Cassini Measures Mars's Parallax to Determine Solar Distance

In September 1672, Giovanni Domenico Cassini in Paris and Jean Richer in Cayenne, French Guiana conducted synchronized observations of Mars during its close opposition to Earth. By measuring Mars's position against background stars from these locations, they calculated a parallax angle of approximately 24 arc seconds. This allowed Cassini to determine the distance to Mars and calculate the astronomical unit (Earth-Sun distance). His estimate was approximately 87 million miles (140 million km), about 7% below the actual value. This measurement provided the first reliable scale of the solar system and supported the heliocentric model's predicted planetary distances.