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    Posted March 5, 2012 by
    Silkeborg Denmark
    This iReport is part of an assignment:
    Your views of space and stars

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    Coming Venus Transit - astronomical event of the year

    Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention (by Galileo Galilei) of the telescope; 1631, 1639, 1761, 1769, 1874 and 1882. I was lucky to photograph the event in 2004 (took the day off - just in case the weather would be nice). The greatest thing about the photo shoot, was the close up shot of the atmosphere ring - never photographed before.

    I hope you all get a chance to see this rare event, I think it will be "The astronomical event of 2012". To inspire you, I show to you this composite photo from 2004, Venus in transit and below the rare photo of the atmosphere ring illuminated by the bright sunlight. Hope you like it.

    As NASA explain: The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible. On average, there are 13 transits of Mercury each century. In contrast, transits of Venus occur in pairs with more than a century separating each pair.

    The last Venus transit was in 2004 so the second event of the pair will occur on Wednesday, June 6 (Tuesday, June 5 from the Western Hemisphere). The entire event will be widely visible from the western Pacific, eastern Asia and eastern Australia as shown in Figure 1. Most of North and Central America, and northern South America will witness the beginning of the transit (on June 5) but the Sun will set before the event ends. Similarly, observers in Europe, western and central Asia, eastern Africa and western Australia will see the end of the event since the transit will already be in progress at sunrise from those locations.

    The principal events occurring during a transit are characterized by contacts. The event begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. The entire disk of the Venus is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arc-min/hr. At contact III, the planet reaches the opposite limb and is once again internally tangent with the Sun. The transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress.

    Frequency of Transits:
    Transits of Venus are only possible during early December and early June when Venus's orbital nodes pass across the Sun. If Venus reaches inferior conjunction at this time, a transit will occur. Transits show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. The next pair of Venus transits occur over a century from now on 2117 Dec 11 and 2125 Dec 08.

    Edmund Halley first realized that transits of Venus could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method proved impractical since contact timings of the desired accuracy are impossible due to the effects of atmospheric seeing and diffraction. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.
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