What is the length of a day on Jupiter in Earth hours?

A day on Jupiter lasts about 9.9 Earth hours. More precisely, Jupiter completes one rotation in roughly 9 hours 55 minutes, making it the shortest day of any planet in the Solar System.

Jupiter’s rapid spin comes from its immense mass and the conservation of angular momentum during its formation. It is a gas giant, so its atmosphere does not rotate as one perfectly solid surface; different latitudes move at slightly different speeds. Astronomers therefore use several rotation systems when tracking cloud bands and storms.

The 9.9-hour figure describes Jupiter’s rotation relative to the distant stars, called its sidereal rotation period. It is not the same as a solar day measured by the Sun’s apparent position, though the difference is small. The Great Red Spot and other atmospheric features can also have their own circulation periods, which should not be confused with the planet’s day. Jupiter’s fast rotation helps flatten it slightly at the poles and contributes to powerful jet streams and its enormous magnetic environment.