassini was very interested in astrology as a youth, and he read all about astrological procedures. It was his “knowledge” about astrology that gained him his first position as an astronomer!
As Cassini got older, however, he switched more and more from astrology to astronomy. Eventually, he began to “disavow” or “denounce” astrology. That is, he started believing and saying that it was flat-out wrong.
- If you disagree with Cassini and me and think that astrology is pretty cool, and that it can tell the future, read Phil Plait’s Bad Astronomy post about it.
- Joe Rhatigan and Rain Newcomb wrote a book called Prize-Winning Science Fair Projects for Curious Kids. One of the projects deals with testing astrology (see page 30), and there is also a discussion of pseudoscience (stuff that sounds science-y but is entirely without evidence or goes against evidence) (see page 31).
Shrinking France
Cassini was the first person to accurately determine longitude (the distance either east or west between any one point on Earth and an arbitrary line we call zero, or the Prime Meridian). He used the method that Galileo had earlier suggested, using eclipses of Jupiter’s moons as a clock.
Using this method, France was accurately measured for the first time—and it turned out to be quite a bit smaller than everyone thought it was. The king joked that Cassini had taken away more of his kingdom than he had won in all of his wars.
- Try out this latitude & longitude game.
- There are more latitude & longitude games (such as “Message in a Bottle”) plus much, much more L & L fun at Mr. Donn’s website.
Cassini’s Name Flies Far 
One of the most exciting unmanned space explorations ever is the Cassini orbiter. It has been circling Saturn for six years now, returning measurements of all kinds plus stunning photos of the rings, moons, and planet. Astronomers have been thrill
ed with all the information, and everybody has been thrilled with the amazing images.
- Check out this great space mission here.
- Learn more about Saturn here.
- Take a virtual fly-by of Saturn and its system here.