In January 1610, in his first attempt to apply the newly invented telescope to astronomy, Galileo discovered the four large satellites of Jupiter. He correctly interpreted their motion as being that of objects circling Jupiter—establishing the first clear proof of celestial motion around a center other than the Earth. The discovery of these satellites played an important role in supporting the Copernican revolution that formed the basis for modern astronomy.
A few decades later the satellites of Jupiter were used to make the first measurement of the speed of light. Observers following their motions had learned that the satellite clock seemed to run slow when Jupiter was far from Earth and to speed up when the two planets were closer together. In 1675 the Danish astronomer Ole Roemer explained that this change was due to the finite velocity of light: The satellites only seemed to run slow at large distances because the light coming from them took longer to reach Earth. Knowing the dimensions of the orbits of Earth and Jupiter and the amount of the delay (about fifteen minutes), Roemer was able to calculate one of the most fundamental constants of the physical universe—the speed of light (about 300 000 kilometers per second).
Galileo’s notes summarizing his first observations of the Jovian satellites Io, Europa, Ganymede, and Callisto in January 1610 were made on a piece of scratch paper containing the draft of a letter presenting a telescope to the Doge in Venice. These observations were the result of Galileo’s first attempt to apply the telescope to astronomical research.
The four great moons of Jupiter are called the Galilean satellites after their discoverer. Their individual names—Io, Europa, Ganymede, and Callisto—were proposed by Simon Marius, a contemporary and rival of Galileo. (Marius claimed to have discovered the satellites a few weeks before Galileo did, but modern scholars tend to discredit his claim.) Io, Europa, Ganymede, and Callisto are names of lovers of the god Jupiter in Greco-Roman mythology. Since Jupiter was not at all shy about taking lovers, there are enough such names for the other eleven Jovian satellites, as well as for those yet to be discovered.
In the century following Galileo’s death, improvements in telescopes made it possible to measure the size of Jupiter and to note that it bulged at the equator. The equatorial diameter is known today to be 142 800 kilometers, while from pole to pole Jupiter measures only 133 500 kilometers. For comparison, the diameter of the Earth is 12 900 kilometers, only about one-tenth as great, and the flattening of Earth is also much smaller (less than one percent). By measuring the orbits of the satellites and applying the laws of planetary motion discovered by Johannes Kepler and Isaac Newton, astronomers were also able to determine the total mass of Jupiter—about 2 × 10²⁴ tons, or 318 times the mass of the Earth.
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.