A diagram by James Ferguson shows Venus crossing the face of the Sun in June 1761. Astronomers fanned out around the globe to watch the event, called a transit. They faced such dangers as war, storms, and hostile locals. The goal was to compare observations to determine the distance from Earth to Venus, which in turn would reveal the distance to the Sun. [Museum of the History of Science, University of Oxford]
In today's world, watching a tiny black dot move across the Sun doesn't seem like much of a reason to risk life and limb, or to spend months or even years away from home. But 250 years ago, several teams of astronomers thought it well worth the risk. They braved war, stormy weather, disease, and other hardships to watch the planet Venus move across the Sun for just a few hours.
Their goal was to use the event, known as a transit, to measure the distance from Earth to the Sun.
At the time, astronomers knew the relative distances between Earth, the Sun, and the other planets. They knew, for example, that Venus was only about seven-tenths as far from the Sun as Earth is. But they didn't have absolute distances. In fact, estimates of the distance from Earth to the Sun varied by tens of millions of miles.
But on June 6th, 1761, nature would provide a way to measure that distance -- the transit of Venus.
The idea was to place teams of astronomers at widely spaced locations. They would note the starting and ending times of the transit, and measure Venus's track across the Sun. By comparing the angle and the timing from different locations, and applying some trigonometry, they hoped to measure the distance to Venus. And with that measurement in hand, they could then calculate the distances to the Sun and the other planets.
That was the plan, anyway. But it was both difficult and dangerous to carry out. More about that tomorrow.
Script by Damond Benningfield, Copyright 2011
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