On 5/6 June 2012 the world witnessed a transit of Venus, a celestial took place that is so rare that it will not be seen again until 2117. Astronomers Without Borders had some special plans for this rare event.
Mercury and Venus are the only planets closer to the Sun than Earth, both moving faster in their orbits and passing us regularly. But rather than crossing directly between us and the Sun, these planets are usually slightly above or below the Sun as we see them. When they line up just right we see the round, black silhouette of the planet slowly crossing the Sun, an even referred to as a "transit." Mercury transits the Sun 13 or 14 times each century. But Venus transits happen in pairs - two transits eight years apart - with more than 100 years between each pair.
Only six Venus transits have occurred since the invention of the telescope in the early 17th century. There were no observers of the first one in 1631 that we know of, and only two who we know saw the transit in 1639. In the 18th century, Sir Edmond Halley described a method for measuring the distance from the Earth to the Sun through observations of Venus transits from widely separated sites. The same had been attempted with transits of Mercury but Venus transits allow for much more precise measurements.
Halley's publication led to expeditions sent around the world by many nations to view the pair of Venus transits later in the 17th century. The same took place with the 19th century pair of Venus transits. No Venus transits occurred in the 20th century. While 20th century methods eventually supplanted the Venus transit method in measuring distances in the solar system, the history of the event is an important link to our past.
Learn more about what a Venus transit is, what you can see at your location, and how to observe it:
AWB-Esri Web App
Transit of Venus Project
Transit of Venus .org
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