Sept. 17, 1789: William Herschel discovers Mimas

Today in the history of astronomy, the seventh moon of Saturn is discovered.
By | Published: September 17, 2025

Gemini Sparkle

Key Takeaways:

  • As of March 2025, Saturn possesses 274 confirmed moons, the highest number among planets in the solar system.
  • Early discoveries, beginning with Titan in 1655, were limited to five moons by the late 18th century, with William Herschel adding Enceladus and Mimas in 1789.
  • The Voyager missions (1980s) and the Cassini mission (2005-2015) significantly increased knowledge of Saturn's moons.
  • Recent research (2024) suggests the possibility of subsurface oceans on both Enceladus (previously known for its geological activity) and Mimas.

With 274 confirmed moons as of March of 2025, Saturn is the solar system planet with the most natural satellites. By 1789, though, only five had been spotted: Titan by Christiaan Huygens in 1655, and Iapetus, Rhea, Dione, and Tethys by Jean-Dominique Cassini between 1671 and 1684. William Herschel would add two more to that count, using his 40-foot reflector to discover Enceladus on Aug. 28, 1789, and Mimas on Sept. 17 of the same year. His son, John Herschel, later named both moons for the Titans of Greek mythology. For centuries, details about the moons were scant, as they were little more than specks in a telescope. Voyager 1 and 2 supplied more information in the 1980s, and the Cassini mission far more during its tour (2005-2015), helping us understand the contrasting worlds. Enceladus is highly geologically active, with a subsurface ocean and erupting geysers. While the heavily cratered and unchanging Mimas was long thought to be inactive, research published in 2024 suggests Mimas also may have a hidden ocean under its surface.