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Mercury will cross the sun's face next Monday (May 9) in a rare "transit" that will be visible from most places on Earth, weather permitting. Main Viewing Guide: Mercury Transit of the Sun on May 9: How to See It and What to Expect
Such transits of Mercury occur just once every eight years on average; the last one happened in 2006, and the next one won't come until 2019. (Venus is the only other planet in our solar system that transits from our perspective, as it and Mercury are the only two planets closer to the sun than Earth. Venus last crossed the sun's face in 2012 and won't do so again until 2117.)
Visibility range: Observers in most of the world will be able to see the May 9 event. Only people in East Asia, Japan, Indonesia, Australia and New Zealand will be completely shut out — and even they can still watch online, as several different webcasts will cover the transit.
SOURCE - SPACE.com