Transit of Mercury
Since the orbit of Mercury brings it in between the Earth and the Sun several times a year, you would think that we would see it pass directly in front of the Sun. Astronomers call this a transit, and in the case of Mercury, it doesn't happen as often as you would think. In fact, Mercury transits only happen once every 7 years or so - 13 to 14 transits a century.
They happen so rarely because the orbit of Mercury is inclined by 7° compared to the orbit of the Earth. So even though Mercury is lined up between the Sun and the Earth, it's either too high or too low to actually pass directly in front of the Sun.
The last three transits occurred in 1999, 2003, and 2006. The next transit won't happen until 2016.
The length of the transit depends on what path Mercury takes as it passes through the Sun. Grazing transits can last for just a few hours, while the longest possible transit can take close to 8 hours.
The next Mercury transit will happen on May 10, 2016.
Filed under: Astronomy
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