Earthbound Views Jupiter and Saturn

In contrast to Uranus and Neptune, Jupiter and Saturn make for easy viewing. On a good, dark night, even a quite modest telescope will reveal the planets' belts. The use of colored filters can enhance bands in Jupiter's atmosphere. Moreover, Jupiter rotates so fast (its day consumes a mere ten hours) that any details you see will perceptibly move across the planet's face if you observe long enough. Its rapid rotation also makes the planet appear noticeably oblate (elongated). It is even possible to observe the near moons (like Io) emerging from behind Jupiter as they orbit.

Although smaller and nearly twice as distant as Jupiter—and therefore appearing much smaller and dimmer than the larger planet—the sight of Saturn through a refractor of at least a 4-inch aperture or a reflector with at least a 6-inch aperture is thrilling. Expect to see the planetary disk and its belts and zones, as well as its celebrated rings (discussed later in this chapter). You may even catch a glimpse of the moons, including Titan, brightest and biggest of Saturn's nine moons (which we will discuss in the next chapter). Titan's atmospheric pressure is similar to Earth's, although its composition and temperature are different. Titan is slightly larger in diameter than the planet Mercury.

Close Encounter

The dark brownish strips across Jupiter are belts, the brighter strips are zones. Belts are dark, cooler regions, settling lower into the atmosphere as part of a convective cycle. Zones are regions of rising hot atmospheric gas. The bands are the result of regions of the atmosphere moving from high pressure to low pressure regions (much as they do on the earth). The rapid rotation of Jupiter confines this movement to narrow belts. The planet does have an atmospheric geography that can be mapped:

V The light-colored central band is the equatorial zone. It may appear white, orange, or yellow.

V The Great Red Spot, a hurricane that has been observed south of the equatorial zone since the invention of telescopes, has a diameter approximately twice that of the earth.

V On either side of the equatorial zone are dark bands called the north and south equatorial belts. At times, you may witness a south equatorial belt disturbance: an atmospheric storm.

V North and south of the equatorial belts are the north and south temperate belts.

V At the extreme northern and southern ends of Jupiter are the polar regions, which are sometimes barely perceptible and sometimes quite apparent dark areas.

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