During the month of June, you will have the chance to see an event that will not happen again in your lifetime. It is the “transit of Venus,” an occasion when the planet Venus can be seen moving across the face of the Sun. These events come in pairs with each occurrence in the pair separated by eight years, and each pair separated by over 100 years. June’s event is the second in its pair, so the next transit will not occur until 2117.
The upcoming transit will start at 3:30 p.m. on June 5 when a dark spot, which is Venus, will be visible at the top of the Sun. The planet will then move toward the four o’clock position. However, the Sun will set for us before the transit ends.
Please keep this warning in mind: Do not look directly at the sun without proper protection. This means you need to use eclipse glasses or No. 14 welders’ glasses. Sunglasses won’t do! In fact you will get a better view on your computer or TV, as the image will be magnified.
In the early 18th century, the scientific community knew the relative distances of the planets from the Sun, but not their absolute distances. For example, they knew that Mars was 1.5 times the Earth’s distance from the Sun, but not how many miles that represented. In 1761, Edmond Halley, of comet fame, came up with a method of calculating the Earth-to-Sun distance using the transit of Venus. If that were known, the scale of the entire solar system would fall into place.
Accordingly, many countries sent members of their scientific communities to far-flung parts of the globe where they could observe the 1761 transit. By this time Halley had died, but he would have been gratified to know that the results garnered from that and the 1769 transit produced a Sun-to-Earth distance that only differs from today’s figure by 0.8 percent!
A very short version of a story from that time that I particularly like is the tale of the Frenchman Guillaume Le Gentil, whom I will refer to as GG. Our intrepid Frenchman left Paris in 1760 for Pondicherry, India. At that time, Britain and France were having one of their wars. So when GG’s ship was almost to Pondicherry and they found the city in British hands, they turned back, forcing GG to observe the transit from a rolling deck. This did not produce very reliable data.
Deciding to wait for the next transit that was due in 1769, GG now shuttled around the area looking for a perfect site. He settled on Manila, but was ordered by France to go back to Pondicherry, which was now in French hands. There he missed the transit because of cloud cover (while Manila was clear). He was shipwrecked twice on his way back to Paris and when he got there in 1772, found he had been declared legally dead – his heirs had divided up his estate, his wife had remarried, and he had no job. All because of his bad transit connections.
Fortunately the king intervened and all was resolved. Le Gentil remarried and it appears lived happily ever after.