Transit of Venus 2012

Watch the June 6 Transit of Venus live here

Transits of Venus are rare and historically important astronomical events. Most of Australia is well-positioned to witness the next transit of Venus on Wednesday 6th June 2012.

Webcast of the transit

Viewing the Transit of Venus across the Sun requires special equipment. Serious eye injuries or permanent blindness can result from looking directly at the Sun with the naked eye or through a telescope. For those without an alternate safe viewing option or if there is bad weather on the day, a live video of the whole event will be broadcast on this page. Coverage will start from 8am, Eastern Australian Time (GMT +10 hours) and will conclude about six and a half hours later after Venus has completed its transit.

The event will be covered by a number of high-quality specialist telescopes in Australia to ensure the best possibility of getting a live feed.

What is a Transit?

A Transit of a planet occurs when the planet passes directly between the Earth and the Sun so that as seen from the Earth, the planet appears to pass across the face of the Sun. Transits can only occur with planets whose orbit is between that of the Earth and the Sun; that is, Mercury and Venus. A transit of a planet is similar to a solar eclipse but the planet appears to be much smaller that the Moon so it cannot cover the Sun and looks like a small black disc slowly crossing the Sun.

How often do Transits occur?

Transits of Mercury occur quite regularly (with about 13 each century) but they are difficult to observe due to the very small apparent size of Mercury. Transits of Venus are much rarer and are more interesting due to the larger apparent size of Venus and due to their historical connections.

Transits of Venus occur in a pattern that repeats every 243 years with pairs of transits eight years apart separated by gaps of 121.5 years and 105.5 years. Venus and the Earth are aligned in the same direction out from the Sun about every 584 days (this is called in conjunction), however a transit does not occur each time because Venus's orbit is usually above or below the Sun in the sky.

Since the phenomena was first recognized there have only been six transits of Venus - 1639, 1761, 1769, 1874, 1882 and the most recent one  in 2004. The 6th June 2012 transit is our last opportunity to observe a transit of Venus, as the next event occurs on 11th December 2117

The Transit of 6th June 2012

For the transit of 6th June 2012, Venus will take about six 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.

Acknowledgements

The UQ Astrophysics team would like to thank the Astronomical Association of Queensland (AAQ) for its invaluable contribution of time and resources for this project.