Venus Transit of 1882
One of the most basic units of measurement in astronomy is called, appropriately enough, the "astronomical unit" or AU. It's the average distance from Earth to the Sun, and the basic unit for measuring the solar system or other objects that have a similar scale.
Today, the length of the AU is known to within a few meters. That's thanks in part to something that happened 125 years ago today: a transit of the planet Venus.
A "transit" occurs when an object passes in front of a star. Today, astronomers are using transits to discover planets in other star systems. But in 1882, they used it help plot the distance from Earth to the Sun. The best measurements of the day varied by several million miles.
The U.S. Naval Observatory sent eight expeditions to photograph the transit -- four in the U.S., and the rest in the southern hemisphere. They snapped about 1400 pictures.
By viewing the transit from different points, they saw Venus at slightly different angles against the Sun. It's like holding up a finger and looking at it with your left eye, then your right -- the finger appears to move against the background as you blink back and forth. How far it moves allows you to measure the distance.
Astronomer William Harkness spent the next decade analyzing the pictures, plus others from a transit in 1874. His work yielded a value for the AU of 92.8 million miles -- about 160,000 miles less than the true distance.
Script by Damond Benningfield, Copyright 2007
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