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June 5th, 2012 saw one of the rarest cosmic events to happen in our lifetime: the transit of Venus. During the event, Venus crosses in front of the sun from the perspective of the Earth, an occurrence that happens in pairs about every century. The last transit to occur was in 2004 but the next time Venus makes its trip will be in 2117 and 2125.
The transit began at 3:09 PM Pacific Daylight Time and lasted seven hours. Every continent was able to view the transit at some point, even some parts of Antarctica. Normally visibility is limited to one area or continent with similar events such as lunar and solar eclipses.
NASA (National Aeronautics and Space Administration) sent out several recommendations in relation to the watching of the transit. They recommended that individuals should not stare at the sun and should use telescopes or eclipse glasses to observe the transit.
Even the Hubble telescope could not look directly at the sun to view the transit of Venus because the sun would damage the lens. Instead, scientists pointed the telescope at the moon. The moon reflects the light of the sun and the shadow of Venus in transit was visible.
The transit of Venus is visible from Earth because there is a perfect alignment between Venus, Earth and the Sun. Venus’s orbit is 226 Earth days in contrast to our 365 day orbit. It would seem that the transit of Venus should be something observable from the Earth at least once a year except that the orbits of Venus and the Earth are slightly titled planes. The tilt of these orbits is what makes the transit so rare an occurrence.
Since the invention of the telescope in the early seventeenth century, there have been eight times that Venus has been visible from Earth as it crosses the Sun. The first of these occurrences in 1631 was not witnessed by anyone (although we know it happened since these events happen in pairs within eight years of each other), but in 1639 people started to notice.
Edmund Halley, best known for the namesake of Halley’s Comet, came up with a plan to use the transit of Venus to measure the size of the solar system, one of the biggest mysteries of science at the time. According to NASA, Halley said that by using the principles of parallax and viewing the transit of Venus from widely spaced locations on Earth it would be possible to triangulate the distance to Venus.
Halley’s predictions set into action what some historians and astronomers call the “Apollo Program of the Eighteenth Century.” In 1761 and 1769 many scientists around the world did their best to record and analyze the transit of Venus. Captain James Cook was sent to Tahiti to watch the transit as well as Jeremiah Dixon and Charles Mason (known for the Mason-Dixon line). Despite everyones efforts, weather and limited development in optics prevented their success. However, the solar system was eventually measured with the aid and development of cameras in the late 1800s.