The world has known many great astronomers, but only a few great astronomy dynasties. One of those celebrates an anniversary tomorrow – the birth of its patriarch 400 years ago.

Giovanni Cassini was born in a small Italian village just across the border from Nice, France. Cassini the first, as he’s often called, was the first of four generations of Cassini astronomers. And he definitely was the most productive.

He’s best known for his discovery of a gap in the rings of Saturn. It’s named the Cassini Division in his honor. Later, Cassini and his son Jacques theorized that the rings were made of “swarms of tiny satellites” moving at different speeds. They understood the truth many years before it was actually confirmed.

More about Cassini and Saturn’s rings tomorrow.

Cassini made many other contributions to astronomy. He was among the first to realize that light travels at a limited speed, although he didn’t believe the speed could be calculated. And he developed a law to explain why we always see the same side of the Moon: The Moon takes the same amount of time to orbit Earth as it does to complete one turn on its axis.

For most of his career, Cassini was director of Paris Observatory. His descendants kept the job in the family for more than 120 years. Cassini the fourth ended the streak in 1793 – when he left the observatory to write the “stellar” history of this astronomical dynasty.

Script by Laura Tuma