Observers around the world watched Mercury make a rare passage in front of the Sun on Wednesday, in an event that will not be repeated until 2016. The event may help shed light on the planet's tenuous atmosphere.Mercury appeared as a tiny black circle just 1/200th the width of the Sun. It began moving across the Sun's face at 1912 GMT (1112 PST).In order for transits to occur, Mercury has to come exactly between Earth and the Sun. If Mercury and Earth orbited the Sun in the same plane, this would happen every 116 days – when Mercury, which orbits the Sun every 88 days, catches up to Earth in its orbit.But because the orbits of the two planets are slightly tilted with respect to each other, Mercury transits occur only about 13 times per century.Mercury's 2006 transit was visible from a broad swath of Earth including the Americas, Australia and eastern Asia.

The Sun set before the end of the transit for observers in the Americas, except for those on the west coast. Observing the transit requires a telescope or binoculars fitted with special filters – looking directly at the Sun can cause permanent vision damage.

View: Full Article | Source: New Scientist