Cassini, using an improved telescope, observed a large feature, now called the Great Red Spot, in the southern hemisphere of Jupiter. He used the motion of this feature around the planet to measure the rotational rate of Jupiter to a high degree of accuracy.

The Great Red Spot on Jupiter has puzzled astronomers ever since it was first seen in 1665. The development of the telescope and improvements in the resolution of telescopes made it possible to see the Spot by the middle of the seventeenth century. The size and the color of the Spot have varied significantly since it was first observed. [kw]Cassini Discovers Jupiter’s Great Red Spot (1665)
[kw]Red Spot, Cassini Discovers Jupiter’s Great (1665)
[kw]Great Red Spot, Cassini Discovers Jupiter’s (1665)
[kw]Jupiter’s Great Red Spot, Cassini Discovers (1665)
Astronomy;1665: Cassini Discovers Jupiter’s Great Red Spot[2180]
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Cassini, Gian Domenico
Astronomy;Jupiter’s Great Red Spot

The English physicist Robert Hooke Hooke, Robert first reported seeing a large oval-shaped feature in the southern hemisphere of Jupiter in 1664. The oval seen by Hooke is believed to have been the Spot, although Hooke did not mention its color. Credit for the discovery of the Spot is generally given to the Italian astronomer Gian Domenico Cassini, who observed it in 1665. At that time, Cassini was a professor of mathematics and astronomy at the University of Bologna, Italy. Cassini had seen spots on Jupiter beginning in 1664, but he quickly realized that these spots were actually the shadows of Jupiter’s largest moons (which Galileo had observed at the beginning of the century). Then Cassini observed an “exceptional” spot,” which he called the “big permanent spot.”

Cassini’s many astronomical discoveries were possible because he was able to observe the sky with new, powerful telescopes Telescope made by Giuseppe Campani Campani, Giuseppe of Rome. Campani, and his brother, Matteo Campani-Alimensis, were experts in grinding and polishing lenses, especially lenses having a very long focal length and only small curvature. Because of their small curvature, these lenses did not suffer from the same optical problems that lenses with sharper curvatures exhibited; thus they provided clearer views of objects in the sky. Campani’s telescopes employed these long-focal length lenses to greatly magnify images, allowing a planet’s details to be seen. Cassini used the telescope built by Campani to study the planets Jupiter, Saturn, Mars, and Venus. Beginning in 1664, Cassini made many important discoveries, which were possible only because of the great magnification and image clarity of Campani’s new telescope.

Gian Domenico Cassini.

(Library of Congress)

Jupiter’s Spot has been continuously present since the time it was discovered by Cassini, and additional spots have been observed as the quality of telescopes improved. The Spot is located in Jupiter’s southern hemisphere, about 22 degrees south of the planet’s equator. It is a gas planet, composed mainly of hydrogen and helium. Thus, the Spot is not actually a spot on the surface of Jupiter, but, rather, a storm high in Jupiter’s atmosphere. What appears to be Jupiter’s surface, when viewed from Earth, is actually a layer of clouds. There are many colored bands and spots visible in these high clouds near the top of Jupiter’s atmosphere, with the Spot being the largest and most easily visible of these features.

The Spot is believed to be a giant, hurricane-like storm, caused by interactions between high and low temperatures and pressures, as are hurricanes on Earth. The top of the clouds in the Spot extend about 5 miles higher than nearby clouds, and they are cooler. On Earth, hurricanes are much smaller in size then the Spot, and they last for only a few days. In addition, hurricanes on Earth are “cyclonic,” that is, they are low-pressure systems. The Spot is “anticyclonic,” that is, it is a high-pressure system. On Earth, hurricanes weaken considerably when they pass over land, so some scientists speculate that the Spot persists because it does not pass over “land.” Other scientists suggest that Jupiter’s internal heat source continues to provide energy to this giant storm, allowing it to persist for centuries.

