Measuring The Astronomical Unit

The fundamental aim of the transit expeditions was to enable the astronomical unit—the AU, the Earth—Sun distance—to be determined. It would be remiss if I did not complete that part of the story.

It already has been mentioned how Simon Newcomb, in 1891, reexamined the 1761 transit data and, using the correct geographical coordinates for the observation sites, showed that the timings were consistent with the actual solar distance. In fact he did this likewise with the 1769 data, handling well over a hundred timings, some of which he had to reject as clearly erroneous.

Newcomb was not the first to attempt this reanalysis. Astronomers did not simply wait for the 1874 and 1882 transits to arrive. In the first half of the nineteenth century various attempts were made to exploit the 1761 and 1769 data. The German astronomer Johann Encke did this, but ended up with an answer making the solar distance somewhat larger than indicated by other techniques; the result was that the transit observations were distrusted until Newcomb demonstrated their veracity.

In both 1874 and 1882 renewed efforts were made to determine the AU through Venusian transits. In the former year the United States alone sent three expeditions to Siberia, Japan, and China to achieve northern sightings, and five groups to New Zealand, Australia, and Kerguelen Island in the Southern Hemisphere, the advent of photography allowing a permanent record of the phenomena to be made. The weather stymied much of the photography, and comparatively little success was met by the Americans or the numerous British, French, and Russian groups, and others. The Germans did better, obtaining clear weather at all six of the sites they had chosen. In 1882 a similar array of astronomers observed the path taken by Venus across the Sun, although American astronomers did not need to venture too far: the whole transit was visible from the eastern two-thirds of North America and all of South America.

Science moves on, though. In 1898 the large Earth-approaching asteroid 433 Eros was discovered. Within a couple of years, astronomers were using parallax observations of Eros in the same way as Mars had been employed earlier. Eros comes much closer to us than Mars, leading to a more accurate evaluation of the AU.

The invention of radar led to the ultimate determination of the AU. Again,Venus has been involved, although in a quite different way. By bouncing radio pulses off that planet and timing the echoes' return, the solar distance now has been measured with a precision unimaginable to Halley, Cook, and all others involved when transits of Venus were considered by many to be the only viable avenue to improved navigation.

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