Saturn has an equatorial diameter of 120,536 km, 9.44 times that of Earth. That makes it the second largest planet in our Solar System, trailing only Jupiter. Saturn, like all of the other planets, is an oblate spheriod. This means that its equatorial diameter is larger than is diameter measured through the poles. In the case of Saturn this distance is quite a bit different due to the planet’s high rotational speed. The polar diameter of Saturn is 108,728 km, meaning that it is flattened by a factor of 9.796%.
Scientist know that Saturn rotates very quickly, but the exact speed of that rotation has been hard to determine because of the thick clouds in the atmosphere. With terrestrial planets, scientists are able to find surface features and basically time how long it takes for that feature to reappear in the same position. This is a simplified description of how they determine rotational speed. The problem with Saturn is that the surface can not be observed. To make things even more difficult, the visible features of the planet’s atmosphere rotate at different speeds depending on their latitude.
The atmosphere of Saturn is broken down into systems. System I is the equatorial zone has a rotational period of 10 hours and 14 minutes. System II encompasses all other areas of Saturn and has a rotational speed of 10 hours 38 minutes and 25.4 seconds. System III is based on radio emissions and has mostly replaced the use of the term System II. It has a rotational speed of 10 hours 39 minutes and 22.4 seconds. Despite these numbers, the rotational speed of the planet’s interior is currently impossible to measure precisely. The Cassini spacecraft found the radio rotational speed of Saturn to be 10 hours 45 minutes and 45 seconds. In 2007, it was determined that the varying radio emissions from the planet did not match Saturn’s rotation rate. Some scientists think that the variance is due to geyser activity on the Saturnian moon Enceladus. The water vapor from these geysers enter Saturn’s orbit become charged, thus creating a drag effect on Saturn’s magnetic field. This slows the magnetic field’s rotation slightly compared to the rotation of the planet. The current estimate of Saturn’s rotation is based on various measurements from the Cassini, Voyager and Pioneer probes. That estimated speed is 10 hours 32 minutes and 35 seconds as of September 2007.
Again, the equatorial diameter of Saturn is 120,536 km and its polar diameter is 108,728 km. It is very important to understand why the difference in these diameters is so large, that is why so much detail is given on the rotational speed of the planet. You can take many of the same factors into account when thinking about all of the gas giants.
Here’s an article about how long a day is on Saturn, and another article about how the storms never end on Saturn.
Here’s Hubblesite’s News Releases about Saturn, and more information from Solar Views.
We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.
Source: NASA