Look skyward, space-lovers: Tuesday, June 5 is the last chance in your lifetime to see the transit of Venus across the face of the sun.
Venus won't visibly pass between the Earth and the sun again until 2117, so get your special sun-viewing glasses now. In North America, the transit will start in the hours before sunset on June 5. Viewers in Asia, Australia, Africa and Europe can catch the transit at sunrise on June 6. The transit will begin at about 6:03 p.m. EDT, 5:04 p.m. CDT, 4:05 p.m. MDT and 3:06 p.m. PDT.
From Earth, Venus will appear to cover just 1/32 of the face of the sun, so you'll need "very good conditions and very good eyes" to view the event without magnification, according to Nick Schneider, an astronomer at the University of Colorado, Boulder. But you still need protective eyewear, because without it, the intensity of the sun's light can cause serious retina damage.
If you have your own telescope with a sun filter, you can set it up to project an image of the sun on a sheet of paper or the ground, Schneider said. Never look directly at the sun without a specialty filter (often available at telescope stores) or #14 welder's glasses (available at specialty welder's stores).
The most satisfying way to view the transit will likely be to go to a local museum or planetarium that has a special sun-viewing telescope set up. For those who can't arrange an in-person viewing, the transit will be streamed online from multiple observatories and telescopes across the globe.
Why so rare?
Transits of Venus occur in pairs eight years apart separated by either 105.5 years or 121.5 years. The last Venus transit occurred in 2004, and the next pair won't happen until 2117 and 2125. The reason transits are so rare is that Venus' orbit is off-kilter from Earth's orbit by about 3.4 degrees. That means that when Venus does pass between the Earth and the sun, it's often too low or high to cross in front of the sun's face.
"We tend to think of our solar system layout as perfectly organized with all the planets going around in the same direction all lined up," Schneider told LiveScience. "The truth is that small amounts of misalignment matter a lot."
There have been only seven transits of Venus since the invention of the telescope in 1610, according to NASA. They occurred in 1631, 1639, 1761, 1769, 1874, 1882 and 2004. [Gallery: Views of Venus]
Captain Cook and the transit of Venus
The 1769 transit was an especially historic one. Several decades earlier, astronomer Edmund Halley realized that by observing the transit of Venus, scientists could finally crack the question of the absolute size of the solar system. At this time, astronomers knew the distance of the planets to the sun as fractions of Earth's distance to the sun, but they had no idea of the absolute distance. Venus could have been a million miles away or a billion.
Halley figured out that by having one person measure the transit of Venus on one side of the planet and another do the same on the other side, scientists would be able to triangulate the absolute distances involved. In 1768, the famous British explorer James Cook set sail for Tahiti to make this crucial measurement.
Given 1700s technology, Cook and his shipboard astronomer weren't able to make perfect measurements; nor were observers stationed elsewhere around the globe. It wasn't until the 1800s transits that scientists were able to finally set the scale of the solar system with certainty. Modern-day astronomers still use transits to make new discoveries, identifying new alien planets when they pass in front of their stars, causing a miniscule dimming of starlight.
This time around, astronaut Don Petit plans to photograph the transit of Venus from his post in the International Space Station, becoming the first person to record a transit from orbit. Schneider is looking forward to these historic photos.
"What I find so poignant about this is that it's the next step made possible by those first halting steps to measure the transit of Venus," Schneider said. "The space age just would not have been possible without those pioneers."