Jupiter and its moons

1.1 Rotation and orbit

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

Jupiter's orbital velocity is about 13 km per second, little more than half that of Mars, allowing Earth to overtake it every 399 days. Jupiter's orbit is less eccentric than Mars's, and its distance at opposition varies between 590 million and 670 million km. Its large size and the reflectivity of its cloud tops make it a consistently conspicuous object, brighter than Mars except when Mars is at its closest opposition distance.

Jupiter rotates in just under ten hours, a shorter period than for any other planet. The value of 9.93 hours quoted in the planetary facts table is the rotation period of the interior, as deduced from the rotation of Jupiter's magnetic field and corresponding periodic variations in radio emissions. The cloud tops that act as Jupiter's visible surface rotate faster than this near the equator, where the cloud top rotation period is 9.84 hours. This corresponds to a westerly (i.e. eastward-blowing) wind of about 150 m per second (540 km per hour!) A notable consequence of Jupiter's rapid rotation is that its shape is perceptibly flattened (Figure 9.1). Its radius measured from centre to pole is only 66,854 km, which is 6.5 per cent less than its equatorial radius.

Figure 9.1: Voyager 1 image of Jupiter showing nearly a full disc[.] The cloud bands run parallel to lines of latitude, and the flattening of the planet's shape towards the poles is apparent[.] The famous Great Red Spot is visible below the centre of the disc[.] The small object above right of Jupiter is its innermost large satellite, lo[.]
NASA NASA

Jupiter's axial inclination is slight, so that the plane of its equator is very close to the plane of its orbit. Jupiter has 40 known satellites. The eight innermost ones orbit almost exactly in the equatorial plane, but the orbits of the next six are inclined at between 27° and 43°. The rest have orbits inclined at between 147° and 165°, so that their motion is retrograde. This family of satellites and its more important members are described later in this chapter.