HISTORY OF SCIENCE AND TECHNOLOGY

GIOVANNI DOMENICO CASSINI (1625 - 1712)

Giovanni Cassini was born in Perinaldo, now in France but in 1625 it was part of Italy. He was schooled in astronomy and mathematics by the Jesuits before taking up the post as Astronomy Professor at Bologna in 1650. Louis XIV of France invited Cassini to Paris in 1669. Cassini became a member of the Academie Royale des Sciences and was promoted to director of the Paris Observatory in the same year. He became a French citizen when he married in 1673.

Cassini is credited with many discoveries. He was the first to observe four of Saturn’s moons; Iapetus in 1671, Rhea in 1672 and Dione and Tethys in 1684. He realised from his observations that Iapetus did not rotate on its axis; it always shows the same side to Saturn.

Turning his telescope on Saturn, Cassini observed that its rings are split into two parts, separated by a gap. The outer ring, known as the A ring is darker than the inner ring, or B ring. This dark gap is now known as the Cassini Division.

In 1662 Cassini published his Tables of the Sun. This work showed precise measurements of the Sun’s movement as observed from Earth. His observations also allowed him to calculate the distance from Earth to Mars and from the Earth to the Sun. He used these distances to make more accurate calculations of the dimensions of the Solar System. This, in turn, allowed him to find the value of the Astronomical Unit (AU). It is to his credit that Cassini’s value of the AU was only 7% short of today’s value.

Cassini also measured Jupiter’s rotation and its day-length. He plotted the movement of the Jovian moons by observing the moons’ shadows on the surface of the planet. His calculations were so precise that they were used by his colleague, Ole Roemer, to help him determine the speed of light in 1675. Cassini was also involved in the publication of a Moon map in 1692. He died in Paris in 1712.

Cassini has the Cassini Crater on the Moon named after him. A Cassini Crater also exists on Mars and Cassini Regio is a cratered region on Iapetus that almost covers one half of the satellite. There is an asteroid named Cassini as well as a spacecraft that was launched by NASA and ESA in 1997, reaching its destination, Saturn, in 2004.

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