Updated: May 24, 2019
Ripples in Saturn's rings tell how long a day is on Saturn. Despite being nine times the size of Earth, Saturn's days are less than half as long as Earth's. In 1993 two astronomers named Mark Marley and Carolyn Porco realized something about Saturn’s rings. “Saturn’s seismic activity changes the planet’s gravity enough to make its rings of tiny, orbiting particles shift in response. Because rotation stirs up the planet’s gas, the frequency of those inner oscillations could be used to figure out how fast the planet spins.”(Science News). Cassini, a spacecraft orbiting Saturn was finally able to get images good enough to test Marley and Porco’s theory. When Saturn vibrates, it creates waves in its wings. NASA scientists were able to measure the waves and use the measurements to calculate the speed of Saturn's rotations.
Info and picture found at sciencenews.org