Transits of Venus are rare and historically important astronomical events. The next transit of Venus will occur on 5 and 6 June 2012, and will be the last Venus transit this century. After 2012, the next transits of Venus will be in December 2117 and December 2125. Most of Australia is well positioned to witness the next transit of Venus on Wednesday 6th June 2012.

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is usually measured in hours (the transit of 2004 lasted six hours). A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is almost four times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth.

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities.

Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the solar system. Observations of the 1639 transit, combined with the principle of parallax, provided an estimate of the distance between the Sun and the Earth that was more accurate than any other up to that time. In addition, the June 2012 transit will provide scientists with a number of other research opportunities, particularly the refinement of techniques to be used in the search for exoplanets.

 A transit of Venus can be safely observed by taking the same precautions used to observe the partial phases of a solar eclipse. Staring at the Sun without appropriate eye protection can quickly cause serious and often permanent eye damage.

 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.

 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

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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.

Safe Viewing of the Transit  

Never look directly at the Sun or you may cause serious and permanent damage to your eyesight. Damage can occur without any feeling that there is something wrong as there are no pain receptors in the retina and the damage may not become apparent for some time.  There are several safe methods of observing the transit.

 1. Projection using a SolarScope

 A SolarScope is a purpose made projection device ideally suited for groups of people to observe the transit.  It works by projecting an image of the Sun onto a screen and it is the image that is viewed. Following the instructions, you can not look directly at the Sun.

 2.  Observing using the internet

 The transit can be safely viewed on the internet on webcasts. The transit will be broadcast live on this page, see above. 

 3. Projection using Binoculars or a telescope

 This involves projecting an image of the Sun through binoculars or a telescope onto a card. Note that with these projection methods you must always look only at the projected images (usually with your back towards the Sun).  You must ensure that the second tube of the binoculars and any telescope finderscope are blocked and no one looks through the telescope or binoculars as this could result in instant blindness. For information on how to use these projection methods safely see http://www.csiro.au/Outcomes/Understanding-the-Universe/Tracking-spacecraft/solar-viewer.aspx

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