Celestial events

A major international program was initiated, and there were great expectations due to improvements of telescopes and observing techniques.

Observations of the transit in 1874 from Nagasaki, Japan.
Photo: Wiki Commons

Johann Encke, a German astronomer, analyzed all the observations and found that the transit in 1769 gave an average of the solar parallax on 8.6040 arc seconds, corresponding to a distance of 152 930 000 kilometers between Sun and Earth.

Combining with the results of the transit in 1769, the derived value was 8.571 arc second and 153 450 000 kilometers. Todays values are 8.794 arc seconds and 149 570 000 kilometers, indicating that Enckes values were too high. But Encke had done what was possible with the material available at the time. Moreover, other measurements not having anything to do with Venus, showed that the distance Earth – Sun was about 149.6 million kilometers.

But a lot of uncertainties remained, and there were great expectations to the next transits in 1874 and 1882. A big international program was initiated, and expeditions were sent out to every corner of the world. Both telescopes and observational techniques had been improved, Louis Daguerre had invented photography and the plan was to take a photo of Venus as it crossed the solar disk, not only focus on the start- and stop-time. The American Congress guaranteed a significant amount of money to eight expeditions, three to the northern hemisphere and five to the south.

But the American expeditions were not as successful as hoped. In Vladivostok were Asaph Hall, the man who later discovered the two small moons around Mars – Phobos and Deimos – was in charge, bad atmospheric conditions occurred and the pictures were not good. In Nagasaki, however, things were better for the Americans. George Davidson, leader of the expedition, got help from The Japanese Hydrographic Bureau and a very competent astrophotographer.

50 plates were taken during the transit, and together with the 47 pictures taken by the expedition in Queenstown, New Zealand and 15 pictures from The Kerguelen Islands in the Indian Ocean, these pictures were the only pictures the American expedition found useful.

Despite all the observations during this transit, it was still not possible to find the length of the astronomical unit with greater accuracy.

6th December 1882 was the last transit in the 1900's – the next one would take place in 2004. For the astronomers, December 6th was literally of astronomical importance!

The American Naval Observatory and Transit of Venus Commission sent eight groups around the word to observe both the 1874- and the 1882 transit. Only eleven plates are still intact from the 1882-expeditions. More pictures
Source: www.usno.navy.mil/library/rare/rare.html

The transit would be visible from easily accessible places – in other words: no reason for travelling to distant corners of the world as in 1874. Some nations such as Russia and Austria did not plan any observations – they believed there were other methods for measuring the astronomical unit.

The Americans planned a lot of expeditions and the enthusiasm for the coming transit flourished. In New York, telescopes were set up and rumours said that the stockbrokers were just as enthusiastic as everyone else. A lot of schools were closed because of the event, and the transit made front page in newspapers all over the country.

The weather forecast was promising, and several interesting observations were made, including the black-drop-effect and the fluorescent ring around Venus caused by the planets atmosphere. A lot of pictures showing the black spot slowly moving across the solar disk were taken, and it was searched for satellites around Venus.

In connection with the transit, new observatories were built: one in Argentina, one in South-Africa and one in Durban. British expeditions were sent off to various places, but the results showed wide variation.

Despite all the observations, the method for finding the astronomical unit had failed. However, the observations were not wasted as the expeditions made the beginning of international research cooperation. But Edmond Halleys belief that astronomers in future transits would be able to estimate the exact unit of the astronomical unit by using the parallax, had yet to materialize.