Every 24 hours, a day passes as Earth rotates around its axis. On Mercury, a day is 58 Earth days long, while speedy Jupiter makes a full rotation in 9 hours and
56 minutes. But for years, no one knew for sure how long a day lasts on Saturn.
Now it’s official: a day on Saturn is 10 hours and 33 minutes. The Cassini space probe made this discovery possible. It spent 3 years collecting data on Saturn. “We could see the system really close up,” says Bonnie Meinke, a planetary astronomer at the Space Telescope Science Institute.
To measure a day on a rocky planet, astronomers pick a distinct characteristic and track it. For most gas giants, they can look at the planet’s tilted magnetic field and calculate the length of a day from its wobble. But Saturn’s surface has no distinct characteristics and its magnetic field perfectly lines up with its axis—so no wobble to study!
The data from Cassini showed a rippling pattern in Saturn’s rings. The pattern was caused by Saturn’s pull as it spun. Astronomers used this ripple to calculate the length of a day on Saturn. “Looking at the rings, those small details give us big clues,” says Meinke.