Astronomy Today

Measuring Units

How was the astronomical unit first measured?

Italian astronomer Gian Domenico Cassini (1625–1712), who is famous for studying the rings of Saturn, was the first astronomer to make a nearly accurate measurement of the length of the astronomical unit. Cassini first measured the parallax of Mars, based on his own observations made in Paris and those of his colleague Jean Richer (1630–1696) in South America. With this information he was able to calculate the distance from Earth to Mars, and from that the distance from Earth to the Sun. Cassini’s measurement was slightly low at about 87 million miles (140 million kilometers), but he was off by less than ten percent of the correct value: 93 million miles (149.6 million kilometers).


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