... Tonight's Sky . ... Clocking Neptune's spin . ... By tracking atmospheric features on Neptune, a University of Arizona scientist has accurately determined the planetт??s rotation, a feat that had not been previously achieved for any of the gas planets in our solar system except Jupiter. A day on Neptune lasts precisely 15 hours, 57 minutes, and 59 seconds, according to the first accurate measurement of its rotational period made by Erich Karkoschka from University of Arizona, Tucson. ...