Distance to the sun discovered by Giovanni Cassini

Born in 1625, Giovanni Cassini was raised and educated in Italy. Invited by Louis XIV, Cassini moved to Paris in 1669 to head the brand-new Paris Observatory. With an improved, high powered telescope that he carefully shipped from Italy, Cassini continued a string of astronomical discoveries that made him one of the world’s most famous scientists. The discoveries include the rotation periods of Mars and Saturn, and the major gaps in the rings of Saturn - still called the Cassini gaps.

The solar distance had a priority in his research program there. Since the value suggested by the measurements in Bologna could have been influenced by variations in atmospheric refraction, it was important use some other method to prove, or disprove the longer Earth-Sun distance scale.

He made the first reasonably correct Earth-Sun distance measurement in 1672 via triangulation. As with triangulation measurement, Cassini first had to establish a base line. Since the orbit of Mars is outside that of Earth, Cassini knew that at some time, Mars would be in a position to provide a proper baseline; accordingly, Cassini decided to measure the Earth-Mars distance to use as his baseline.

He determined the parallax of Mars using observations he made in Paris and those made by Jean Richer in Cayenne, French Guiana. The distance to the sun has always been regarded as the most important and fundamental of all galactic measurements, Cassini’s 1672 measurements, however, was the first to accurately estimate that distance.
Distance to the sun discovered by Giovanni Cassini