Sic transit Venus

An overcast morning in Salthill yesterday meant no sign of the transit of Venus – I did get to see the Moon briefly.

When Sir Edmund Halley proposed that the solar parallax (and hence the distance from the Earth to the Sun) could be calculated by using the transit of Venus, it triggered a huge scientic endevour. Halley published his proposal in 1716, but the next transit was not due for another 45 years.

The transit of Venus occurs when the planet Venus passes in front of the Sun, as seen from the Earth. It isn’t big enough to cause a solar eclipse – it only dims sunlight on Earth by around a thousandth of a percent. What makes the transit interesting is that it is (a) rare and (b) entirely predictable. It  occurs in a regular sequence every 243 years – 121.5 years before the first transit, an 8 year gap until the next and then 105.5 years until the next transit, another 8 years, and then back to 121.5 again.

Using parallax (explained here) required scientists located on different locations on Earth to measure the time it took Venus to cross the surface of the sun. Depending on where they were located, the times would be slightly different. Using the times, and the distance between the locations where the measurements were taken, would enable calculation of 1 Astronomical Unit (i.e. the distance between Earth and the Sun). Scientists were lucky in 1761 – advances in the manufacture of both telescopes and clocks meant that fairly accurate observations and timing could be made. The scientists were not so lucky when it came to travelling – long journeys by ship to often hostile locations were necessary to make the observations. The results from the 1761 observations were not entirely satisfactory, but had proved a good practice run for the following transit 8 years later. For the 1769 transit, Captain James Cook was dispatched to Tahiti to observe the transit from there (he also had some business in Australia) and didn’t return to England with his results until 1771. Other scientists battled the cold of northern Canada and Norway to make their observations.

Of course, having sailed and trekked for months to their chosen point, there was no guarantee that that a scientist would actually get to see the event. The unluckiest scientist of all must surely be Guillaume Le Gentil, who set sail from France in 1760 to observe the 1761 transit from India. Alas, war broke out between France and Britain (who occupied most of India) so he had to sail to Mauritius instead. In early 1761, he attempted again to sail to India from Mauritius, but was forced to turn back. He decided to observe the transit (in June 1761) from Mauritius. Alas, despite the clear sky on the day, he had decided to make the observation from his ship (which was rolling about in the water) so he could not make any calculations. Having sailed halfway round the world, he decided to wait for the second transit, a mere eight years away. In the meantime, there was peace between France and Britain, so he was able to build an observatory (on solid ground this time) in the area in India where he had originally planned to observe the first transit. On the day in question, after a month of cloudless skies, it was overcast, and he saw nothing.

I was thinking of Guillaume yesterday morning when I was standing on the Prom in Salthill, just after 5am, hoping that the cloud would break to allow me to photograph the second 2012 transit. It didn’t and I saw nothing of the sun until that evening. Typically, yesterday evening was warm and sunny, but for that once in a lifetime event in the morning, there was nothing but rain and cloud. I can’t complain, though – Guillaume’s fruitless journey took 11 years of his life, whereas it only took me about 8 minutes to drive down to Salthill.When Guillaume returned to France, he found that he had been declared dead and that his wife had remarried. I didn’t take any chances – I brought my wife with me to the Prom (though she spent most of the time sleeping in the car).

I didn’t even get to feel the Mayo earthquake.

This is the second time in a week that I’ve gotten up at 4.30 am for a photo expedition, and both times were a washout. And speaking of washouts, it has been raining solidly for the last 12 hours, with a month’s rain predicted to fall tomorrow. Ain’t the summer great ?