Venus in Transit

Next Wednesday, Finland will experience a rather neat and rare astronomical event, though not one so easy to notice. The transit of Venus is essentially the same as a solar eclipse, though instead of the moon blocking out the sun by passing between it and Earth, it’s Venus that’s blocking out the sun. Only, not so much.

Faraway Venus appears so small from Earth that we see it only as a small dot as it crosses the face of the sun. So, I imagine the transit won’t exactly be spectacular. But it is rare -- it won’t happen again for another 105 years – and it does have a unique scientific pedigree. 

When I heard that the transit of Venus was coming up, I immediately thought of James Cook and Tahiti. Naturally. Any excuse to think of a place like Tahiti.

The most recent transit, 2004.
Photo: Jen Herold.

What makes the solar transit of our neighboring planet so interesting is its role in a geometry exercise of astronomical proportions. Over 400 years ago, Johannes Kepler, a founding father of astronomy, revolutionized the field by discovering a more accurate model of how planets move around the sun. He was thus able to predict when Venus and smaller Mercury would next transit the sun, but died a year before the first of these events took place, in 1631, and no one was prepared to observe it. The opportunity to gleam some important data was lost.

When the next transit of Venus occurred in 1639 (they usually come in pairs eight years apart, then again after 105 or 122 years), it was seen by two astronomers in England, who then took a stab at measuring part of the solar system.

Because these two Englishmen observed the transit from slightly different angles (from villages about 40 kilometers apart), Venus appeared to each to take a slightly different path across the sun.

From this, they could use fairly basic geometry to calculate the distance of Northern England from that big yellow orb in the sky. The resulting estimate of almost 60 million miles (97 million kilometers) was way off the mark, though closer to the actual distance of 93 million miles (150 million km) than anyone had come up with previously. 

An early observation of a transit of Venus, 1639.

What was needed for a more accurate measurement was more and better observations, and from more widely separated spots on earth. That and, of course, another transit of Venus. This wasn’t scheduled to occur again until 1761 and 1769, by which time scientists around the world were ready. Astronomers were dispatched to such widely scattered places as Norway, Newfoundland and Madagascar to measure the transit on June 6, 1761.

Two of those taking part in this international effort were English astronomers by the names of Mason and Dixon, who successfully observed the transit from the Cape of Good Hope. This pair went on to gain wider fame by accurately marking the southern limits of the colony of Pennsylvania. This more down-to-earth feat led to one of their names, in the form of Dixie, being applied to such diverse items as disposable paper cups, an all-girl country band, a type of jazz, not to mention a large part of the future United States where I come from.

For the next transit, eight years later, some astronomers traveled even further afield. The Royal Society of London organized an expedition to the amazingly agreeable destination of Tahiti, which had been “discovered” by an Englishman only two years before. The very able James Cook, a master navigator and mapmaker, was picked to lead the voyage, the first of his three explorations of the Pacific.

I love the idea of an expedition setting out on three-year journey to exotic parts of the world just to observe a single brief celestial event with no other purpose than the advancement of pure science. I would have signed up for that trip in an instant.

Aboriginals encountering 
Cook's men in 1770.

But, there was a second, secret and slightly less noble, though just as romantic, "assignment for Cook. The British Admiralty instructed him to search for Terra Australis Incognito before returning home to England. This mythical continent was rumored to exist in the far reaches of the southern ocean and would have made an epic addition to the British Empire.

After leaving Tahiti, Cook spent a year sailing around and mapping New Zealand, proving it was a pair of islands and not part of any legendary continent. He also discovered the eastern coast of Australia, ran aground on the Great Barrier Reef, and observed original Australians and their habits, which included cooking shellfish on the beach, apparently an ancient Ozzie custom that has now been elevated (or reduced?) to a fun-loving cliche.

He, however, did not succeed in his secret task of finding a gigantic landmass in the vastness of the South Pacific, the original mission did contribute to a much-improved measurement of our solar system.

Next week’s transit will be another opportunity for scientists somewhere to gather data. For us laypeople it will be just a chance to watch Venus obscure a small part of the sun. That is if our slice of the sky is not obscured by clouds.

Of course, no one can safely view it without proper eye protection. I’m hoping to use the safety glasses handed out to us by Air France back in 1999 on a flight to Paris from the States that was landing just as a total solar eclipse sweep across the City of Light.

The transit should be visible in Finland beginning at one a.m. Wednesday morning (three hours before sunrise in Helsinki) and lasts until almost eight. I hope I can catch some of it. It would be a shame not to. Something tells me I won’t be around for the next one in 2117.