Today in the history of astronomy, Saturn’s ‘yin/yang moon’ is discovered.

| Published: October 25, 2026

This portrait of Giovanni Cassini poses him in front of the ongoing constuction at the Paris Observatory, and the Campani refractor that he used to discover Iapetus. Credit: L. Cossin, CC BY-SA 4.0, via Wikimedia Commons

In 1669, French-Italian astronomer Giovanni Cassini arrived at the still-under-construction Paris Observatory as its new director. He quickly talked Louis XIV’s minister of finance into buying a 34-foot Campani refractor for the observatory, and on Oct. 25, 1671, used that refractor to discover Saturn’s moon Iapetus . The second saturnian moon discovered, after Christaan Huygens’ 1655 Titan find, Iapetus appeared brighter at some parts of its orbit and darker at others.…