Transit of Venus June 8, 2004

THE DOT THAT CHANGED OUR VIEW OF THE WORLD

by Knut Jørgen Røed Ødegaard

The size of our Solar system and the distances to the stars have only been known for a few hundred years. Observations of a dot that in 1631 moved across the solar disk started a change in knowledge. In the last few centuries astronomers have used the rare transits of Mercury and Venus to measure the distances in our Solar system and to other stars.

Mercury on the solar disk on May 9, 1970 as seen from Trondheim, Norway. Somewhat later Mercury crossed the large sunspot on the right.
Photo: Steinar Thorvaldsen.

No transits were observed before the invention of the telescope in 1610. transits of Mercury cannot be observed at all without optical aid since the planet is so small and far away. Transits of Venus can be seen with the eye, but they occur so rarely that nobody detected them. To see a transit of Venus the dangerously intense solar light would have to be moderated by fog or by using coloured glass. One would also have to know exactly when to look.

A TINY SURPRISE

The very first transit was observed in 1631 by the french astronomer Gassendi. Mercury transited the solar disk. At first he did not realize that it really was Mercury and not a sunspot. The astronomers believed taht Venus was large enough to cover one fourth of the soalr disk. They knew that Venus was smaller, but estimated its angular size to be about one tenth of the Sun.

Gassendi saw a tiny black dot moving. It was moving a lot faster than sunspots should do. Finally he realized that the dot actually was the planet Mercury. The angular size of Mercury is only about one 150th of the Sun and much, much smaller than expected!

For the first time the astronomers got an opportunity to estimate the size of an other planet. They would later on use other transits to measure the distances in space.

Mercury entering the solar disk on May 9, 1970. The pictures are taken from the Solar observatory on Harestua in Southern Norway at 05:12:42, 05:14:40 and 05:15:59 CET.
Photo: Truls Lynne Hansen et al..

HOW LARGE IS THE WORLD?

Even decades into the 18. century the size of our Solar system was poorly known. The relative distances between the planets, the Sun and the Earth were known, but the astronomers had no way to measure the absolute scale.

In 1649 a transit of Venus was observed for the first time. In 1716 Edmond Halley (famous for the comet) a paper describing how transits could be used to measure the distance to the Sun and therefore the distance scale for the whole Solar system. Halley realized that transits of Venus in particular would start to slightly different times around the globe. The reason is that the observers would view the phenomenon in slightly different angles. The size of the Earth was known and the times when Venus transited the Sun could therefore be used to find the distances.

Several expeditions were sent to observe to transit of Venus in 1761. The phenomenon was observed from South-Africa, Canada, Sibiria and Madagascar. Various unfortunate circumstances made the observations almost useless.

Even more expeditions were prepared for the transit of Venus in 1769. Among them was Captain James Cooks famous journey to Tahiti. Even though France and England had just been in war, the french government instructed their navy not to attact Captain Cooks ship, since its purpose served the whole humanity!

An other expedition was sent to Vardø in Northern Norway. Christian VII, king of Denmark and Norway was aware that the transit of Venus in 1769. The king wanted the transit observed with great precision in the northermost part of his kingdom.

The danish ambassadeur in Vienna asked the royal astronomer Miksa Hell if he would travel to Vardø to lead an astronomical expedition. The king would pay all expenses. The transit of June 3, 1769 would last from 21.09 until 03.25 and in Europe it would only be visible from the northernmost Scandinavia. Hell accepted the request.

Hell and his expedition spent the very rough winter in Vardø and got useful weather conditions during the transit. He made measurements showing the solar distance to be 151.2 kilometres, rather close to the actual value of 149.6 million kilometres. The sensational results made Hell a rather famous astronomer.

For the first time the astronomers had an accurate value for the solar distance and distances to other celestial objects could be derived. Today the mean distance to the Sun is used as a standard unit of distance - the Astronomical Unit.