|David Peck Todd, photograph of the 1882 Transit of Venus|
- 1631 (not witnessed) & 1639
- 1761 & 1769
- 1874 & 1882
- 2004 & 2012
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|
And a live webcast of the 2012 Transit from the Exploratorium:
Transit of Venus | exploratorium.edu/venus