On Tuesday, the planet Venus will pass between Earth and the Sun, appearing to cross the face of our star in a rare event known as a "transit." The next transit of Venus to be visible from Earth won't happen until December 2117. In other words: you don't want to miss this. Here's everything you need to know to catch a glimpse.
What is a Transit?
"Transit" is just the word astronomers use to describe one cosmic body passing in front of another from a third object's perspective. Two weeks ago, the Moon made a transit of the Sun, crossing between our star and the Earth in an annular eclipse. When scientists scan the Milky Way for planets outside our solar system using the Kepler telescope, they generally do so via the "transit method" (depicted here), detecting planets by looking for dips in the brightness of a distant star that occur when an orbiting planet passes between it and Kepler's line of sight.
The transit of Venus, therefore, simply describes the event wherein Venus passes in front of the Sun as viewed from Earth. But Venus and Earth do not orbit the Sun in the same amount of time (the Earth takes 365 days, Venus 225); nor do they orbit the Sun in the same plane. As a result, it's rare for the three bodies to align in such a way that Venus' path across the Sun is visible from Earth. How rare? The planets' orbits are such that transits occur in pairs separated by 8 years, followed by over 100 years of no transits whatsoever. The most recent transit, for example, took place in June 2004, but the event before that occurred all the way back in 1882, and the next transit won't be visible from Earth until 2117.
When and Where?
Depending on where you live, the transit will take place on either June 5th or June 6th. Regions labeled in white on the map featured here (click to enlarge) will be able to witness the event in its entirety. Regions labeled in blue and green will be able to catch part of the transit beginning at sunset on June 5th, or sunrise on June 6th, respectively. Regions labeled in grey won't be able to watch the transit with their own eyes, but that shouldn't prevent them (or anyone else for that matter) from watching it live online — more on that below.
For those in the U.S.: Venus will make its first appearance on the Sun's face on the afternoon of Tuesday, June 5th, a few minutes after 22:00 UTC (around 6:03 p.m. EDT, 3:06 p.m. PDT), and exit the solar disc approximately seven hours later.
For everyone else (or anyone looking for exact transit times): NASA recommends using this handy web applet.
The Black Drop Effect
The transit of Venus is a long-drawn affair; it takes almost twenty minutes for the planet to even creep entirely into view along the Sun's outer edge.
When it finally does appear, Venus will be visible in the form of a black disc, about 1/32 the diameter of the Sun. That's large enough for you to forego the use of binoculars or a telescope (with proper eye protection, of course — more on that below), but if you live in the Western Hemisphere, and you do have access to a properly outfitted telescope, these first twenty minutes are your chance to spot a phenomenon known as "the black drop effect," whereby Venus appears to make its entrance onto the solar disc in the shape of a teardrop. (The image featured here shows the teardrop effect as photographed during 2004's transit).
Skygazers in the green-labeled region of the Eastern Hemisphere will be able to catch a glimpse of the teardrop effect as Venus exits the solar disc. Those living in the white regions of the map featured above will be able to witness both black drop events.
How to Watch Safely
Once it's moved entirely into view, Venus will spend about seven hours coursing its way across the face of the Sun before slipping out of sight, giving you plenty of opportunities to catch a glimpse of the event — but you'll want to make sure you take proper safety precautions. Fortunately, safety measures for watching the transit of Venus are basically the same as watching a solar eclipse.
Looking directly at the sun — even with sunglasses — can permanently destroy your vision. DON'T DO IT. Instead, use a solar filter. These come in a variety of forms, from wearable solar shades, to attachments that you can affix to telescopes and binoculars. They'll cut the brightness by enough that you'll be able to catch glimpses of the Sun without frying your retinas.
If it's last minute, or buying a solar filter is out of the question, another great option is to meet up with your local astronomy club. Someone is bound to have the right equipment for watching the transit.
Poke a small hole in an index card with a pencil point, face it toward the Sun, and hold a second card three or four feet behind it in its shadow. The hole will project a small image of the Sun's disk onto the lower card... Experiment with different size holes. A large hole makes the image bright but fuzzy; a small hole makes it dim but sharp.
More advanced projection methods involve the use of a cardboard box, or the room of a house with a sun-facing window. In a pinch, you can even use a welding helmet — just make absolutely sure it's housing number 14 welder's glass — anything less is unsafe. You can click through for more helpful viewing tips from NASA.
You don't have a solar filter. Your local astronomy club disbanded years ago. There are clouds in the way. You live somewhere in the grey area from the map above. You're too lazy to make a a pinhole projector. Whatever. You're in luck. One of the best ways to catch the transit is to watch it online. We recommend checking in with NASA and SLOOH.