A transit occurs when Venus passes between Earth and the sun. Depending on your geographical location, on June 5 or 6, Venus will appear as a small black circle crossing the sun.
Transits rarely occur because they depend on the convergence of two planets’ relative orbits, but that also makes them predictable: They come in pairs, eight years apart. The 2012 transit is the pair of the 2004 transit.
The time between the occurrence of the pairs alternates between 121.5 years and 105.5 years. The next transits won’t occur until 2117 and 2125.
Just as looking directly at the sun would injure human eyes, so would training the Hubble directly on the sun injure its own sensitive components.
What to do?
Why, use the moon as a mirror, of course.
As sunlight passes through Venus’s atmosphere, certain wavelengths are absorbed. Scientists will read the absorption spectrum that travels all the way through the atmosphere to determine the atmosphere’s composition. They’ll position Hubble to focus on the moon, where the non-absorbed sunlight wavelengths end up.
The Hubble will focus on the Tycho crater and will view the transit through its Camera for Surveys, Wide Field Camera 3 and Space Telescope Imaging Spectrograph.
Because spectral signatures are quite faint—only 1/100,00th of the filtered sunlight is reflected off the moon—the Hubble must remain focused on the transit for seven hours for an accurate and complete observation.
Astronomers were concerned about the effect of Earth’s blocking Hubble’s view of the moon for 40 of each 96-minute orbit. They weren’t confident that Hubble could recapture and refocus on the same spot for the entire seven hours.
A dry-run in January dispelled their worries—Hubble performed like a champ.
reported by Discovery News May 7, 2012