Course of the transit of Venus - Source: Thomas Tuchan, Volkssternwarte Laupheim
Reading aloud On June 8, 2004, star friends can follow a very rare celestial event. Venus will pass by the sun between 7:19 and 13:23 as a black dot. Most recently, this so-called Venus transit took place in 1882, and it was not until December 8, 2125, that the next passage of Venus from the sun was visible from Central Europe. Even with the simplest of aids, the sky spectacle can be safely tracked. A Venus transit arises exactly like a solar eclipse. Venus, earth and sun are in a plane directly behind each other. When Venus steps between the earth and the sun, it becomes visible on the solar disk. This position is so rare, as the orbit of Venus is inclined to the Earth's orbit by 3.4 degrees. The Venus runs therefore usually a bit north or south past the sun.

This time, however, Venus will walk across the solar disk. It only appears as a small point, even if the planet with a diameter of 12, 100 kilometers is almost as big as the earth. The dense carbon dioxide atmosphere of the neighboring planet causes the temperatures to rise up to 500 degrees Celsius. The air pressure on the surface corresponds to the pressure in more than 900 meters water depth, and from the sky of the inhospitable planet it rains sulfuric acid.

Rafael Lang of the Volkssternwarte Laupheim warns that the mini solar eclipse, which takes place on 8 June, must not be observed directly with the eyes or even with unprotected telescopes. Otherwise there are serious eye injuries, including blindness. The easiest way to observe it is to use a pair of special goggles, which were sold by the millions on the occasion of the solar eclipse on August 11, 1999. However, Venus is only recognizable as a tiny dot in this way.

Stern friends who are no longer in possession of such glasses or who want to see more, Lang recommends to turn to a public observatory. There, telescope observations are offered by means of special filters or sun projectors under expert guidance. Lang demonstrates the principle of such a sun projector: The image of the sun is thrown from a telescope onto a screen and can thus be safely observed. display

Stargazers can try that out for themselves: A binoculars or a telescope is attached to a tripod and aligned with the sun based on the shadow cast by the instrument. The image of the sun is then projected onto a piece of cardboard lying on the floor. The distance between the eyepiece and cardboard and the sharpness of the instrument are varied until a sharp image is formed.

The observations of Venus travelers have a long tradition. For the first time in 1639 two amateur astronomers succeeded in following the rare celestial event. The astronomer Edmond Halley developed a method for determining the distance of the Sun for the Transit 1761: The transit begins and ends for two distant observers through the different observation angles at different times. From this the sun distance can be calculated.

Several expeditions have been sent all over the world to watch the transit. However, the clocks were still too inaccurate for a precise measurement at that time. In addition, it is difficult to estimate the exact beginning and end of the transit.

A German expedition to Sumatra could not observe the transit because of bad weather. Not to have to go back eight years later on the long journey, the participants decided to wait until the next transit on the island. But the observations of the next Venus transit from 1769 and 1874 brought no clear result. It was not until 1882 that an approximately accurate determination of the sun's distance of 149, 595, 000 kilometers was achieved.

Even today, Sternfreunde can go in the footsteps of the earlier explorers and try to determine the sun's distance. The European Southern Observatory has launched a competition for amateur astronomers. Scientifically, however, the results are unlikely to be of any importance, because radar measurements enable astronomers today to measure the mean distance of the sun very accurately: it is 149, 597, 870 kilometers.

ddp / bdw - Oliver Schmid

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