The apparent displacement of an object across the sky when seen from two separate locations is called its parallax. The distance of the object can be measured if a suitable baseline of known length is used, the directions of the object being measured from each end of the baseline (triangulation).
Giovanni Domenico Cassini (1625-1712) was born in Perinaldo in Liguria in northwest Italy. He studied mathematics and astronomy at the Jesuit College (est.1640) at Genoa. In 1645 he was offered a position in the Panzano Observatory at Liguria and in 1650 he became professor of astronomy at the University of Bologna. Beginning in 1664 he gained access to a series of greatly improved telescopes. He measured the rotation periods of Mars and Jupiter, discovered four of Saturn’s moons (Iapetus, Rhea, Tethys and Dione), and in 1675 observed the gap in Saturn’s rings named after him.
In 1669 at the instigation of his chief minister Jean Baptiste Colbert (1619-83), Louis XIV (76; r.1643-1715) invited Cassini to Paris to oversee the construction of the Paris Observatory. On its completion in 1671 Cassini was given the post of its new head. He then decided to become a French citizen and changed his name to Jean-Dominique Cassini.
Cassini knew that in August and September of 1672 Mars would be in opposition, i.e. at its nearest to Earth and thus provide optimum conditions for measuring its distance. The parallax of celestial objects is small and so the observations are required to be from widely separate locations (geocentric parallax). It happened that that time Colbert was trying to establish a colony at the mouth of the Cayenne River in South America and so ships were sailing there regularly. In 1671 Jean Richer (1630-96) was sent to Cayenne with instructions to make observations leading to the measurement of the parallaxes of the Moon, Sun, Venus and especially of Mars.
Because Mars is moving, Cassini and Richer had to time their measurements precisely, i.e. they had to know how time in Cayenne compared with time in Paris. While at the University of Bologna, Cassini had studied Jupiter’s moons and drawn up improved tables of their motions. In 1666 he had noticed that a moon’s reappearance from behind Jupiter would be seen simultaneously from any point on Earth and could therefore serve as a time signal.
Richer returned in August 1673. When Cassini had completed his analysis of the data, he reported that from the parallax of Mars and using Kepler’s Laws he had derived a distance to the Sun of 87 million miles (≈140 million km), today’s value is ≈93 million miles (≈150 km).
In England, John Flamsteed (1646-1719) decided to employ a method Tycho had used to measure parallax in which one observer would suffice to measure a diurnal parallax, i.e. a change in position as observed from one single location on Earth’s surface at two different times of day. Flamsteed’s results were largely in agreement with Cassini’s.
In 1676 Ole Christensen Romer (1644-1710), a Danish astronomer also working at the Paris Observatory, noticed that the times of the eclipses of Jupiter’s moons disagreed with Cassini’s predictions. Romer speculated that that light took longer to reach Earth when Jupiter was further away. He proceeded to calculate the speed of light to be 140,000 miles (≈225,000 km) per second; somewhat lower than today’s value of ≈186,000 miles (≈300,000 km) per second.
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