On June 5th or 6th (depending on where you are on Earth) the planet Venus will move across the face of the Sun. Transits occur in pairs 8 years apart, with a gap between pairs. The 2012 transit follows the 2004 transit, if you miss it the next one is due in December 2117. Johannes Kepler, using data from Tycho Brahe predicted that Venus would pass in front of the sun in December 1631, but this transit was not visible from Europe. Eight years later, the clergyman and astronomer Jeremiah Horrocks predicted and then observed a transit on December 4, 1639. The transit is beautifully recorded in the local parish church – in a stained glass window.
Historically transits of Venus were essential to determine the size of the solar system. The distance to planets was known in terms of the Earth – Sun distance, the astronomical unit, but nobody had an exact value for that. Edmond Halley worked out how to calculate the exact distance, which was by timing the contacts of Venus with the Sun from different locations on the Earth. The 1761 transit was observed by nearly 200 scientists around the world. In St Petersburg, the distinguished Russian scientist Mikhail Lomonosov, observed that the planet grew a brief fuzzy halo of light when it was right at the edge of the Sun and correctly deduced that Venus has an atmosphere. Unfortunately many of the measurements of 1761 weren’t good enough – one observatory’s clock was running at least 10 seconds slow, another had so many guests wanting to see what was going on that the floor shook so much that no measurements could be taken and many French and British observers had cloud in the east. One French astronomer even undertook a 5 month journey to Siberia to observe the transit – and had to watch out for being mistaken for a magician by the locals.
By 1769 many expeditions were undertaken to observe the transit, since it was visible from western North America, the Pacific and eastern Asia. Most well known would have to be Capt. James Cook who took an eight month journey to Tahiti and set up an observatory at Point Venus. Providence, Rhode Island has a Transit Street, it’s at the location of a temporary observatory that was set up to watch the transit.
The next transit, on December 6, 1882, made the front pages of newspapers. Thousands of photographs were taken and calculations from the resulting data gave a value of the astronomical unit of 92,702,000 plus or minus 53,700 miles. It’s easier to remember 93 million miles or 150 million km.
To see the next transit from Ireland will be tricky enough as it will be underway at dawn on June 6th. Maximum visibility of the event will be obtained by going as far North and East as possible, from the Republic you are advised to head to Skerries.
This episode of “What is the Stars?” was first broadcast during the week of May 28, 2012. Listen live on RTÉ’s lyricfm on Mondays and Fridays at 22:45.
If you are uncertain how to say “Tycho Brahe” listen to this youtube video, recommended to me by a Danish friend.