(p.2) Finding the AU: How the Transit of Venus Tells Us Our Distance from the Sun By 1619, German astronomer Johannes Kepler had figured out the relative distances of all the planets from the Sun. For example, if the Earth’s distance from the Sun is one astronomical unit (AU), then Venus’s distance from the Sun is .72 AU, Mars’s is 1.5 AU, and so on. However, no one knew the value of AU, so the absolute distances between the celestial spheres was not known. In 1716, English astronomer Edmond Halley proposed a method for calculating our distance from the Sun—the astronomical unit—using the transit of Venus. The underlying principle behind Halley’s method is something called parallax, the shift in position that comes from viewing an object from two different points. (What is parallax? Try this.) Imagine two different people, one on each pole of the Earth, viewing the transit of Venus. The person on the North pole sees Venus following one path across the Sun. The person on the South pole sees Venus follow a slightly higher path, one that’s shifted a little to the north. Because we see the Sun as a circle, these two different paths will have different lengths. Halley proposed that an easy way to measure the difference between the lengths of these two paths would be to time the transits, using the four phases of the transit—the first, second, third, and fourth contacts—as indicators. With the two different paths known, the distance between the Earth and the Sun can be pretty easily calculated using trigonometry and Kepler’s third law of planetary motion.