Transit of Venus

How far away is the sun?

To answer this simple question took a huge amount of cooperation in what was to be the first international scientific collaboration of its kind. Nowadays international scientific projects are commonplace, but in the 18th century it took a rallying call to observe an a rare astronomical event. This event had been predicted to occur in 1761 and again in 1769 and the scientific community was determined to take advantage of this prediction to collect data for a mathematical solution to find the distance to the sun first proposed over 50 years earlier.

Captain James Cook’s observations of the transit of 1769.

The Astronomer Royal Edmund Halley had published a paper in 1691 that showed how observations of the transit of Venus, seen from different locations on Earth, could be used to triangulate the distance from the Earth to the sun. Halley wasn’t able to witness a transit in his lifetime as he died in in 1742, but when the next pair of transits came as predicted in 1761 and 1769, nations sent scientists all around the world to take measurements. The measurements taken during the 1761 transit were not precise enough, but in 1769 further attempts were made, including measurements taken by Captain James Cook in Tahiti on his way to Australia. The calculations from Halley’s paper were used by a French astronomer Jerome Lalande who arrived at a distance just over 2% greater than our current value of 149.6 million km – an highly accurate measurement considering the technology of the day. The measurements were taken again during the next pair of transits in the 19th century using photography which produced a more accurate value.

The transit is so rare because the Venutian orbit is not in the same plane as the Earth’s orbit and so the Earth and Venus line up with the sun in specific recurring intervals of 8 years, 121.5 years, 8 years, and 105.5 years. In fact, this alignment has only occurred 7 times since the invention of the telescope: in 1631, 1639, 1761, 1769, 1874, 1882, and the last occurrence, 8 years ago, in 2004. The 2012 transit of Venus will be the last chance to view this phenomenon for 105.5 years. (Here is a small clip explaining more from the BBC with Bang Goes the Theory’s Liz Bonnin holding the solar system in her hand!)

On June 5-6, 2012, Venus will pass in front of the Sun again and if the clouds hold off we should be able to see it in the UK, but only if we are up early enough. The transit will already be in progress at sunrise (at 4:45 am) on June 6 in the UK and will continue until 5:56 am.

Note: Take care when observing the transit. NEVER look directly at the sun. The best way is to project an image of the sun using binoculars or a pinhole and watch the transit on the projection. Here’s a nice selection of methods  (via @standupmaths).

The transit of Venus from 2004.

During this transit some new scientific research is to be performed. From past missions to Venus scientists have detailed data of the Venutian atmosphere and even the weather that occurs there. They hope to compare these  data with spectral analysis of the thin slither of sunlight that will pass through the atmosphere of Venus as it transits the sun. By analysing the light that has passed through the atmosphere it should be possible to identify the elements within the atmosphere and compare it to what is already known from the data from the probes. Even the Hubble space telescope is will be taking measurements of  the transit although it needs to observer the sunlight reflected off the moon as direct sunlight would damage the telescope’s optics.

Scientists studying the Venus transit are hoping to be able to apply their findings to analysis of the light from a distant star as an exo-planet transits. The Kepler space telescope is currently finding a few new exo-planets each month orbiting other stars in our galaxy and in nearby galaxies.  By using these results and applying the science to transits of exo-planets orbiting other stars the hope is that observations should reveal in more detail the atmospheric composition of these planets.

Other articles and resources:

A really detailed post on the transit by @BadAstronomer Phil Plait.

The Guardian has some nice articles about the transits:

• Last chance to see
• The story of the 18th century transit.
• Transit of Venus: a measure of distance
• Watch the Transit and dream of other worlds

Viewing the transit from the international space station.

Times of the transit round the world  from timeanddate.com

A video from SixtySymbols.com (a Physics relative of the Periodic Videos project by Nottingham University)

The Royal Society’s pages on the transit.

Scientific American has some excellent resources as well.

Other Sources:

• Bill Bryson; A Short History of Nearly Everything
• John Gribbin; Science A History

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