Often we're told about a particular astronomical event — eclipses and planet line-ups, for example — that happen only rarely. But Venus crossing the face of the Sun on June 5–6, 2012, takes "rare" to a new level. Don't miss next week's chance to see this, because you'll never have another chance in your lifetime.

June's celestial spectacle, called a transit of Venus, happens only four times every 243 years. However, the spacing between each occurrence is very uneven: it's 121½ years, then 8 years, then 105½ years, then 8 years again. The last transit occurred in June 2004 — and after this June's event there won't be another until December 2117.

Fortunately, unlike the narrow, fleeting path of visibility for a total solar eclipse, the upcoming transit by Venus will last for about 6½ hours and can be seen from more than half of Earth's surface.

As the map here shows, North Americans are positioned to see at least a portion of it on the afternoon of June 5th. Unfortunately, almost everyone in South America will miss out. On the other side of the globe (click on the map), portions of the transit are observable at or after sunrise on June 6th from Europe, northeast Africa, west and south-central Asia, and western Australia.

The best-positioned skygazers are those in eastern Asia, eastern Australia, Alaska, New Zealand, and all of the Pacific from Hawaii westward. They have ringside seats for watching the entire transit, including the crucial events around both its start and finish.

Seen from anywhere on Earth, the transit starts within a few minutes of 22:10 Universal Time (6:10 p.m. EDT) on June 5th and ends within a few minutes of 4:45 UT June 6th. The two tables below offer local times for a sampling of cities positioned to see at least parts of the event.

Transit of Venus Event Times — June 5, 2012
City	Ingress begins	Ingress complete	Deepest transit	Sun's altitude
Calgary	4:05 pm	4:23 pm	7:26 pm	19°
Chicago	5:04 pm	5:22 pm	8:26 pm	0°
Dallas	5:05 pm	5:23 pm	8:26 pm	0°
Denver	4:05 pm	4:23 pm	7:26 pm	9°
Halifax	7:03 pm	7:21 pm	—	—
Los Angeles	3:06 pm	3:24 pm	6:26 pm	18°
Mexico City	5:06 pm	5:24 pm	—	—
Miami	6:05 pm	6:22 pm	—	—
New York	6:04 pm	6:21 pm	—	—
Québec	6:04 pm	6:21 pm	—	—
Seattle	3:06 pm	3:24 pm	6:26 pm	23°

Transit of Venus Event Times — June 6, 2012
City	Deepest transit	Sun's altitude	Egress begins	Egress complete
Bangkok	8:32 am	36°	11:32 am	11:50 am
Berlin	—	—	6:37 am	6:55 am
Bombay	7:03 am	13°	10:05 am	10:23 am
Cairo	—	—	6:38 am	6:56 am
London	—	—	5:37 am	5:55 am
Moscow	5:31 am	4°	8:37 am	8:54 am
Rome	—	—	6:38 am	6:56 am
Stockholm	3:30 am	0°	6:37 am	6:54 am
Tehran	6:02 am	2°	9:07 am	9:24 am

Fred Espenak provides more extensive listings for cities in the United States, Canada, and elsewhere in the world. You can also find precise times and circumstances for your exact location by clicking your position on the map here.

During its historic dash, Venus will appear as a black silhouette 58 arcseconds wide, about 3% of the Sun's diameter. That's easily large enough to be seen with any kind of optical aid — and many observers should be able to glimpse the black dot with their eyes alone. (Is your eyesight sharp enough? Try this test, suggested by Baltimore observer Herman Heyn: Draw a black dot exactly 2 mm across on white paper. Put the paper in good light and stand 23 feet, or 7 meters, away. Can you spot the dot?)

Important: You'll need to take careful precautions when attempting to view the transit. There are several good ways to do this safely. You can view through special "eclipse shades" (not regular sunglasses) or a dark rectangular arc-welder's glass (#13 or #14). Or, you can set up your telescope or even tripod-mounted binoculars to project the Sun's image onto a white card or other flat surface. Solar filters are also made to fit over the front of your telescope. Check out these safe-viewing options recommended by the editors of Sky & Telescope.

Transits Then and Now

The first known observation of Venus crossing the Sun was on December 4, 1639. A young English astronomer named Jeremiah Horrocks had predicted the event after refining calculations by Johannes Kepler. "Horrocks completed his calculations in October 1639, barely a month before the transit," explains historian Eli Maor. "He hurriedly alerted a few friends, urging them to observe the rare event with utmost care. He knew that the transit would provide astronomers with an opportunity to measure Venus's apparent diameter, a task nearly impossible to achieve at any other time due to the planet's intense glare."

