Figure 1 - A couple viewing the transit in 1769!
On June 5, 2012, you will see the planet Venus as it moves across the face of the early morning sun. This astronomical oddity has played a very important role over the last few centuries in giving scientists a way to understand the size of the solar system.
There is some evidence that the ancient Babylonians saw and recorded on a tablet something about Venus and the Sun in the 16th Century B.C., but the record is not clear. It is fair to say though that Galileo Galilee with his telescope, in 1610, was the first human to actually see Venus as more than just a bright point of light in the sky. Johannes Kepler, meanwhile, was shaking up the world with his meticulous use of astronomical data assembled by Tycho Brahe. He predicted that Venus would pass in front of the Sun on December 6, 1631, but unfortunately the transit was not visible from Europe at all.
The first recorded sighting of this transit was by British cleric, Jeremiah Horrocks, and his friend William Crabtree, on December 4, 1639—only because Horrocks had mathematically predicted it, using better data than Kepler did.
Scientists discovered they could use the transit to figure out the size of the solar system! How do we know that the actual distance from Sun to Earth is 93 million miles and not, say, 153 million or 23 million? In 1663, mathematician Rev. James Gregory suggested that a more accurate calculation of the Earth-Sun distance could be made during the transit of Venus. It turned out to be harder than anticipated, but during subsequent transits, the focus on Venus led to an even more exciting discovery.
During the transit of June 5, 1761, observed by 176 scientists from all over the world, Russian astronomer Mikhail Lomonosov (1711-1765) discovered a very strange thing. Instead of the very black disk of Venus sliding into the Sun's bright edge, it actually grew a brief, beautiful halo of light all around its dark edge. This is exactly what you would expect to see if Venus had an atmosphere!
In 1769 many international expeditions planned observations. The most famous expedition was led by Capt. James Cook, who set up an observation post in Tahiti with his ship, the Endeavour. The expedition astronomers, with help from a detachment of Royal Marines, set up an observatory on a high point of ground above the bay (still known as "Point Venus") and made many transit measurements. They later discovered New Zealand, got stuck on the Great Barrier Reef for several weeks, and explored many of the then unknown coasts of Australia. Cook and his crew completed their trip around the Earth and reached England safely and in triumph. The expedition established Cook's fame as a mariner and explorer.
The next transit, on December 6, 1882, made the front pages of every national and international newspaper! Thousands of photographs were taken with improved calibrations. Only a few astronomers were trusted to carry out the complex calculations from the resulting data. In 1896, Simon Newcomb's value, a distance from Earth to Sun of 92,702,000 plus or minus 53,700 miles, was adopted by the international scientific community. Today most textbooks report the Astronomical Unit (or AU) as "93 million miles."
The Venus transit has continued to yield fascinating new information for scientists and the public. Take this unique opportunity to make your own observations and calculations.