When a planet passes between the Earth and Sun so that the planet's disk crosses the Sun and blocks a small amount of the Sun's light, the planet is transiting the Sun. To transit the Sun, a planet must be closer to the Sun than Earth, so only Venus and Mercury can transit the Sun as seen from Earth.
Mercury transits are relatively common and occur on average 13 times every 100 years. Venus transits are rare. Venus transits occur in pairs that are 8 years apart. These transit pairs are separated by either 105.5 or 121.5 years. The last Venus transit was on June 8, 2004. After the 2012 transit, the next two transits of Venus will be on December 11, 2117 and December 8, 2125.
Times for the 2012 Venus Transit
The 2012 Venus transit occurs on June 6, 2012 Universal Time (UT) date. Universal Time, also called Greenwich Mean Time, is the time in the Greenwich, England time zone. The transit is early enough on June 6 UT date that it starts on June 5 UT date. In some time zones it occurs on June 5. Hence the transit date is often given as June 5/6. Times for the transit are given in UT. Here are conversions from UT to worldwide and North American time zones.
The 2012 Venus transit begins at about 22:10 UT on June 5 UT date. Greatest transit, when Venus is closest to the center of the Sun's disk, is at about 1:30 UT on June 6 UT date. The transit ends at about 4:50 UT on June 6.
Where the 2012 Venus Transit Is Visible
The 2012 Venus transit will be centered over the western Pacific region. The entire transit will be visible from eastern Asia, most of Australia, Alaska, and northwestern Canada. The Sun will set on June 5 with the transit in progress as seen from North and Central America and from the northwestern portions of South America. The Sun will rise on June 6 with the transit in progress as seen from western Asia, most of Europe, and eastern Africa. The 2012 Venus transit will not be visible from eastern and southern South America, western Africa, the Antarctic, and most of the Iberian Peninsula.
Observing the Venus Transit
Looking directly at the Sun at any time is unsafe and can cause blindness. Never look at the Sun with the naked eye or any optical instrument lacking a high quality solar filter.
To safely observe the Sun use a pinhole, or small optical instrument to project the solar image onto a piece of paper or cardboard. Venus is much smaller than the Sun, so the Venus transit should be just barely visible on a projected image without optical aid or magnification. With some magnification the image of Venus will be more easily visible.
It is also possible to safely observe the Sun, and therefore the Venus transit, with a properly filtered small telescope. Use only a high quality solar filter sold by a reputable manufacturer. Good solar filters can be expensive, but beware of poor quality solar filters.
Sky and Telescope magazine has these good articles on observing the Sun safely: Solar Filter Safety and Viewing the Sun Safely. The information on observing the Sun safely in the NASA article, Eye Safety During Solar Eclipses, also applies to observing the Venus transit.
Further Reading
NASA Eclipse web site, eclipse.gsfc.nasa.gov/eclipse.html, accessed January 3, 2012
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
