Born Ecclefechan, (Dumfries and Galloway), Scotland, 17 December 1837
At the United States Naval Observatory [USNO], William Harkness reduced the USNO photographic observations of the nineteenth-century transits of Venus, producing the only valid solar parallax based on that technique. He carried out research in positional astronomy, photography, spectroscopy, and instrumentation design.
Harkness was the son of Reverend Dr. James Harkness, physician and Presbyterian minister, and Jane (née Weild) Harkness. The family immigrated to the United States from Scotland in May 1839, settling in New York City, then in Fishkill, New York. Harkness attended the Chelsea Collegiate Institute in New York City and private schools in Fishkill Landing and Newburgh. He entered Lafayette College, Easton, Pennsylvania, in 1854, but transferred to Rochester University in 1856 when his family moved to Rochester, New York. Harkness graduated in 1858 with an A.B.. degree, then worked as a legislative reporter, first for the Albany, New York, Atlas and Argus, then in 1860 for the Harrisburg, Pennsylvania, Daily Telegraph. He returned to Rochester and received his M.A. degree in 1861 and ultimately an LL.D. in 1874.
Harkness studied medicine at the New York Homeopathic Medical College and obtained an M.D. degree in 1862, after which he served as a surgeon in the Union Army during several major battles of the Civil War. In 1862, he was appointed an aid at the USNO and, in the following year, a professor of mathematics in the navy. During his service on the Monitor-style warship Monadnock from 1865 to 1866, