Viewing Venus

UK astronomy enthusiasts are in for a serious case of Venus envy next week, as the planet transits the Sun. People in other parts of the world will have a good view, but while the 2004 transit was seen across the UK, this year’s – the last until 2117 – mostly happens after nightfall in these parts. Only the final stages will be visible at sunrise on 6 June, but that’s not stopping intrepid observers, who will be hoping that Britain’s infamous summer weather proves kind. Here’s a map of events if you would like to join in.

Or, if an early rise doesn’t appeal, why not concentrate on past transits instead? There are recorded observations for six transits from 1639 onwards (here’s a map of where some took place) and it’s a rich history of ambitious voyages, international collaboration and competition, precision instrumentation and personal sacrifice.

The most famous transit expedition is that of Captain James Cook, who sailed to Tahiti to observe the 1769 transit – although the main aim of his voyage was to claim the Southern Lands for the British Crown. From today, there’s a small display on the transit at the rear of our Exploring Space gallery, featuring one of the astronomical quadrants made for the Royal Society’s 1769 expeditions. These were used to establish the observers’ locations and help check timings, so that measurements from around the globe could be correlated in an attempt to establish the Earth-Sun distance.

We can’t say for sure that this instrument was the one used by Cook – his account of the voyage states that the quadrant was taken and dismantled by the Tahitians, and had to be hastily repaired; this one bears no obvious signs of mending. We do know that one of the clocks made for timing the transits, now on display in our Making the Modern World gallery, accompanied Cook on his third South Sea voyage in 1776.

Although less celebrated today, the two 19th century transits (1874 and 1882) also saw major expeditions, and were widely reported in the public sphere. This image shows the set up for the New Zealand station of Britain’s 1874 expeditions. It’s not clear if it was taken there,  or at the Royal Observatory in Greenwich, where the equipment and observing huts were assembled and tested prior to departure.

In the 21st century, there is no longer an imperative to mount large-scale co-ordinated expeditions, as the Earth-Sun distance is readily measured by other means. But it’s still an opportunity for interesting science: the Hubble telescope is going to use the Moon as a mirror to observe the Venusian atmosphere, contributing to the search for extrasolar planets. It’s also a great chance for people around the world to take part in amateur observations, which can be done without expensive equipment.

You can read more about previous transit expeditions in recently-published books by Andrea Wulf and Nick Lomb. And if you are going to try and see the transit for yourself, here’s how to do it safely. Hopefully you’ll have more luck than Guillaume le Gentil, the most desperately unfortunate transit-chaser ever. Happy viewing!