By Patricia Reaney
More than three centuries ago on a cold afternoon in the tiny English village of Much Hoole, a British astronomer witnessed for the first time an event that would not happen again for more than 100 years.
Jeremiah Horrocks, a 20-year-old farmer’s son who was completely self-educated in astronomy, calculated correctly that on November 24, 1639 Venus would align between the Sun and Earth and could be seen as it moved across the solar disc.
The event first predicted by the German astronomer Johannes Kepler in 1627 has occurred only six times since the invention of the telescope.
Amateur astronomers and the general public throughout Europe and most of Asia and Africa will have the chance to observe the phenomenon, the first for 122 years, on June 8.
The next transit will not take place until June 6, 2012 and will not be visible in Britain and other parts of Europe.
“Horrocks got it dead right,” said scientific historian Dr Allan Chapman of the University of Oxford.
Beginning at 0519 GMT for six hours, Venus will cross the path of the Sun and, weather permitting, appear as a dark black dot about 1/30th the Sun’s diameter.
The event will be visible for all of the morning in most of Europe and Africa, in the middle of the day in the Middle East and across Russia and India, and during the late afternoon from the Far East and Australia.
Just as Horrocks used the transit to formulate fundamental facts about the solar system, including calculating the astronomical unit, or distance of the Earth from the Sun, amateur astronomers and schoolchildren around the globe can take part in a transit observation experiment organised by the University of Central Lancashire in northern England.
Using a transit calculator on the university’s website (www.transit-of-venus.org.uk) they will be able to measure the distance from the Earth to the Sun just as Horrocks did.
When the transit starts, Venus will appear as a dot on one side of the Sun. By taking measurements as the planet tracks its way across the Sun and using the transit calculator, amateur astronomers will be able to take measurements of the time it takes Venus to get from end to the other.
“Through that, you will be able to calculate how long it took to cross the Sun. You put those details into the transit calculator and it will give you a figure of the distance from Earth to the Sun which is what Jeremiah Horrocks did in 1639 when he first observed the transit of Venus,” said a spokesman for the university.
Although Kepler had predicted that Venus would transit the Sun in 1631, he never lived to see it. He had not calculated that it would occur again in 1639.
“This is put down entirely to Horrocks’s own originality,” said Chapman. “From a mixture of his own meticulous observations of Venus, the Sun and the planets…he realised that the inferior conjunction of 1639 when Venus passes between the Earth and the Sun would neither be above it or below it but bang across its middle.”
He told his friend, a cloth merchant named William Crabtree, that it would happen.
“He predicted this transit, he then went on with Crabtree to draw a number of fundamental facts about the nature of the solar system, which frankly, were they done today, would be classed as Nobel Prize-winning discoveries,” Chapman added.
By the time of the 1769 transit, more than 100 years later, co-ordinated international expeditions had been organised to witness the event across the globe. Captain James Cook travelled to the South Pacific to view it from Tahiti.
When the next ones occurred in the 1870s and 1880s observatories and powerful telescopes were used to witness the event that happens four times in every 243 years.
There are two December transits, eight years apart, and then 121.5 years later there are two June transits, also with an eight year gap. After another 105.5 years the cycle begins again.
Scientists have told anyone who plans to witness the transit of Venus not to try to view it with the naked eye, or through a telescope or camera because it can cause damage to the eyes and blindness.
They said indirect projection is the safest way to observe the sun.
“The transit of Venus will be a spectacular and memorable event. It represents a fantastic opportunity to fire the next generation of astrophysicists with enthusiasm for scientific discovery,” said Professor Gordon Bromage, head of the University of Central Lancashire’s Centre for Astrophysics.