This article was provided by Lee Macdonald.

On Wednesday 6 June 2012 the planet Venus will make a rare passage in front of the Sun as seen from Earth. Venus and Mercury are the only two planets that orbit closer to the Sun than we do and both planets occasionally pass in front of, or transit the Sun’s disc. During a transit Venus appears in silhouette as a dramatic black disc in front of the Sun’s brilliant gaseous ‘surface’ (known as the photosphere). Venus is almost as large as Earth and during a transit is at its closest to us, so it has an angular diameter of approximately 1 arc minute, compared to 30 arc minutes or half a degree for the Sun.

In the photo above, Venus is about to leave the Sun’s disc during the 2004 transit. This photo was taken from France by Lee Macdonald on 2004 June 8th using a Canon 300D DSLR camera attached to a Meade 90 mm ETX Maksutov telescope equipped with a full-aperture solar filter.  (The grey spots on the image are caused by dust on the camera’s image sensor.)

Previous Transits of Venus

Horrocks

Jeremiah Horrocks observing the Transit of Venus

Transits of Venus are extremely rare events.  They occur in pairs 8 years apart separated by more than a century.  The most recent transit occurred in 2004, but the last pair before that occurred in 1874 and 1882 – when Queen Victoria was on the British throne!  The first transit of Venus to be observed took place in 1639; it was observed by Jeremiah Horrocks and William Crabtree from the north-west of England after having been predicted by Horrocks. 

This year’s transit is the second of the current pair and the next will not occur until 2117.  If you missed the transit in 2004, this year’s event will be a once-in-a-lifetime experience!

In the past, transits of Venus had great scientific value as during the eighteenth and nineteenth centuries astronomers used timings of the ingress and egress (entry and exit) to and from the Sun’s disc made from different locations on Earth to determine the distance to the Sun using the geometric effect of parallax.  Once astronomers knew the distance from the Earth to the Sun, known as the Astronomical Unit, they could use Kepler’s and Newton’s laws to determine the distances to all the other known planets and thus determine the size of the Solar System

 Where is the best place to see the Transit of Venus?

Transits of Venus remain fascinating to watch and this year’s should be no exception.  Unfortunately, the 2012 transit is not well placed for viewing from Britain.  As seen from London in the south of the UK, the Sun rises at 04:45 BST on 6 June, little more than an hour before Venus leaves the solar disc at 05:55 BST.  From Edinburgh in the north, sunrise is at 04:31 BST and so across the UK we will be able to see at most the last hour and a half of the transit.  To see the event in its entirety would mean a trip to the Far East.  To observe the transit from the UK, you will need a site with a clear horizon to the north-east, as the Sun will be very low throughout what is left of the transit.

transit of Venus 2012

Transit of Venus 2012 (courtesy of Fred Espenak, NASA)

The Transit of Venus should be seen really well in other parts of the world, for example in Eastern Australia and New Zealand. The British explorer, Captain Cook, sailed on the Endeavour to observe the transit of Venus in Tahiti in 1769. He then went on to circumnavigate New Zealand and find the ‘terra australis incognito’, Australia.

Replica of the Endeavour, Queenstown (photo credit: John Hill)

 Safe observations of the Sun with small telescopes

solar telescope

H-alpha solar telescope (at Glastonbury Festival)

 If you are trying to observe the transit, NEVER look directly at the Sun with a telescope or binoculars.  The only really safe way to observe solar events is to project the Sun’s image onto a piece of white paper or card held 30 centimetres (12 inches) or more behind the eyepiece.

 There is a section on the Sun|trek site to provide a guide to amateur astronomers who want to observe the Sun and also to teachers who would like to carry out some solar astronomy at school (see: http://www.suntrek.org/classroom-resources/observing-the-Sun-safely.shtml).

 

 For further information, see also:-

 Macdonald, Lee, How to Observe the Sun Safely (Springer, 2003).  (A second edition is due to appear from the same publisher later in 2012.)

 Macdonald, Peter, ‘The transit of Venus, 2012 June 5-6’, Journal of the British Astronomical Association, Vol. 121, No. 3, 2011 June, pp. 135-42.