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Guest Steven van Roode

The Transit of Venus: Predictions for 2012 June 5-6

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Guest Steven van Roode

Planetary transits start when the planet’s disk is externally tangent with the sun (ingress, exterior). From then, the planet may be discerened as a little black dent in the solar limb, gradually growing bigger until the entire planet is seen on the solar disk (ingress, interior). During the next five to six hours, the planet will traverse the sun’s disk until the planet’s disk will touch the opposite solar limb (egress, interior). The transit ends when the planet’s disk is externally tangent with the sun (egress, exterior).


For all four contacts, the local time, the position angle of Venus, the angular distance to the sun’s centre and the sun’s altitude and azimuth are given. If the sun’s altitude is negative, this means that the sun is below the horizon and, subsequently, the particular contact will not be visible from the specified location. This is indicated by an asterix. The azimuth is the compass direction, measured eastward from the north. The approximate times of setting and rising of the sun on June 5 and June 6 are given in local time. Instead of sunrise and sunset, you can also choose to show the start and end of more ideal observing conditions, when the sun is at least 8° above the horizon. Finally, the mean cloud amount in June is given, indicating the probablity that clouds might interfere with your observation of the transit of Venus. The time zone offset and daylight saving time are selected automatically, but you can change these if you want to.


You can compute the local circumstances for any instant between ingress and egress using the slider (Safari and Opera only). Not only are the position angle, angular distance and the sun’s altitude and azimuth updated when you move the slider back and forth, the position of Venus on the solar disk at the selected instant is also indicated in the diagram right to the map.





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High school teacher Mogens Winther combined his own images of the Venus transit, obtained in Denmark, with images captured in Australia by a GONG network telescope. The result, shown here, demonstrates parallax--Venus' shifting position as viewed from two widely separated locations on Earth. This is what 18th and 19th astronomers tried so hard to measure; the parallax angle could be used to calculate the distance to Venus and, thus, the size of the solar system.




Edited by wiley

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