1°) The transit of Venus: a stroke of luck for teachers

The transit of Venus is an exceptional event: it occurs twice in eight years, and then nothing happens for over a century. Its observation is therefore a privilege reserved to only one person in three: obviously, no witness remains today of its last manifestation in 1882.

A transit is rather like a solar eclipse caused by a planet rather than the Moon. It only happens with Mercury and Venus as they are situated between us and the Sun. If the plane of the Earth’s orbit and those of these two planets were identical, a transit would occur every 584 days for Venus and every 116 days for Mercury. However, as each planet’s orbit is inclined in relation to those of the others, an alignment takes place only when the Earth and one of these heavenly bodies are simultaneously situated on the line of the nodes, as illustrated in figure 1 for the case of Venus.


Figure 1: Alignment of Earth and Venus on the line of the nodes during transit of Venus.

Unlike eclipses, transits can pass unobserved because of the small apparent size of these planets as compared with the size of the Sun. Therefore the phenomenon of transits remained unknown until their discovery by the scholar Kepler in seventeenth century; he made the discovery by means of calculation rather than observation. Although hardly impressive visually, transits are of fundamental significance in the history of science: in 1631, they confirmed the soundness of Kepler’s calculations (and therefore of his laws), allowed the Sun-Earth distance to be measured, and stimulated the first international scientific cooperation.