Distance to the Sun
Year of Discovery: 1672
What Is It? The first accurate calculation of the distance from the earth to the
sun, of the size of the solar system, and even of the size of the universe.
Who Discovered It? Giovanni Cassini
Why Is This One of the 100 Greatest?
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!
How Was It Discovered?
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.
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