Our understanding of the universe depends on two foundations, our ability to measure the distances to faraway stars, and our ability to measure the chemical composition of stars.
The discovery that allowed scientists to determine the composition of stars is described in the 1859 entry on spectrographs. The distance to the sun has always been regarded as the most important and fundamental of all galactic measurements. Cassini’s 1672 measurement, however, was the first to accurately estimate that distance.
Cassini’s discovery also provided the first shocking hint of the truly immense size of the universe and of how small and insignificant Earth is. Before Cassini, most scientists believed that stars were only a few million miles away. After Cassini, scientists realized that even the closest stars were billions (if not trillions) of miles away.
Born in 1625, Giovanni Cassini was raised and educated in Italy. As a young man he was fascinated by astrology, not astronomy, and gained widespread fame for his astrological knowledge. Hundreds sought his astrological advice even though he wrote papers in which he proved that there was no truth to astrological predictions.
In 1668, after conducting a series of astronomical studies in Italy that were widely praised, Cassini was offered a position as the director of the Paris Observatory. He soon decided to become a French citizen and changed his name to Jean Dominique Cassini.
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. These discoveries included the rotational periods of Mars and Saturn, and the major gaps in the rings of Saturn, still called the Cassini gaps.
Cassini was also the first to suspect that light traveled at a finite speed. Cassini refused to publish his evidence, and later even spent many years trying to disprove his own theory. He was a deeply religious man and believed that light was of God. Light therefore had to be perfect and infinite, and not limited by a finite speed of travel. Still, all of his astronomical work supported his discovery that light traveled at a fixed and finite speed.
Because of his deep faith in the Catholic Church, Cassini also believed in an Earth-centered universe. By 1672, however, he had become at least partially convinced by the early writing of Kepler and by Copernicus’s careful arguments to consider the possibility that the sun lay at the center.
This notion made Cassini decide to try to calculate the distance from the earth to the sun. However, it was difficult and dangerous to make direct measurements of the sun (one could go blind). Luckily, Kepler’s equations allowed Cassini to calculate the distance from the earth to the sun if he could measure the distance from the earth to any planet.
Mars was close to Earth and well-known to Cassini. So he decided to use his improved telescopes to measure the distance to Mars. Of course he couldn’t actually measure that distance. But if he measured the angle to a spot on Mars at the same time from two different points on Earth, then he could use these angles and the geometry of triangles to calculate the distance to Mars.
To make the calculation work, he would need to make that baseline distance between his two points on Earth both large and precisely known. He sent French astronomer Jean Richer to Cayenne in French Guiana off the north cost of South America. Cassini stayed in Paris.
On the same August night in 1672, at exactly the same moment, both men measured the angle to Mars and placed it exactly against the background of distant stars. When Richer returned to Paris with his readings, Cassini was able to calculate the distance to Mars. He then used Kepler’s equations to discover that the distance to the sun had to be 87 million miles (149.6 million km). Modern science has found that Cassini’s calculation was only 7 percent off the true distance (just over 93 million miles).
Cassini went on to calculate the distances to other planets and found that Saturn lay a staggering 1,600,000,000 (1.6 billion) miles away. Cassini’s discoveries of distance meant that the universe was millions of times bigger than anyone had dreamed.
The sun’s diameter is 1.4 million km (875,000 miles). It is approximately 109 times wider than the earth.