The 2012 Transit of Venus

David Peck Todd, photograph of the 1882 Transit of Venus

 The last Transit of Venus of the 21st century will happen in the coming days, June 5 or 6, 2012, depending on where you are in the world. This is a phenomena in which Venus passes directly between Earth and the Sun, and is observable as a small black dot gliding over the face of the Sun over a six hour period. Transits occur in pairs separated by 8 years (2004/2012) followed by gap of about 110 years (the next one is in December 11, 2117!). Only six have been observed since the invention of the telescope:

• 1631 (not witnessed) & 1639
• 1761 & 1769
• 1874 & 1882
• 2004 & 2012

Those dates provide some sense of historical scale involved in this phenomena, which is partly the point I'm getting to ...  The Transit of Venus has been a historically significant astronomical event because alignment of the Sun, the Earth, and Venus has allowed astronomers to calculate distances between the Earth and the Sun, and from that to estimate the scale of the solar system. As this blog post by Karen Masters, an astronomer, explains: 

Basically when Venus crosses the Sun we know that [Venus], the Sun and the Earth are all in a straight line. Very slight differences in the viewing angle from two observers on the Earth can then be used along with our basic knowledge of trigometry to measure the distance to the Sun. For over 100 years, the distance to the Sun measured this way was the most accurate measurement we had.

From knowing the distance to the Sun, we can use slight changes in the apparent position of nearby stars as the Earth orbits the Sun to get their distances (more triangles – this is called the parallax method), and from those stars we calibrate methods which use stars of known or estimated brightness to estimate distances to nearby galaxies, and we jump from distances to nearby galaxies to more distant galaxies and eventually the whole universe. The distances to faraway galaxies have taught us that the universe is expanding and started in a Big Bang around 15 billion years ago, and even if we go to the observations that suggest the universe contains a mysterious “Dark energy” (which won the 2011 Nobel Prize in Physics), they are ultimately based on us knowing the distance to the Sun.

At least two kinds of scale involved in thinking about recorded histories of the Transit of Venus: the quantifiable measurement of enormous distances between planets in our solar system, and the imaginative scale of the historical and technological leaps we have to negotiate mentally to comprehend the human span between each attempt. The Exploratorium Museum has created visuals of the mathematical measurements involved, and an article in the Telegraph interprets them in this way: "Each crossing is an opportunity for precision measurement and a pause to contemplate the astonishing scale of successive human observation." When Captain Cook sailed from Britain and 'discovered' New Zealand and Australia, his trip had been partly partially motivated by a scientific desire to observe the 1769 Transit from Tahiti. So what makes the Transit of Venus a compelling event is not only a kind of astrological drama, but that it is a touchstone for thinking the significance of measurement to the history of ideas.

NASA, photograph of the 2004 Transit of Venus

This is partly what I trying to get at in my posts on scale and mathematical abstraction. An exhibition titled Dark Sky at the Adam Art Gallery in New Zealand, for example, takes the occasion of the Transit of Venus as a point of departure to explore the relation between art and science, both historically and today. On the day of the transit Geoffrey Batchen, a historian of photography and co-curator of the show will discuss the differences between 1874 and 2012 viewings with William Tobin, an astrophysicist. The differences involved aren't just about more sophisticated telescopes or photographs—they involve on the alterations that scientific discovery through the twentieth century has had on a collective social imagination. A couple of videos that speak to this point; a reanimation of the 1882 Transit of Venus from 147 glass plate photographic negatives:
  

And a live webcast of the 2012 Transit from the Exploratorium:

Transit of Venus | exploratorium.edu/venus