Key Takeaways:
- Between 1671 and 1684, Giovanni Cassini identified four moons of Saturn—Iapetus, Rhea, Tethys, and Dione—collectively designating them "Sidera Lodoicea."
- Rhea, discovered on December 23, 1672, is notable as the second largest of Saturn's moons and the fifth closest to the planet.
- Cassini's discoveries were predated only by Titan (1655) and were not succeeded by further moon findings for over a century, until William Herschel's observations in 1789.
- Subsequent discoveries, including 128 small moons confirmed in March 2025, have cumulatively increased Saturn's known moon count to 274.
Across the span of 1671 through 1684, Italian-French astronomer Giovanni Cassini discovered four moons of Saturn while working under the patronage of King Louis XIV. Cassini collectively named the four “Sidera Lodoicea” – the “Stars of Louis.” But they’re better known as Iapetus, Rhea – his second find, discovered Dec. 23, 1672 – Tethys, and Dione. Rhea is the fifth closest moon to its planet, and the second largest of Saturn’s moons.
Only Titan, discovered by Christaan Huygens in 1655, predated Cassini’s finds, and after his 1684 discovery of Dione, none would be found until William Herschel spotted Mimas and Enceladus over a century later, in 1789. Hyperion followed in 1848 and Phoebe in 1898, with groups of moons and individuals added regularly since then. Most recently, in March of 2025, 128 small moons around the planet were confirmed, bringing Saturn’s total moon count to 274.
