On June 5th/6th 2012 people all over the world will watch the planet Venus transit across the Sun. Planet Hunters is all about spotting planets as they move across the face of a star so we thought it would be good to share the event with everyone. Venus will pass directly between the Earth and Sun on the night of June 5th and the morning of June 6th. This historic event can be seen from many parts of the world and will not happen again for 105 years!

Live Webcast

During the Transit of Venus a live video feed of the event will be shown here, courtesy of the GLORIA Project. The webcast shows periodic images of the Sun's disk during the event. As well as displaying live images from Tromsø, Sapporo and Carins (in Norway, Japan and Australia respectively) the live webcast will also be accompanied by commentary in both English and Spanish.

When and Where To See the Transit

This map shows the parts of the Earth where you can the Transit of Venus. For many, the Transit will only be partly visible and hence the need for a live webcast of the event from Australia, Japan and Norway.

Remember: never try to view the Sun directly with your eyes or through a lens. To learn many ways to safely view the transit check out this article from TransitofVenus.org

The transit takes several hours in its entirety and is broken down into specific events. First Contact is when the disc of Venus first touches the disk of the Sun, Second Contact is when the whole of the disc of Venus is silhouetted. Third contact happens when the disc of venus again touches the edge of the Sun as it exists the disc of the Sun. Fourth Contact marks the end of the transit. These contacts are marke din numerals on the diagram below, along with the time of point of greatest transit.

For the 2012 Transit of Venus these events occur at the following times (all times given in Universal Time):

• First Contact: 22:09:38
• Second Contact: 22:27:34
• Greatest Transit: 01:29:36
• Third Contact: 04:31:39
• Fourth Contact: 04:49:35

These diagrams, the numbers, and more, are available at NASA's observers handbook site.

History

Since the invention of the telescope, 400 years ago, Venus has crossed the face of the Sun just 7 times. The Sun, Venus and Earth do not align very often, allowing us to observe a transit. The orbits of bodies in the Solar System are tilted and produce a repeating pattern of transit events on different cycles. Transits of Venus repeat in patterns of 8 years, 121 years and 105 years. The 2004 transit of Venus was part of an 8 year cycle with this years event, and the next event will be in 2117, 105 years from now.

In 1627, Johannes Kepler (Planet Hunters aficionados will recognise the name!) became the first person to predict a transit of Venus, by calculating that a transit should occur in 1631. Unfortunately he also calcukated that it would not be visible from Europe and so no one was able to observe it at the time. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. However, Jeremiah Horrocks, a British physicist and clergyman, made corrections to Kepler's equations for the orbit of Venus and realised that they came in 8-year pairs - meaning that the 1639 transit would not be a near miss, but the real thing. He worked with a partner, William Crabtree in Manchester, to make observations of the event for the first time. He made the first calculation for the distance to the Sun, and the most accurate at the time.

For the 1761 and 1769 transits of Venus, the scientific community undertook a major international, global campaign to measure the distance to the Sun. In 1716 Edmond Halley (for whom the famous comet is named) had devised a method for determining the distance to the Sun by observing a transit of Venus. Halley never saw the event, he died in 1742, but the world was ready and willing to make the most of a once-in-a-lifetime observation. Observers were sent around the world to watch the transit from different locations. One such observer was Captain Cook, who sailed the HMS Endeavour to Tahiti to measure the transit, and who went on afterwards to explore the Southern Hemisphere and discover Australia.

The collected observations of the 1761 and 1769 transits allowed French astronomer Jérôme Lalande to calculate the Astronomical Unit with more precision that ever before. He found a value of 153 million km, the value today is 149.59 million km. This enabled the size of the Solar System to be determined more accurately than had been possible before.

In 1874 and 1882 transits were observed with even greater precision, allowing American astronomer Simon Newcomb to arrive at a value of 149.59 million km. Even though we use even more precise methods today, this is essentially the correct distance to the Sun. Newcomb used data from the current and previous transits to arrive at this value.