Giovanni Domenico Cassini | Biography

Cassini was born in Perinaldo, Italy, near Nice, France. He studied with the Jesuit priests and astronomers Giovanni Riccioli (1598-1671) and Francesco Grimaldi (1618-1663) before becoming an astronomy professor at the University of Bologna at the age of twenty-five.

While at Bologna Cassini observed the moons of Jupiter carefully enough for an extended time to publish an ephemeris, a table which tracked the moons in their orbits. Other astronomers using the ephemeris noticed a discrepancy: when the Earth and Jupiter were far apart, the moons appeared to take longer to pass in front of Jupiter than Cassini had listed. Some scientists postulated that the light reflected from the moons needed extra time to travel the increased distance between the Earth and Jupiter. Working from this postulation in 1676, Olaus Roemer used Cassini 's ephemeris to calculate the speed of light.

In 1669 King Louis XIV of France invited Cassini to direct his new Paris Observatory. Between 1671 and 1684 Cassini discovered four moons of Saturn in addition to Titan, which Christiaan Huygens had detected in 1656. He was also the first to notice a dark gap that split the famous ring around Saturn. It has since been called the "Cassini Division." He also correctly predicted the rings were made of objects too small to be seen separately.

Cassini is best known for his calculations concerning the size of the solar system. He first established a parallax of Mars based on the observations of the planet he simultaneously made in Paris with those made by his colleague Jean Richer (1630-1696) in French Guiana. This value of the distance from the Earth to Mars allowed Cassini to calculate the astronomical unit (a.u.) or the distance from the Earth to the Sun. His figure of eighty-seven million miles is about seven percent too low. However, when compared to his predecessors' estimates (Tycho Brache believed the distance to be five million miles while Johannes Kepler estimated it at fifteen million miles), it is clear that Cassini's astronomical unit gave the world its first accurate assessment of the vast size of the solar system.

Despite his contributions, Cassini remained notoriously conservative in his scientific outlook and was one of the last of his generation to accept Nicholas Copernicus's heliocentric view of the solar system. Cassini retired from the Paris Observatory after going blind in 1710, and was succeeded in his post by his son and later his grandson.

On October 17, 1997 an unmanned spacecraft blasted off from Kennedy Space Center, headed for Saturn. It will arrive at Saturn in 2004, and will carry out an ambitious program of observations of the planet and its moons. It will also release probes to study the atmosphere and surface of Titan. Appropriately enough, it is named "Cassini."