On June 5, 2012, a rare astronomical event will occur: a transit of Venus. This just means that Venus will be between the Earth and Sun, so that Venus will appear as a small dot on the Sun’s surface. The relative motion of Venus and the Earth will cause the silhouette of Venus to drift across the Sun’s face over a period of several hours. Venus transits happen in pairs, eight years apart, so that the 2012 event is the second of a pair. Once we have a pair of these transits, there will be no more for the next 105 years. This odd timing is due to the vagaries of the orbits of the Earth and Venus. If both orbits were exactly in the same plane, there would be a transit whenever Venus and Earth were in line with the Sun; roughly once every two years. As it happens, in almost every Earth-Venus-Sun alignment, Venus is in the sky either above or below the Sun’s disk.

Astronomers will observe the June event mostly as a curiosity, but the Venus transits that occurred in the eighteenth century were of intense interest, and the efforts to observe them constituted the biggest scientific enterprise of the century. Britain, Austria, and France each launched expeditions to observe and time the transits from several locations throughout the world.

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Physicist Richard Feynman, in his Lectures on Physics, asked us to imagine that somehow all knowledge about nature, except for a single fact, was to be destroyed. What fact, he asked, should scientists choose to pass on to their successors? His choice was “Everything is made of atoms”. This, Feynman said, is the most important single piece of scientific knowledge that we have.

I thought of this recently when I read Steven Weinberg’s book The Discovery of Subatomic Particles (an excellent book, by the way, and written for a general audience). At various places in the book, Weinberg describes the experiments and reasoning that lead from simple speculation about atoms to the eventual certainty that they exist. This post is a summary of that chain of thought.

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