The Spot varies in both size and color from year to year. It is an oval measuring about 17,000 miles long and 9,000 miles wide, so large that it could contain three Earths. The Spot rotates counterclockwise, with a period of 6 days. Similar structures have been seen in the atmospheres of Saturn and Neptune.

Once Cassini recognized that the Spot traveled around the planet as Jupiter rotated, he knew he could measure how long it took for the Spot to travel completely around the planet and thus determine the planet’s period of rotation on its axis. The value he obtained for Jupiter’s rotation period was 9 hours and 56 minutes, results he published in 1665. Cassini’s value is within a few minutes of the best value obtainable with modern instruments.

Cassini continued to observe Jupiter throughout his career. In about 1690, he was the first person to report that Jupiter’s atmosphere displayed “differential rotation,” the motion of some features around the planet at slightly different rates than others. Cassini may also have seen the effects of a comet impacting Jupiter. Between December 5 and December 23, 1690, Cassini observed a feature that appeared in the planet’s atmosphere. That feature is similar to features observed in 1994, when more than twenty fragments of the comet Shoemaker-Levy 9 hit the planet. Japanese astronomers Isshi Tabe and Junichi Watanabe have interpreted Cassini’s drawings to indicate that he observed the effects of a similar comet impact in 1690.

It was not until 1878 that Jupiter’s Great Red Spot was named. It had changed into a very intense red color, a change noticed by many observers around the world. The colors in Jupiter’s clouds are still not completely understood, however. Scientists believe the reddish color results from chemical compounds containing sulfur and phosphorus, but the reason why the colors keep changing is not understood.

The Spot has persisted for more than three hundred years, providing evidence for how long storms can last in the atmosphere of Jupiter. The survival of a single storm for this long has forced planetary scientists to develop new ideas about how storms develop, evolve, and survive, since their persistence on gas giant planets clearly is much different from storms on rocky planets.

Cassini’s discovery of the Spot allowed him to develop techniques to obtain precise measurements of the rotation rates of Jupiter, Saturn, and Mars by observing the time it takes for a feature to move completely around the planet. These techniques are still employed by contemporary astronomers to measure rotation rates of planets, moons, and asteroids. In recognition of Cassini’s studies of gas giant planets, the National Aeronautics and Space Administration (NASA) named its Saturn orbiter the Cassini spacecraft, which was launched in October, 1997.

• Asimov, Isaac. Jupiter, the Largest Planet. New York: Ace, 1980. A 247-page, well-illustrated account of Jupiter, from the earliest discoveries of its distance, size, and satellites to recent discoveries regarding its atmosphere, composition, and the Spot.
• Beatty, J. K. “A Comet Crash in 1690?” Sky and Telescope 93 (April, 1997): 111. Summarizes a new Japanese interpretation of Cassini’s 1690 sketches of Jupiter, suggesting he saw the results of a comet impact.
• Débarbat, S., and C. Wilson. “The Galilean Satellites of Jupiter from Galileo to Cassini, Rømer, and Bradley.” In Planetary Astronomy from the Renaissance to the Rise of Astrophysics. Part A, edited by René Taton and Curtis Wilson. New York: Cambridge University Press, 1989. An excellent account of Cassini’s contributions to the measurement of the positions and eclipses of the moons of Jupiter.
• Rogers, J. H. The Giant Planet Jupiter. New York: Cambridge University Press, 1995. A comprehensive account of the discoveries about Jupiter and its satellites, written for a general audience. Begins with a review of the telescopic observations and continues through the modern era of results from spacecrafts.
• Schorn, Ronald A. Planetary Astronomy: From Ancient Times to the Third Millennium. College Station: Texas A&M University Press, 1999. Includes sections describing Cassini’s numerous contributions to the observations of the planets, particularly Jupiter and Saturn.

Gian Domenico Cassini; Galileo; Robert Hooke; Christiaan Huygens; Johannes Kepler. Cassini, Gian Domenico
Astronomy;Jupiter’s Great Red Spot