Thursday, December 31, 2009

Rotation: A New Twist

With all the bands and surface features of the biggest jovian planets, you’d think it would be relatively easy to calculate rotation rates “by eye.” Just look for a prominent surface feature and time how long it takes that feature to make one trip around.
Well, it’s not so easy. Because these planets lack solid surfaces, different features on the surface actually rotate at differing rates! This differential rotation is not dramatic in the case of Jupiter whose equatorial region rotates only slightly faster than regions at higher latitudes. East-west winds move at about 190 miles per hour (300 km/h) in Jupiter’s equatorial regions, and at a zippy 800 miles per hour (1,300 km/h) in the equatorial regions of Saturn. It turns out that the best way to clock the rotation rates of these planets is not to look at their atmospheres, but to measure something tied to the planets’ cores. The periods of fluctuation in the radio emission (which arise from the planets’ magnetic fields) are taken to be the “true” rotation rate.
While Neptune and Saturn are slightly tipped on their axes similar to the earth (30, 27, and 24 degrees, respectively), Jupiter’s axis is nearly perpendicular to the plane of its orbit; the planet tilts from the perpendicular a mere 3 degrees. The true oddball in this respect is Uranus, which tilts 98 degrees, in effect lying on its side. The result of this peculiarity is that Uranus has the most extreme seasons in the solar system. While one pole experiences continuous daylight for 42 Earth years at a stretch, the other is plunged into an equal period of darkness.
It’s interesting to note that if the earth were tipped on its axis like Uranus, a city like Atlanta would experience 70 days when the sun never rose, and 70 days when the sun never set. The North Pole would have 6 months of darkness, and 6 months of sunlight.
On the vernal and autumnal equinoxes, day and night in Atlanta would still each last 12 hours.