Yet Horrocks almost missed the chance to make history — he spotted the planet's dark silhouette using a small refractor only a half hour before sunset! Click here to learn how Maor carefully researched and retraced Horrocks's historic accomplishment.

Years later, Edmond Halley realized that careful timings of such transits, made from widely separated locations on Earth, could be used to determine the distance to Venus and thus, in turn, to the Sun and all the other known planets.

Halley's realization led to an all-out international blitz by astronomers to observe the transits of 1761 and 1769. Notable among these was the unsuccessful quest by French observer Guillaume Le Gentil, who spent 11 years traveling but missed seeing either event. Finally returning to Paris, he found that he'd been declared dead, his wife had remarried, and his relatives had divvied up his estate. Captain James Cook had better luck: on his first voyage, he observed the 1769 transit from Tahiti at a location still known as "Point Venus".

Many modern observers, both professional and amateur, will again return to the Pacific to witness the upcoming transit. Some are heading to Tahiti to stand where Cook and his crew had some 2½ centuries before; others are going to Hawaii, where the event concludes less than a half hour before sunset.

Weather Prospects

For those wishing to witness the entire spectacle, from ingress through egress, success will depend not only on location (basically eastern Asia and the western Pacific) but also on the weather.

"Within that zone," says meteorologist Jay Anderson, "one country stands out above all others for its clear weather in early June: Australia." He notes that some climate stations in Queensland and the Northern Territory report having June days that are sunny, on average, at least 90% of the time.

In the United States, the best clear-sky prospects are in the Southwest and along parts of the Pacific Coast. The odds get steadily worse eastward.

For a more complete weather picture, check out Anderson's website.

More About the Transit

NASA's transit website (compiled by Espenak) provides a wide assortment of maps, tables, and technical details. So do the comprehensive transitofvenus.nl website, amassed by Steven van Roode and François Mignard; Chuck Bueter's transitofvenus.org, and Jay Pasachoff's transitofvenus.info.

There's no shortage of online resources to let you relive past events and to plan for the one in June. Here on SkyandTelescope.com, for example, you can see how observers worldwide fared during the 2004 transit, which favored Europe, the Middle East, Africa, and most of Asia. Historian William Sheehan offers tales from the 18th and 19th centuries, and Anthony Misch describes how he and Sheehan created an animation of the 1882 transit using archived images. Sky & Telescope's January 2012 issue offers a detailed preview by Espenak and Anderson.

Here are more transit-related resources worth checking out:

• Lou Mayo of NASA's Goddard Space Flight Center (astronomer2go (AT) verizon.net) has established a Transit of Venus Working Group (TOVWG) to coordinate public-outreach efforts. He calculates that his efforts may reach 50 million people by the time of the event.

• Larry Marschall's CLEA (Contemporary Laboratory Experiences in Astronomy) online/DVD student lab exercise on the transit illustrates neatly how transit observations can be used to measure the astronomical unit. The manual with the exercise tells the history and method.

• Jean-Eudes Arlot of the Institut de Mecanique Celeste (IMCCE) at the Observatoire de Paris in France is coordinating a public-outreach program that will use pooled observations to re-measure the astronomical unit.

• Tying the event to the cutting edge of modern extrasolar-planet studies, a group of astronomers plans to study the light skimming through Venus's atmosphere during the transit — "firstly, to use Venus as an example of a transiting exoplanet. Astronomers will use the transit to test the techniques they have developed to analyse the composition, structure and dynamics of exoplanetary atmospheres. Secondly, they will be able to make simultaneous Earth- and space-based observations of Venus's atmosphere. These joint observations will give new insights into the complex middle layer of Venus's atmosphere, a key to understanding the climatology of our sister planet."

• Filmmaker Maarten Roos is making a documentary film about transits of Venus.

• Jay Pasachoff (Williams College) has uploaded a 22-minute lecture about the transit of Venus, with historical discussions and contemporary science.

• The Canadian composer Victor Davies has written an opera, Transit of Venus, based on the play of that title by the Canadian playwright Maureen Hunter. It's about the expedition of Le Gentil (here's a plot summary). It was performed by the Manitoba Opera in 2007 and is available for further performances.

More S&T coverage of the transit of Venus: See This Week’s SkyWeek Video Your Viewing Guide to the Transit of Venus Will You See the Black Drop? Safe Sun Viewing for the Transit Time-Traveling Transit How to Photograph the Transit Where to View the Transit Online