It's not hard to reason this one but you migh...
Contrary to popular belief, a small black mole crossing the bright disk of the Sun is not God's attempt to keep evil spirits at bay! Instead, it is an extremely rare astronomical phenomenon called Transit of Venus.
This event is so rare that only two transits of Venus occur every 125 years! We're lucky to have two such transits taking place in our time. In case you missed the first one back on June 8, 2004, you'll be able to catch this event unfold on June 5 or 6 (depending on your time zone) this year. Jeremiah Horrocks of Lancashire was the first to observe such an astronomical rarity way back in 1639.
Essentially, a transit implies that a smaller body passes in front of a larger one. In a way, this is very much like a solar eclipse except due to Venus' tiny size in comparison, only a small portion of the Sun is obscured. This causes the little 'black mole' to move across the Sun's surface.
Since the only planets that pass between the Earth and the Sun are Mercury and Venus, they are the only ones who can produce a transit with respect to our planet. Transits are extremely rare because the orbits of all three planets are extremely large and it is very rare for all of them to be perfectly aligned.
For a Venus transit to occur, two things need to happen simultaneously:
-Venus must be in inferior conjunction with the Sun--or located directly between the Earth and the Sun.
-Venus must be crossing through Earth's orbital plane.
The entire phenomenon lasts from anywhere between three to seven hours, but the exact time taken depends on the path of the Sun. In 2004, the transit lasted for over five hours, and this time scientists are predicting a similar time frame.
Another extremely interesting thing that occurs during this period is the 'Black Drop Effect.' This happens right at the beginning and the end of the transit when Venus is just entering, or exiting the Sun's disk. The light from the Sun is refracted by Venus' dense atmosphere, making it look stretched out!
This rare phenomenon took place in Indian skies today at the crack of dawn. We'll never get this opportunity again--not for another 125 years. Now, that's what we call a once in a lifetime experience!
-Pictures courtesy Thinkstock-
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