Jupiter (CML)

Central meridian longitude · three rotations, one planet

Jupiter is not solid and does not turn as one body, so astronomers keep three clocks for it at once. System I covers the fast equatorial belt, turning in 9h 50m 30s; System II the temperate latitudes on either side, where the Great Red Spot sits, at 9h 55m 41s; System III the magnetic field at 9h 55m 30s, which the IAU treats as the planet's true rotation because the field is anchored to the interior rather than the visible cloud. What each system reports is the central meridian longitude — the line of Jupiter facing Earth this moment. Observers use it to know when a feature will next come round, and the two-minute gap between System I and System II is the visible fact that the equator overtakes the rest of the planet.

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