The length of a day on Mars – the time it takes Mars to complete a rotation on its axis – has been known since 1666. It was even accurate to within 3 minutes!
The length of a day on Saturn was a mystery until 2019.
It was extremely difficult to measure for two reasons. First, because without a solid surface, Saturn had no geographical features by which to measure rotational speed. And Saturn has a funky magnetic field that makes alternative ways of measuring hard. Saturn’s day was only measured when NASA’s Cassini probe studied the planet’s icy, rocky rings and their interaction with the planet.
