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Venus to Make Final Passage Across Sun Until 2117

Indian children use cardboard eclipse glasses as they prepare to watch the transit of Venus in Allahabad, India, June 5, 2012.
Skywatchers around the world will have the chance to witness one of the galaxy's rarest astronomical events - Venus passing between the Sun and the Earth.

The six-and-a-half hour long passage begins Tuesday at 2209 UTC. Skywatchers on all seven continents will be able to view it - except for a large portion of South America and western Africa.

Venus will appear as a small, black dot as it crosses in front of the Sun. However, experts warn to never look directly at the Sun - doing so can permanently damage eyesight or cause blindness.

The passage will be shown on a number of websites including NASA's and the European Space Agency's.

Transits of Venus always occur in pairs, eight years apart, with more than a century separating each pair.

Tuesday's transit bookends the 2004-2012 pair and will be the last time the phenomena occurs until 2117. The last passage occurred in 1874.

Observations of transits in previous centuries allowed astronomers to calculate the distance between the Earth and the Sun - some 150 million kilometers.

The Earth-to-Sun measurement gave scientists the final piece of information they needed to accurately determine the size of the solar system for the first time - and ultimately, the sizes and distances of everything in the cosmos.