Rare event: The transit of Venus

Like occultations, described in the last article in this series, there is another class of celestial phenomena that hold attraction to the astronomer. Known as ‘transit’ it represents the passage of a planet across the solar disc. 

Viewing from the earth there are only two planets that can come in between the Sun and ourselves: Mercury and Venus. The others move in orbits that lie outside ours, around the Sun and so we never can see them transit across the solar disc.

In a sense the transit of Venus is similar to what happens at a solar eclipse. In the case of the eclipse we have the Moon travelling across the solar disc. Because it is comparatively so near, the Moon covers the disc and leads to a total eclipse of the Sun. 

In fact, it is a coincidence that the Moon’s disc almost exactly covers the Sun’s disc as seen from the Earth. Venus is much further away from us than the Moon and so it looks smaller as it travels across the Sun. It looks like a thick black dot moving across the Sun’s bright yellowish disc.

How often do we see a transit of Venus? The answer is that currently transits occur in pairs separated by eight years. However, this pattern is repeated after periods of 121.5 and 105.5 years. This apparent irregularity is to do with the parameters of the orbits of Earth and Venus. For us, the first of these transits happened in 2004 on June 8 while the second one is due in 2012, on June 6. Afterwards there will be a long wait of over a century before the next transit. So make the most of the 2012 event!

The story of the French astronomer Guillaume Le Gentil may be mentioned in the context of how colonial astronomers were prepared to travel far to see transits. Le Gentil was deputed by the King of France to observe the transit of Venus in 1761.

There was a competition between the English and the French to observe this phenomenon. The two nations were at war and to avoid the war zone, Le Gentil had to follow a circuitous route and arrived too late for the observation. He waited in Pondicherry for eight years to watch the second transit, but was unsuccessful as the sky was overcast that day. (Even the English failed to watch the event).

Le Gentil’s ill fated expedition had further twists to it: on his way back he was twice ship-wrecked and reached Paris to find that because of his long absence he had been declared ‘legally dead’, and his property given away to his successors!  Also, his wife had remarried.

The modern astronomers may not face such tribulations on an observing trip but they surely share with Le Gentil the desire to observe this spectacular and relatively rare event.