On June 5, 2012, the last Transit of Venus will happen in your lifetime. Unless, of course, you plan on living until December 2117 to catch the next one. Because it is the “last chance of a lifetime,” folks are clamoring about when and how to observe it. But I’ve found that many people have even more basic questions like “What exactly is happening?” “Why should I care?” and “Won’t Hubble get the pretty pictures for us?”
First off, the event is that the planet Venus will pass directly between Earth and the Sun. For several hours, Venus will appear as a dark dot moving across the Sun’s brilliant face. The precise alignment required for such a transit happens very rarely: pairs of Venus transits are separated by eight years, with 105.5- or 121.5-year separations between pairs. The first of this current pair occurred in 2004. The 2012 transit of Venus is only the eighth since the telescope was invented in 1609.
Historically, Venus transits were very important in measuring the precise distance to the Sun. A single measurement of a transit provides the relative sizes and distances to Venus and the Sun. Multiple measurements from multiple places on Earth enable astronomers to triangulate the true distance to the Sun. In the 1700s and 1800s, no other astronomical observation could provide this measurement, and grand expeditions were funded to observe Venus transits.
Today, we have radar measurements of the distance to Venus, so transits are not the unique opportunity they once were. For Hubble, however, the 2012 transit of Venus will be a unique opportunity of a totally different dimension.
Exciting discoveries can be made by studying an extrasolar planet that transits in front of its star. Some of the star’s light passes through the planet’s atmosphere, and we can determine the composition of that atmosphere. The same argument applies to the transit of Venus. However, since we know the composition of Venus’ atmosphere, observing its transit can provide an invaluable calibration and substantiation of the techniques used on extrasolar planets.
But, Hubble never looks at the Sun. Our star’s dazzling light would irreparably damage the telescope’s optics. Instead, Hubble will use the Moon as a giant mirror, studying its reflected sunlight to glean information about the Venus transit. It is a difficult observation, but one that mimics some of the complexity of observing extrasolar planets.
Who would have thought that the path to searching for habitable planets in the universe would involve examining Venus in a lunar mirror?