Once the size and mass are known, it is possible to calculate another fundamental property of a planet—its density. The density, which is the mass divided by the volume, provides important clues to the composition and interior structure of a planetary body. The density of Earth, a body composed primarily of rocky and metallic materials, is 5.6 times the density of water. The mass of Jupiter is 318 times that of Earth; its volume is 1317 times that of Earth. Thus Jupiter’s density is substantially lower than Earth’s, amounting to 1.34 times the density of water. From this low density, it was evident long ago that Jupiter was not just a big brother of Earth and the other rocky planets in the inner solar system. Rather, Jupiter is the prototype of the giant, gas-rich planets Jupiter, Saturn, Uranus, and Neptune. These giant planets must, from their low density, have a composition fundamentally different from that of Mercury, Venus, Earth, Moon, and Mars. Jupiter Through the Telescope Jupiter is a beautiful sight seen with the naked eye on a clear night, but only through a telescope does it begin to reveal its magnificence. The most prominent features are alternating light and dark bands, running parallel to the equator and subtly shaded in tones of blue, yellow, brown, and orange. However, these bands are not the planet’s only conspicuous markings. In 1664 the English astronomer Robert Hooke first reported seeing a large oval spot on Jupiter, and additional spots were noted as telescopes improved. As the planet rotates on its axis, such spots are carried across the disk and can be used to measure Jupiter’s speed of rotation. The giant planet spins so fast that a Jovian day is less than half as long as a day on Earth, averaging just under ten hours. During the nineteenth century, observers using increasingly sophisticated telescopes were able to see more complex detail in the band structure, with wisps, streaks, and festoons that varied in intensity and color from year to year. Furthermore, observations revealed the remarkable fact that not all parts of the planet rotate with the same period; near the equator the apparent length of a Jovian day is several minutes shorter than the average day at higher latitudes. It is thus apparent that Jupiter’s surface is not solid, and astronomers came to realize that they were looking at a turbulent kaleidoscope of shifting clouds. Although the face of Jupiter is always changing, some spots and other cloud features survive for years at a time, much longer than do the largest storms on Earth. The record for longevity goes to the Great Red Spot. This gigantic red oval, larger than the planet Earth, was first seen more than three centuries ago. From decade to decade it has changed in size and color, and for nearly fifty years in the late eighteenth century no sightings were reported, but since about 1840 the Great Red Spot has been the most prominent feature on the disk of Jupiter.