If you can take a moment to look at the sky with an eclipse viewer or break out that pinhole projector, you can join a very exclusive astronomical club. On Tuesday, June 5, in most of California, you can watch the planet Venus slowly pass in front of the sun from 3:04 p.m. to 9:46 p.m. PDT.
What is it?
The transit of Venus is among the most rare of planetary alignments. Venus passes in front of the sun twice every century. Transits appear in close "pairs," so although you might have seen this special occurrence in 2004, it will not be visible again until December 2117.
Without optical magnification, Venus will appear as just a small dot against the sun. Using a pair of binoculars or a small telescope will provide a much more satisfying experience. But don't attempt to view the phenomenon, using any method, without special equipment, described below. Looking at the sun for even a small amount of time can burn your retina.
Where to watch?
NASA has invited the public to join them at the Exploration Center at its Ames Research Center, at Moffett Field, near Mountain View, beginning at 3:04 p.m. There will be activities, solar filters and telescopes, and a chance to meet with Kepler mission scientist Natalie Batalha, who will discuss the mission as well as the transit event's significance.
The Peninsula Astronomy Society is also holding a public viewing at the Foothill College Observatory in Los Altos from 3:30 p.m. until 7 p.m. The event is free; parking is $3.
The Lick Observatory on Mt. Hamilton has telescopes, viewers and a time-lapse film on the 1882 transit of Venus using 147 glass-plate images from the Lick archives. Tickets are $9.50 and must be purchased in advance.
In the East Bay, the Chabot Space & Science Center will have telescopes, solar viewers and an astronomer on hand. General admission is $15. The center's recent solar eclipse viewing event sold out, so you might want to order tickets online soon.
The Lawrence Hall of Science will have telescopes on its plaza and demonstrations in the planetarium.
There's also a phone app for iPhones and Androids to help you time your viewing and take part in an international astronomy experiment to determine the distance to the sun.
Find more viewing events throughout the world at NASA.
How to watch?
Looking directly at the sun, or through unequipped cameras, binoculars or telescopes can cause eye damage. Special solar eclipse viewers or welding glasses at a strength of 14 can protect your eyes, sunglasses cannot. While looking through a camera viewfinder is dangerous, looking at a digital projection is fine.
A historical perspective and current research
Since the invention of the telescope in 1608 there have been only seven Transits of Venus, and Tuesday's will make eight. The transit of 1631 went unobserved, and the 1639 event, the first ever observed, may have been seen by only two people. Talk about an exclusive club; more people have walked on the Moon.
For 18th and 19th Century Transits of Venus, expeditions were sent out across the globe to observe Venus' path across the Sun—including one led by Captain James Cook, in 1769. The observations were compared and used to triangulate the actual distance from the Earth to the Sun, and by extension of scale to all the other planets in the Solar System.
Not only is this phenomenon of historical scientific importance, it's even an opportunity for scientific investigation today. Tuesday's transit will be observed by researchers to refine techniques for detecting the presence of extrasolar planets (exoplanets): planets orbiting stars other than our Sun. Heck, there may even be an alien civilization who has detected the presence of Venus—or the Earth, for that matter—by observing a transit…
Here's a time-lapse video of 1882's transit of Venus observed from the summit of Mount Hamilton, in Santa Clara County, created from plates made by Massachusetts astronomer David Peck Todd.