Online Astronomy eText: The Planets / The Sky
The Rotation of Saturn
Page last updated Mar 30, 2015

The Overall Rotation of the Planet
     Since Saturn does not have a solid surface it is difficult to define a rotation period for the planet as a whole. Different portions of its "visible surface" represent different circulation systems in its atmosphere and move around its axis at different rates, depending on whether they have westward or eastward motions relative to the overall rotation of the planet. "System I", which refers to the Equatorial region, has a rotation rate of around 10 hours 14 minutes, while "System II", which refers to non-Equatorial regions, has a rotation rate of around 10 hours 39 minutes. There is also a "System III" rotation, which refers to the rate of rotation of the planet's radio emissions, which was once thought to refer to the rotation of the "body" of the planet, and at the time of the Voyager flybys (around 1980) was 10 hours 39 minutes and 22.4 seconds. However, by the time the Cassini spacecraft reached Saturn in 2004 the System III rotation period had increased (meaning the rate of rotation had slowed) to 10 hours 47 minutes and 6 seconds, indicating that it is not associated with the "real" rotation of the planet, because there is no conceivable way that such an immense object could have its real rotation change so rapidly. Subsequent study showed that because the magnetic field of Saturn is almost perfectly aligned with its axis of rotation, it hardly changes its direction relative to the plasma field surrounding the planet, making the radio emissions relatively insensitive to the actual rotation of the planet.
     There is, however, another method of determining the rotation rate: measuring the oblateness (the distension of the equatorial radius relative to the polar radius) caused by the planet's rate of rotation. The calculations required are difficult, because obtaining an accurate value requires accurate estimates of the change in the internal density of the planet as you move from its surface to its center, which cannot be directly determined for a gaseous planet. But by comparing the results of models of Saturn to models of Jupiter, whose rotation rate is very well known, and taking advantage of the fact that the composition of the two planets is undoubtedly more similar than different, a rotation rate was obtained in early 2015 that is thought to be accurate to about 45 seconds: namely, 10 hours 32 minutes and 45 seconds. This is nearly 7 minutes shorter than the Voyager estimate, and over 14 minutes shorter than the Cassini estimate, so although it would be nice to have an accuracy of better than 45 seconds, it seems a considerable improvement. However, once it was realized that the System III rotation rate was probably meaningless, efforts were made to use other methods to determine the rotation rate of the planet, and a compilation of earlier results published a few years ago (10 hours 32 minutes and 35 seconds) differs from the new measurement by only 10 seconds, which is considerably less than the 45 second uncertainty of the new result, so (1) the previous compilation must have been more accurate than anyone had any right to expect, and (2) the new result isn't as spectacular news as might be presumed by someone reading news reports about it. But it is encouraging to know that the new result agrees with the older one, if only because it means there is no need to change the calculations on the page about how rotation rates differ from day lengths (as discussed in the next section).
     Regardless of these uncertainties there is no doubt that Saturn has the second-fastest rotation rate of any planet, both in terms of its rotation period and the speed of its rotation at the Equator, which is just under 10 kilometers per second or about 35000 kilometers per hour (= just under 6 miles per second or about 22000 miles per hour). Only Jupiter has a faster rotational velocity or rotation period.

Difference Between Rotation Period and Day Length
     Since the rotation period of Saturn is uncertain, the difference between its rotation period and day length is for all practical purposes unimportant; but as discussed at Rotation Period and Day Length, given its rapid rotation and long orbital period the difference between the two values is only about one second. So no matter where you are on Saturn, whatever the local rotation period is, the length of the day is one second longer.

The Rotational Tilt of Saturn
     The axis of Saturn's rotation is tilted relative to its orbit by just under 27 degrees. This means that insofar as Saturn has seasons they would be similar to those on the Earth, being relatively minor at the Equator, relatively extreme at the Poles, and in-between at in-between latitudes. However, since Saturn's temperature is over 200 degrees below zero Fahrenheit, even its warmest summers are very cold by Earthly standards, and seasons as we know them probably do not exist.
     Despite the lack of obvious seasonal effects, the tilt of Saturn's axis does produce a very easily observable effect, because the ring system of the planet and the orbits of virtually all of the planet's moons are very closely aligned to the planet's Equatorial plane. Therefore, as discussed at The Rings of Saturn, when we see Saturn near one of its polar summers we see the rings from above or below as a broad, spectacular oval; whereas when we see Saturn near the start of its spring or fall (as we did in 2008 and 2009), the rings are barely visible due to our nearly edge-on view. In addition, there have been exceptionally large storms observed on Saturn at times corresponding to its northern hemisphere summer, but since the year on Saturn is 30 Earth years long, very few of those possibly seasonal changes have been more or less accurately observed, and whether they are actually seasonal, or just accidentally aligned with the seasons, is as yet unknown.