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The Transit of 6th June 2012

For the transit of 6th June 2012, Venus will take about six hours and a half hours to travel across the face of the Sun. Venus must be above the horizon for the transit to be visible. Australia is one of the best places on Earth from which to observe the transit as the entire transit will be visible from eastern and central Australia. The transit will have started before sunrise for those observing from Western Australia.

The predicted path of Venus across the Sun’s disc is shown in the diagrams above for locations on the east coast of Australia. Venus will travel in a straight line across the Sun. However, because the Sun appears to rotate as it crosses the sky, Venus will appear to move in an inverted “U” shape when viewed from Australia.

Timing for the transit is given in terms of “contacts”.

First Contact (C1) Venus first touches the Sun.

Second Contact (C2) Venus is just inside the Sun on the way “in” (ingress).

Third Contact (C3) Venus is just inside the Sun on the way “out” (egress)

Fourth Contact (C4) Venus last touches the Sun

The table above gives the times of the transit for major Australian cities

Timing of the transit and the “black drop”

It is now possible to predict the timing of the transit contacts with great precision, because we now have accurate information on the distance to the planets and the Sun. When early astronomers were trying to measure the distance to the Sun, it was necessary to time the transit contacts as accurately a possible. However they had great difficulty with this because of what has become known as the “black drop” effect.

This effect occurs immediately after second contact and again immediately before third contact when Venus appears to be connected to the Sun’s limb (edge) by a narrow dark zone. The black drop effect is thought to be due to image blurring from atmospheric distortion and equipment diffraction coupled with solar limb darkening.