The rotation period of Uranus

The recent fly-by of Uranus by the Voyager 2 spacecraft provided a singular opportunity to measure one of the fundamental but poorly known physical properties of the planet, its intrinsic rotation period. Earth-based photometric and spectroscopic estimates of the `atmospheric' period vary greatly, with values ranging from ~12 to 24 h (refs 1-4) estimates of the period based on the dynamical flattening of a rotating body range from ~15 to 17 h (refs 5, 6). Here we use the Voyager planetary radioastronomy⁷ and magnetometer⁸ observations at Uranus to derive a period of 17.24 +/- 0.01 h. The greatly improved precision of this measurement provides useful constraints on models of the planet's internal structure.