Equatorial Radius of Jupiter

A gas giant has no surface, so its radius is quoted at a chosen pressure level — conventionally where the atmosphere reaches one bar. Jupiter spins once in 9 h 56 m, the fastest of any planet, and the resulting bulge is visible in any decent telescope: the equatorial radius exceeds the polar by 4638 km, a flattening of 1/15 against Earth's 1/298.

Because the rotation is differential, with the equator circulating faster than mid-latitudes, planetary scientists use System III, tied to the rotation of the magnetic field, as the definitive rotation period. Eleven Earths would fit across the disc, and the Great Red Spot alone has been wider than our planet — though it has been shrinking for a century and a half.