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Fri, 11 May 2012

Get ready for the Transit of Venus

How the transit should appear shortly after sunrise – a photo adapted from the 2004 transit. Photo by Robin Scagell

A Transit of Venus takes place when Venus crosses in front of the Sun, so we can see the globe of the planet silhouetted against the Sun's disc. A transit will take place on Wednesday 6 June 2012. You might think that this would be a regular occurrence – after all, Venus passes between the Sun and the Earth every so often as it orbits the Sun closer in than Earth. But the two orbits are inclined by a few degrees, so a transit can only take place when both planets are close to the point where the planes of the orbits cross.

This happens only twice every century – or to be more precise, eight years apart, with a gap of up to 121 years between the pairs. The last Transit of Venus was in 2004 and the next transit will not be until 2117.

However, unlike in 2004, when the UK was fortunate enough to see the whole even lasting several hours, and enjoyed clear weather over much of the country, the 2012 event will be much more difficult to observe because most of the event takes place before the Sun rises from the UK. Only the last hour or so will be visible – and at sunrise. So by the time most people wake up, it will all be over.

What time to look

As seen from the UK the event finishes at 5.55 am BST, at which time Venus has left the Sun's disc altogether. How much of the transit you may see, weather permitting, before that time depends on where you live and the time of sunrise. The more northerly and easterly you are, the earlier sunrise will be.

The line of equal sunrise times on 6 June lies roughly parallel with the east coast, roughly from Cromer to Edinburgh, where the sunrise is just after 4.30 BST. To the west of that line, sunrise is later. For the most densely populated part of the UK, it is between 4.40 and 4.55 am. The best place to be for an early sunrise is the tip of Shetland, where the Sun rises around 3.30 am BST. You can see a chart of sunrise times on a map of the UK at the HM Nautical Almanac Office website.

Third contact, when Venus touches the edge of the disc, is at 5.37 BST and fourth contact, when Venus leaves the disc altogether, is 5.55 BST. So most parts of the UK will see only the stages when Venus is partially off the Sun's disc. For other timings, see the HM Nautical Almanac Office's Transit of Venus page.

Where to look

You may need to choose your observing site in advance. Contrary to popular belief, the Sun does not usually rise in the east and set in the west, except on a few days in the year. In June it rises roughly northeast. You don't want to find that there's a tree or a building in the way when you get up specially to watch the event. You'll need to work out exactly in which direction the Sun rises from your location, which is at an azimuth (degrees measured round from north) of around 43–51º from the UK, where 45º is northeast.

To do this, go to this online sunrise calculator and choose your location from a list of towns around the UK. Exact precision is not vital. You will be taken to a page where you can choose the date and also set it to show the azimuth. Knowing the azimuth, either plot the sunrise azimuth on a map or, if you have Google Earth on your computer, go to your chosen observing site with that. There is a ruler tool that allows you to draw a line with any chosen azimuth so you can work out in which direction the Sun will rise.

How to observe

Great care is needed when observing the Sun, even when it is low down. Its brightness can easily blind you, particularly when viewed through optical aid. The only safe ways are to use a filter specifically designed for solar observing, or to project the image through binoculars or a telescope onto a white screen. On this occasion, the pinhole in a cardboard box method used for observing partial eclipses will not work as Venus will be too small to be seen. For further information on safe solar observing, see our comprehensive online guide. There is also advice on the Royal Astronomical Society's Transit Page, which contains more information about the event and also locations where public observations are being arranged.

If you are planning to observe the event, you might consider going to a local open space and inviting people from the neighbourhood.

Tales of derring-do

Past observations of Transits of Venus involved tales of high adventure and discovery. Tony Sizer, lecturer at the Royal Observatory, Greenwich, presented a talk on the subject at a recent SPA meeting which you can view online. Or to hear the full story, get the DVD of a lecture by the inimitable Dr Allan Chapman, given before the 2004 transit, available from the SPA Shop.

Below is a simulator which you can customise for your location to show the appearance of the transit. Note that you will need Flash player to see it, and that the Sun appears deep red when it is below your horizon.

Please install latest Flash Player to run SunAeon Venus Transit 2012


Added by: Robin Scagell