Why the Length of Saturn's Day is So Hard to Measure 

By Sarah Zhang on at

The length of a planet's day seems like such a simple number, a basic piece of information that should accompany any solar system diagram. But probes flying past Saturn have turned up oddly conflicting numbers.

Scientist have now calculated the most definitive length of Saturn's day yet, which is 6 minutes shorter than previously thought.

It turns out that measuring how fast Saturn spins is tricky, especially compared to the other gas giant, Jupiter. Ken Croswell explains how Jupiter's spin was easily calculated in Science:

No one disputes Jupiter's spin rate of 9 hours and 55 minutes, because deep beneath that planet's atmosphere, its core generates a magnetic field, the axis of which is tilted compared with the spin axis; therefore, as the planet turns, the magnetic field sweeps around like the beam of a lighthouse, sending out radio waves that make it easy to measure how fast the hidden core spins.

Astronomers are out of luck with Saturn though, whose magnetic and spin axes are aligned. Scientists writing in Nature report how they calculated the planet's rotation in a more roundabout way. Saturn's equator bulges at it spins, distorting its gravitational field. By looking at gravity distortion data collected by the space probe Cassini, the team could go back and calculate the length of Saturn's day: 10 hours and 32 minutes and 45 seconds with an uncertainty of 46 seconds.

Why does this matter? Croswell notes it completely changes what we know about winds on Saturn. Wind speeds are measured relative to the planet's rotation. Saturn's old rotation seemed to imply winds blew only one way on Saturn, which is pretty bizarre. The new number suggests that mystery was simply created out of human error. [Nature, ScienceNow]

Top image: NASA/JPL/Space Science Institute