Introduction.

The distances to the nearest stars have been found using the method of Annual Parallax combined with a precise knowledge of the Earth-Sun distance.

On June 8^th 2004 there will be a rare astronomical event - a transit of Venus across the face of the Sun. This transit will be seen all across Europe and presents an excellent opportunity for cooperative experiments to measure the Earth-Sun distance.

In this paper we show how to analyse data from previous transits and suggest simple observations to perform in 2004.

Warning: Never look directly at the Sun. Never look at the Sun through any optical instrument (telescope, binoculars, lens etc.). Just one glance will destroy your eyesight. The only safe way to observe the Sun is by projecting its image onto a screen

The transit of Venus.

A transit occurs when Venus is in inferior conjunction with the Sun. That is when Venus is situated between the Sun and the Earth.

If the plane of the orbit of Venus around the Sun coincided exactly with the plane of the orbit of the Earth, that is to say the ecliptic plane, there would be a transit at each inferior conjunction and transits would occur every year. However this is not the case as the plane of the orbit of Venus makes an angle of 3.4° with the ecliptic plane. This explains why transits occur only rarely.

The ecliptic latitude of Venus must necessarily be smaller than half the apparent diameter of the Sun, that is less than 16 minutes of arc (16 / 60°), for the observer to be able to see Venus moving across the face of the Sun. Venus must therefore be close to a node of its orbit, that is to say near the intersection of its orbit with the plane of the ecliptic. The angle in the diagram is exaggerated for clarity.

As a consequence of these conditions, transits occur in pairs 8 years apart, each pair occurring approximately every 120 years.

The next pair of transits will take place on June 8^th 2004 and June 6^th 2012 and the previous pair took place on December 9^th 1874 and December 6^th 1882.

The transits of June 6^th 1761 and June 3^rd 1769 were marked by the first major international campaign of observation in many places around the world.

The campaign of observation in 1761 and 1769.

Observers were posted at latitudes as far away from one another as possible, so as to improve the precision of the calculations. There had to be a substantial number of them to be sure of good weather conditions and even to be assured that they were to reach their destinations in time. The selected places were often in far distant regions and the voyages at that time were at risk from tempests and warfare between nations, as was the case in the Indian Ocean where England and France were waging war.

It must be emphasized that, in 1761, for the first time an international scientific cooperation had been set up. It coordinated more than 130 different expeditions all around the world to observe the transit.

In 1769, there were observers in Pondichéry in Madras, at St Domingue, in California, in the Bay of Hudson, in Tahiti, in Lapland at Vardö, at Cajanebourg in the peninsula of Kola and at Iakoutsk in Siberia. Altogether there were 151 observers in 77 different locations. All these expeditions had very different challenges, some of them were dramatic, and the results didn't always live up to expectations!

The transits always take place either in June or December when the declination of Venus is either approximately +23º or -23º.