Because of the peculiarities of the orbits of the planets around the sun, Venus transits are visible from Earth in pairs separated by eight years. The upcoming transit is the second in a pair that began with one in 2004, which at the time was the first visible in more than 121 years. [Venus Transit of 2004: 51 Amazing Photos]
The 2004 transit provided scientists with the first opportunity to view such an event with modern equipment and telescopes.
"Modern solar telescopes captured unprecedented view[s] of Venus’s atmosphere backlit by solar fire," astronomer Tony Phillips wrote on the Science@NASA blog. "They saw Venus transiting the sun's ghostly corona, and gliding past magnetic filaments big enough to swallow the planet whole."
While scientists are excited for this year's Venus transit, they don't stand to learn quite as much as their counterparts did in the 18th century, when a pair of Venus transits in the 1760s have been described by modern historians as "the Apollo program of the 18th century," according to Phillips.
Then, astronomers had no way of measuring the absolute size of the solar system, until the Venus transits presented an opportunity to triangulate the distance to Venus by comparing measurements made from different vantage points on Earth. Scientists spread far and wide around the planet — famous explorer James Cook went to observe from Tahiti — however, bad weather and technical glitches prevented measurements of the accuracy scientists had hoped for.
Now, researchers will be able to observe the transit with an accuracy 18th-century scientists could only dream of. In addition to studying the planet Venus, scientists plan to use the transit to test techniques they hope to wield studying alien planets beyond our solar system.