1. What is the transit?

On 8^th June, there was a rare astronomic event- the transit of Venus in front of the Sun disc. This event could be seen in Europe, Africa and Asia. It is a great opportunity for mutual experiment, for measuring the distance between the Earth and the Sun, popular as “astronomic unit”. The transit is used for finding out planets when they passed in front of stars.

A great number of heavenly bodies which goes round one another have different visual diameters for example – double stars, the Sun and its planets or a planet and its satellites. The smaller and darker component lies in interior side, watched from the Earth, then it is possible the component to pass in front of the bigger body. We called this “transit”. The most famous transit is the solar eclipse, when the Moon goes in front of the Sun. As the two heavenly bodies had one and the same visual diameter, the Moon hides the Sun and we see the solar eclipse. The similar transit we have when a satellite or its shadows goes in front of the disk of the planet – for example Galileo’s satellites or its shadows on Jupiter and Titan on Saturn. These events we called “shadow’s transit”

2. When does the Venus transit come?

The Venus transits are quite interesting but rare phenomena. It happened when the watcher front the Earth sees the Venus in front of the Sun. The Venus goes round the Sun quicker than the Earth and goes in front of the Sun in straight line and with one and the same speed. The transit happened when Venus had down-connection with the Sun when Venus lies between the Sun and Earth. Then Venus can be see as a small black circle on the Sun disc, going from east to the west end. When Venus passed down to the Sun, Venus is the closest to the Earth and it is possible the transit to be seen with our eyes (through dark glass!).

If the plain of the Earth’s and Venus’s orbits were the same, the transit could be seen with every down-connection with the Sun. They would repeat periodically after 548 days (sinodic period of Venus). But the Venus orbit makes corner 3.4⁰ with   the plain of the ecliptic. That’s why the transit happens so rare and irregularily. In order to be seen Venus on the Sun disc, ecliptic broadness of Venus have to be smaller than the half of the visual sun diameter, or 16 minutes(16 / 60°). That’s why Venus has to be close to the knot of its orbit or the cross point of the orbit’s plain with the ecliptic. The corners of the diagram are exaggerated many times.

Our Earth goes through the Venus orbit’s knots at the beginning of June and December every year. Transit of Venus is possible only when the Earth and Venus are in one and the same Venus orbit’s knot. Only in case the Sun, the Venus and the Earth are on one straight line, which connect them. From the other side the transits are possible only at the beginning of June and December when the Earth is in one of the Venus orbit’s knots. If there are down-connection of the Venus to the Sun and the Earth is in the knot of Venus orbit but the Venus is not in this knot – transit is impossible. Also in the same case if the Earth is not in the Venus orbit knot but Venus is – transit is impossible. As we see, the conditions for Venus transit want the exact position of the Venus and the Earth in connection with the Sun.

Such positions are happened to be in exact periods of time, the lasting of which we can find through the relation between sinodical period and the Earth year. It can be seen, that five synodics periods of Venus are almost equal to eight years (584.5=2920 days; 365,25.8=2922 days). That shows the Venus transits are possible after 8 years, but not after every 8 years.

As we have in mind that 152 sinodics periods of Venus are almost equal to 243 earth’s years, in this way after a transit of 8 years, the next could happen after 235 or 243 years. But the passages come with going-up and going-down knots of its orbits and that is way they come in the next order: …8, 105½, 8, 121½, 8, 105½, 8, 121½,… years. Passages of Venus after 1600 were and will be:

*It’s taken from the book “Venus” A. Petrov     

Following of this conditions, transit happened in pairs – in every 120 years. This pair will be on 8^th June 2004 and 6 June 2012, the previous pair was 9 December 1874 and 6 December 1882. Till now, the transits of Venus that were watched are in 1639, 1761, 1769, 1874 and 1882 as they caused a great interest. The purpose was the corner dimensions of Venus to be found.