A Rare Sunspot
Venus crosses between Earth and the Sun every 19 months. Its orbit is tilted slightly with respect to Earth’s orbit, though, so most of the time Venus passes just above or below the Sun. Occasionally, though, the geometry is just right, so Venus passes across the Sun.
Such transits happen in pairs, with the two halves of the pair separated by eight years. There was a transit eight years ago, so the June 5 transit is the last half of a pair. The pairs are separated by either 105.5 or 121.5 years. So after this year, the next transit isn’t until 2117.
The June 5 transit begins about 5:04 p.m. CDT, when Venus first touches the edge of the solar disk. Venus will be fully immersed in the Sun’s disk about 18 minutes later, and will remain inside the disk for another six hours. The Sun will set before then across the Lower 48 states, but the entire transit will be visible from Alaska and Hawaii.
To view it safely, look through special solar eclipse glasses or dark welder’s glass (number 14 or darker). Many groups, including McDonald Observatory, will offer viewing through telescopes equipped with special filters.
An Astronomical Ruler
Astronomers used the transits of the 18th and 19th centuries to develop a basic ruler for measuring distances throughout the solar system: the astronomical unit. But it was hard — and often dangerous — work. More »
Transit of Venus.org
Information on the current transit, viewing tips, and the history of transit science
NASA Transit Page
Viewing times for more than 100 American cities
Transit Event Locations
Clickable map with information on dozens of transit-watching events
Special transit-watching events at the Observatory, near Fort Davis, Texas
American Philosophical Society Museum, Philadelphia
The museum will present 10 days of transit-related activities, including an exhibition of scientific instruments used to observe the 1769 transit.
Transit of Venus: 1631 to the Present
By Nick Lomb, 2011. A richly illustrated history of transit expeditions and observations.