Transits of Venus, in which the planet Venus crosses the face of the Sun as seen from Earth, are rare phenomena that occur at intervals of 105.5 years, 8 years, 121.5 years, and 8 years, respectively. Two such transits occurred in the 19th century, in 1874 and 1882. The importance of these transits was that by precisely timing the motion of Venus accross the Sun, they provided a method of determining the solar parallax, and thus Earth-Sun distance and the scale of the solar system, one of the great unsolved problems in the history of astronomy at the time. In a broader sense the worldwide efforts to observe this phenomenon are important because of the debate over photographic techniques, and because they are part of the overall history of the determination of the astronomical constants. Eight American expeditions were fitted out in 1874, organized by the Transit of Venus Commission, with Simon Newcomb as Secretary. The U. S. Congress appropriated funds totalling an astounding \177,000 for the expeditions. Although Newcomb considered the result of the 1874 observations disappointing due to inherent difficulties in the method, at the urging of Naval Observatory astronomer William Harkness, in 1882 Congress once again appropriated some \10,000 for improving the instruments, and \$75,000 for sending eight more expeditions. Despite reservations about the method, Newcomb (now Superintendent of the Nautical Almanac Office) led an expedition to South Africa, and among the four northern stations, Asaph Hall led the expedition to San Antonio, Texas. The value of the observations made by the 1882 American expeditions also caused considerable dispute. Newcomb's statement in his Reminiscences (1903) that the 1882 results were never officially published has been taken to mean that no results were ever produced. In fact Harkness spent a considerable part of his career analyzing the data, producing a value for the solar parallax, and putting it in context of other astronomical constants. This he did in his lengthy monograph "The Solar Parallax and its Related Constants," published in 1891 in Washington Observations. Newcomb in fact used this value in his famous volume The Elements of the Four Inner Planets and the Fundamental Constants of Astronomy (1895), but gave it a much lower weight than most other methods. The next transit of Venus will occur, as Harkness put it "when the June flowers are blooming in 2004," followed by the second of the pair in 2012. Since the astronomical unit has now been determined by radar with an uncertainty of a few kilometers, the scientific value of the phenomenon will be limited. But the coming transits of Venus will still be eagerly anticipated as one of astronomy's rarest occurrences, occurrences that at one time stimulated herculean efforts and great controversy.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1995