Another distinguishing characteristic of the giant planets is their rapid rotation; their days are short, ranging from 10 to 17 hours. The rapid rotation of the giant planets distorts their shapes. If they did not rotate at all, these fluid bodies would be perfectly spherical. Rapidly rotating planets bulge at the equator and have an overall flattened appearance. This flattening is called oblateness, and it is often expressed as a fraction of how flattened the sphere is—that is, the difference between a planet’s equatorial radius (RE) and its polar radius (RP), divided by its equatorial radius. A perfect sphere has an oblateness of 0, while a flat disk has an oblateness of 1. Saturn’s oblateness is especially noticeable, because its equatorial diameter is almost 10 percent greater than its polar diameter (see Figure 10.3 and Table 10.1). In comparison, the oblateness of Earth is only 0.3 percent.