The Transit of Venus

Anticipating the return of the Beagle any day now, on June 3^rd, Darwin was having a quiet day at home.  He writes:

“Staid quietly at home, & in the evening walked to the Lagoa. Called on a Mr Roberts, one of the endless nondescript characters of which the Brazils are full. — broken down agents to speculation companies; officers who have served under more flags than one: &c &c to all of whom I am charitable enough to attribute some little peccadillo or another.” (June 3)

Okay, so there is no direct connection between Darwin and the Transit of Venus, but none-the-less that is my topic for today.  I just couldn’t pass up writing about his “once-in-a-lifetime” event, and if you bear with me, I will draw (what I think is) a significant connection before this post is up (plus a couple of smaller ones, too). In fact, I would go so far to say that Darwin’s voyage on the Beagle may never have happened, had it not been for the Transit of Venus. But first I have to start at he beginning…

About every 120 years, the solar system treats us to a rare celestial event – the passage of Venus between the Earth and the Sun.  From the Earth, this passage appears as a black dot that moves across the face of the Sun – what astronomers call a transit. Because of the geometry of the solar system, a pair of transits (spaced about 8 years apart) only happens about every 120 years. The only transits that were ever observed by human beings happened after the invention of the telescope and include those that occurred in 1639, 1761, 1769, 1874, 1882, and 2004. (The 1874 transit did occur in Darwin’s lifetime, however, it was not visible in England and, as far a I know, Darwin never mentions it in his writings. He died shortly before the 1882 transit, which would have been visible from his house. )

The last transit of our century will occur this Tuesday, June 5^th (from about 3pm to 9pm here in the Pacific Northwest). I hope to see it, though the weather forecast is not very promising. If not, maybe my grandkids will get a chance to see the next one in 2117.  If you have the opportunity to observe it on Tuesday (and can get to a site with appropriate protection for viewing the sun) I would highly recommend it.

2004 Transit of Venus (NASA):

Anyway – back to history. Prior to the 18^th century only one man definitively saw a Transit of Venus– that was Jeremiah Horracks in 1639 (though it is possible his friend William Crabtree also caught a glimpse of it, as well).

Jeremiah Horrocks observing the Transit of Venus in 1639 (by Eyre Crowe)

In the early 1700’s, an aging Sir Edmond Halley, knowing the next transit was still many years away, realized it’s significance to the world of science.  Halley noted that if astronomers were able to make accurate measurements of the timing of a transit in the 1760’s, that it would be possible to accurately calculate the distance from the Earth to the Sun.  And since (thanks to Kepler) we knew the relative distance of the planets from the sun, knowing that distance would allow scientists to calculate the distance of all the planets from the sun, and ultimately to measure the size of the solar system (and the known universe at that time).   Effectively it would add a scale to the “blueprints” of the cosmos. In the Age of Enlightenment this was an opportunity not to be missed.

There was only one catch – for the calculation to be effective, multiple observations of the transit had to be made from opposite ends of the Earth.  Astronomers, as they had done in the past, could not just watch from their observatories and rooftops.  They would have to travel to remote locations, and share their results with one another in order to determine the Earth-Sun distance.  The idea of doing this in 1761, was pretty much the equivalent with suggesting we send scientists to Mars and Venus today.  It required years of travel, financial and political backing, and international cooperation.

Over the next 45 years, Halley’s message remained at the forefront of astronomy, standing as a challenge to the world of science. And when the 1760’s approached, the scientific community picked up the gauntlet and met the challenge. Astronomers were sent to the South Atlantic, India, Indonesia, Africa, northern Sweden, Siberia, and Hudson’s Bay… to name just a few locations.  The most famous of these expeditions was the first voyage for Captain James Cook, whose primary mission was to go to Tahiti and measure the transit. On the HMS Endeavour with Cook where two scientists – the astronomer Charles Green and the botanist Joseph Banks.

Postage stamp commemorating Captain James Cook’s observation of the Transit of Venus:

The adventures of these globe-trotting scientists make for wonderful stories  – including tragedies, comedies and heroic successes. (For those interested in these tales, I would highly recommend Andrea Wuff’s brand new book Chasing Venus). But I think one of the most significant things to come out of the 18th-century Transits of Venus, was the birth of the scientist/adventurer – the natural philosopher who traveled the world to better understand it, rather than just experimenting in their own backyard. Other than a few minor exceptions, this appears to be the first time that scientists began to travel the world en masse.

Many of the transit expeditions brought back new charts and maps of unexplored lands, as well as plant and animal specimens and written descriptions of exotic natural histories (from places such as Madagascar, Australia and New Zealand). The potential benefit of the traveling scientist was not missed by the military and government officials (not to mention the scientists themselves) and the idea stuck. The world never looked back.

From this point forward, the Royal Navy generally included a ship’s naturalist (often the dual role of the ship’s surgeon). In addition, they encouraged and even trained officers to make scientific observations (see excerpts from one such training manual in Charles Darwin – Geology Teacher).  The same was true of French expeditions after the 1760’s (France was the world leader in organizing efforts in to observe the 1761 transit). Even Alexander von Humboldt’s journey, that so inspired Darwin, was a German manifestation of this change in “policy”.

So this is why I say that Darwin’s trip may never have happened without the Transit of Venus.  It was the transit expeditions that made having a scientist along on an expedition a good idea – only about 50 before the Beagle left Plymouth with a young naturalist on board.

There are two other smaller connections between Darwin and the transit that I want to mention.  One of the prime viewing spots for the 1882 transit was in South America, so many countries sent teams of astronomers to locations that were familiar to Darwin.  One team, that included the photographer William Bell, stopped over in Rio for a little while during the trip and visited the Botanical Garden (see Gunpowder and Breadfruit: Rio’s Botanical Garden for a picture Bell took while on the Transit Expedition).

Another team that observed the 1874 and 1882 transits in New Zealand included an explorer who grew up hearing his father’s tales of scientific adventures – Leonard Darwin (one of Charles’ sons). Leonard was a photographer on the 1874 expedition and led the 1882 trip.  Alas – in both cases he was not able to make the observation due to cloudy skies.

Leonard Darwin and his transit viewing station from 1882 (from the University of Queensland Hume Photograph Collection):

So there you have it – my linkage between Darwin and the Transit of Venus.  You don’t necessarily have to travel the world to see it, but if you are able to see the transit be sure to think about all those scientists who risked their lives in the name of understanding our place in the universe, and how they set a precedent that changed the world of science forever. (RJV)