(Published in GralsWelt 31/2004)
A rare astronomical event
On June 8, 2004 and then on June 6, 2012, a rare astronomical phenomenon awaits us: Venus, visible from Earth, will pass the solar disk.
Such a “Venus passage” presupposes that Venus is at a node *) of its orbit in the conjunction Sun / Venus. This happens only four times in a cycle of 243 years, and then twice in a row. The last passage of Venus was in 1882, there was none in the 20th century, and the next on June 8, 2004 has the following times:
Entry: 7:20:45 CEST (Central European Summer Time)
Anyone who has lifted their dark protective goggles since the solar eclipse of 1999 can see Venus as a small, dark point wandering across the solar disk; observation with unprotected eyes is strongly discouraged.
A telescope would be better, of course with a suitable sun visor. Viewing the sun through binoculars or telescopes without suitable solar filters can lead to blindness.
This rare celestial phenomenon prompts us to look back in history and tell of the passages of Venus in the past.
The meaning for astronomy
Edmund Halley (1656-1742), a friend of Isaac Newton, who became famous for his calculation of Halley's comet, recognized that by precisely observing a passage of Venus from various places on earth, the distance to the sun could be determined, which at that time was not possible was exactly known.
The earth-sun distance is a basic unit of astronomy, the exact knowledge of which is of the greatest importance for astronomical calculations, astronomical navigation, etc.
At the same time, Halley knew that he would not experience the next Venus passages in 1761 and 1769, and that he would have to leave the observation of these to his successors.
Earlier passages of Venus had taken place before Halley's birth, e.g. in the years 1631 and 1639, each December. They were not observed because they were not visible from Europe and there were no astronomers with suitable instruments in the other hemisphere.
When the next round took place in June 1761, only a few, unsatisfactory measurements were possible. Europe was in the middle of the Seven Years War (1756-63). This was not just a European war, for the struggle between England and France also extended to colonial areas; actually it was already a world war.
In addition, there was still a lack of experience in handling the new optical instruments. Some Russian and Swedish explorers went north, and an Englishman wanted to observe from India; However, he only got as far as the Cape Colony because his ship got into a battle with a French warship and was damaged.
A prerequisite for a successful measurement of the earth-sun distance when passing through a planet are measurements at at least two points on the earth's surface, which should be as far apart as possible to the north or south.
In the age of science
After the Seven Years' War there were good conditions for international scientific cooperation. Many people in the Enlightenment were interested in scientific questions, and in the age of the great seafarers no one could doubt the usefulness of astronomical knowledge for navigation.
Last but not least, the monarchs also felt that they had spent far too much money on war and destruction, and that it was now time to do something for peaceful purposes as well; a situation similar to that after the Second World War: in 1957 the "International Geophysical Year" began, in which 67 countries worked together in jointly organized research companies.
Even after the Seven Years' War there were princes and wealthy merchants who supported research trips as patrons. We would like to go into some of the remarkable adventurous research trips as part of the first concerted scientific action, the worldwide observation of the Venus passage:
Expedition to San José
A French / Spanish expedition traveled to Central America. The scientific direction was Jean-Baptiste Chappe Auteroche (1722-1769), member of the French Academy of Sciences. Two Spanish naval officers (Doz and Medina) were responsible for the ship's command. In December 1768 the journey began in Cadiz. In 77 days the Atlantic was crossed and landed in Mexico. Now a forced march of 1,500 km led over Mexico City and the inaccessible Sierra Madra to the Pacific. There it went on with a mail boat to the Franciscan Mission San José on the Baja California.
Here the instruments had to be set up and adjusted, the exact location coordinates and the local time had to be determined, etc.
Then an epidemic broke out and the Spaniards wanted to leave San José. Chappe knew there was not enough time to change position and stayed with his team. He was then able to carry out the important measurements of the passage of Venus in clear weather and in extreme heat.
Then the research group was attacked by the disease (presumably cholera), which killed 19 of 28 expedition members, including Chappe, far from home.
The Jesuit father Maximilian Hell (1720-1792) received an invitation from the Danish king to travel to Vardö (then Danish, now Norwegian) on the Barents Sea.
On land and on water, Hell and his companions fought their way to the far north and wintered in the remote settlement in the extreme cold. Hell's main concern was the valuable instruments that he needed to protect from damage from the cold; For example, he filled the dragonflies on the spirit levels with high percentage alcohol so that they would not freeze and burst.
When the big day came, he was lucky with the weather because the clouds broke up just in time and allowed for good observations.
A bad fate met the Göttingen scientist Georg Lowitz, whom Catherine II (1729-1796) sent to the Caspian Sea to observe the passage of Venus.
Rebel Cossacks captured him as a surveyor for the tsarina and cruelly murdered him.
Observations in Tahiti
The famous explorer James Cook (1728-1779) was also sent out with the astronomer Charles Greene on the Endeavor, a converted coal ship weighing only 370 tons. After crossing the Atlantic and circumnavigating Cape Horn, the expedition landed in Tahiti on April 13, 1769.
The Venus observations could take place with a clear sky.
On the return voyage, which Greene did not survive, the South Seas (New Zealand, Australia) were explored, and on July 12, 1771, England happily reached.
The big arithmetic
After the data from the various observers were available, the evaluation could begin. In total, measurements from 151 scientists who had worked at 77 stations were available.
The result was not unsatisfactory. A value between 8 ½ and 10 ½ arc seconds was determined for the solar parallax **) (today's value 8.80 arc seconds). For the scientists, however, that was not yet accurate enough.
In order to get more exact results, the next Venus passages had to be awaited in the years 1874 and 1882.
Then, with much improved technology, it was finally possible to determine almost exactly the value for the distance from earth to sun ***) that is still valid today.
In our time the Venus passages have largely lost their sensational significance for planetary astronomy, but no star lover should miss the opportunity to experience this rare natural spectacle.
*) Knot: Section of the path of a celestial body with the apparent path of the sun (= ecliptic), seen from the earth.
**) Parallax: In our case, the difference in angle that results when an object (e.g. the sun or the moon) is measured on the one hand from the equator and on the other from a pole, i.e. the "equatorial horizontal parallax".
***) Today's value 149,579,870 km.
(1) Herrman, Joachim "dtv-Atlas zur Astronomie", Deutscher Taschenbuchverlag, Munich 1973.
(2) Littrow, Jos. Joh. / Stumpff, Karl "The miracles of heaven", Ferd. Dümmler, Bonn 1969.
(3) Schmidt, Arno “The more beautiful Europe”, Works II / 1, Hoffmannsverlag 1989.
(4) “Wettlauf zur Venus”, ZDF film in the “Expeditions” series, broadcast on April 16, 2001, 7.30 p.m.