Astronomy enthusiasts around the world are gearing up for Tuesday’s celestial show: the transit of Venus across the face of the Sun. The small black dot of Venus, silhouetted against the bright Sun, will be visible with safe solar telescopes and, to those with especially good vision, with the naked eye when protected by eclipse glasses. If the weather permits, visitors can watch the first two hours of the transit at the National Air and Space Museum. Some people may have witnessed the last transit, in 2004, but the next transit won’t be until 2117. So it’s a rare astronomical event, occurring in eight-year pairs spaced more than a century apart. But what’s the big deal about a dot crossing the Sun? The transit of Venus is more than a good show. The transits of the 18th century provided the “yardstick” upon which much of modern astronomy is based. The German astronomer Johannes Kepler had worked out the relative positions of the planets in 1619 – for instance, that Venus’s orbit is about 70% of the size of Earth’s – but no one had made an accurate measurement of the absolute distance between the Earth and the Sun. Without understanding this fundamental distance, astronomers could not grasp the enormous size of the Sun, nor start to measure the distance to the nearest stars. Parallax is a technique of using widely spaced observing sites to estimate distance using trigonometry. It is a critical part of human vision, called depth perception. Test it out for yourself by holding up a finger in front of you, and closing alternate eyes. The finger appears to jump back and forth against the more distant background. Parallax is only useful for distances which aren’t enormously larger than the distance between the observing sites (the baseline, which is the distance between the eyes for human depth perception). It may help you estimate the distances to objects in a room, but it won’t help you figure out which of two mountains is closer. Similarly, astronomers need a large baseline to have any hope of measuring stupendous astronomical distances. The first scientific determination of the distance between the Earth and the Sun was done in 1671-1673 by the astronomer Giovanni Cassini, who used parallax to measure the distance to Mars. He measured the position of Mars against the background stars from Paris while a colleague, Jean Richer, simultaneously measured its slightly different position from French Guiana. He knew the baseline – the distance between their observing sites – and therefore he could calculate the distance to Mars. Using Kepler’s orbits, Cassini calculated the distance to the Sun. He found a distance of 140 million kilometers, which was within 7% of the true value.
The English astronomer Edmund Halley suggested using the transit of Venus as a more precise cosmic yardstick, though he did not live to see it work. For the transits of 1761 and 1769, astronomers traveled all over the world, getting as long a baseline as possible, to implement his idea. It was the first major international collaboration of scientists for astronomy. They carefully timed how long it took for Venus to cross the Sun’s disk, which helped fix Venus’s path across the Sun. Observers in far-flung locations recorded slightly different paths taken by Venus across the Sun (see figure). They used parallax to find the distance between the Earth and the Sun. Their measurements were accurate within about 2%: they found a distance of about 153 million km (95 million miles), while modern radar observations, again using Venus, pin the distance down at 150 million km (93 million miles). These distance measurements set the groundwork for much of future astronomy.
Watch a recorded lecture to understand how we use transits to find planets around other stars.