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Astronomers have been working to determine the length of Saturn’s day since William Herschel published “On the rotation of the Planet Saturn upon its Axis” in 1794 (the same year that Kentucky and Vermont got their stars added to the US flag, and Eli Whitney patented the cotton gin). Herschel’s value was 10h 16m, which is close to the modern “System I” reference frame, which is useful for Saturn’s fast equatorial jet stream. Astronomers soon realized the midlatitude jets had a longer rotation period, which led to System II. But the most important reference frame is the bulk rotation of the planet itself, and this has been extremely difficult to pin down.

The problem is that gas giants are just that, all gas, such that there are no mountains or other solid features to track. And, both Jupiter and Saturn have alternating jet streams. For Jupiter, the planet’s magnetic field is tilted like on Earth, and in situ spacecraft like Voyager and Galileo have accurately determined its internal rotation rate, called System III. Saturn’s magnetic field is quite different, with no measurable tilt in its external field. Although its internal field is probably tilted, strong differential motion of a conductive layer is thought to be responsible for shielding this from view. During the Voyager era in the 1980s, the fall-back plan was to use Saturn Kilometric Radiation (SKR), which are radio waves from the magnetosphere. This initially led to 10h 39m, which is still the official IAU rotation rate for Saturn. However, Cassini found this signal drifts all the way to 10h 45m, and in fact is different in the northern and southern hemispheres, so it cannot yield Saturn’s rotation period.

In 2009, Read (Oxford), Dowling (UofL), and Schubert (UCLA) realized that for Saturn’s alternating jet streams to be stable, the longest atmospheric waves, called Rossby waves, had to be stationary in the planet’s rotation frame. By using Cassini data to determine cloud-tracked wind speeds and temperature fields, they determined a rotation rate of 10h 34m, which is now called the System IIIw rotation period (“w” for “wave”). These atmospheric waves are affected by large mass anomalies deep inside Saturn.

Back in 1982, planetary scientist David Stevenson (Caltech) realized that Saturn’s rings are a sensitive seismograph, if only faint waves in the inner (C) ring (see figure) could be observed. Cassini spent 13 years in orbit around Saturn and was able to observe dozens of such ring waves. In 2019, a group led by Chris Mankovich (UCSC) modeled these and came up with a planetary rotation rate of 10h 34m, independently confirming the value obtained 10 years earlier via atmospheric waves. The effort now is to refine this fundamental measurement down to the seconds.

Watch also the interview with Prof. Dowling on WDRB - 01/27/2019

The interview with Dr. Dowling starts at 13:08 minutes.