28 Distance to the Sun
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.
Fun Facts: The sun’s diameter is 1.4 million km (875,000 miles). It is
approximately 109 times wider than the earth.
More to Explore
Brush, Stephen. The History of Modern Astronomy. New York: Garland, 1997.
Core, Thomas. The Distance from the Sun to the Earth. New York: Dover, 2002.
Hinks, Arthur. New Measur