The planet Mercury will make a rare trip in front of the sun on Wednesday for the first time since 2003 — an event that's not expected to happen again until 2016.
The Mercury transit, as astronomers call it, occurs when the planet passes between the Earth and the sun in a five-hour journey.
The black dot in this picture of the sun is Mercury. The planet made a rare pass in front of the sun on May 7, 2003.
(NASA Goddard Space Flight Cente)
It's a rare occasion: a transit can happen only when Mercury is crossing Earth's orbital plane at the same time that it catches up to Earth in its orbit around the sun. This happens about a dozen times per century.
The transit of a planet across the face of the sun is relatively uncommon: we can only see Mercury and Venus's transits from Earth because we're further out in the solar system looking in.
Like Mercury, Venus orbits closer to the sun than Earth does, and it also has transits. But its longer orbital period makes Venus's transits even rarer, with only two occurring this century. One was in 2004, and the other will take place in 2012.
Mercury's transit this month will be visible from various places on Earth, including the Americas, Australia and eastern Asia.
For Europe, Africa and the western two-thirds of Asia, none of the transit will be visible, since the sun will have set some hours before the transit gets underway.
Mercury will start moving across the sun's face at 2 p.m. ET, or 11:12 a.m. PT.
Mercury will leave the sun's face at about 4:10 p.m. PT. Researchers say the sun sets just before the whole event finishes in eastern, central and most of western Canada, but viewers in B.C. should be able to see the whole thing.
In order to observe the transit, a telescope with a special filter made for solar viewing is required.
Researchers warn observers not to look at the sun without proper eye protection. Some sites, such as NASA, are carrying webcasts of the transit.
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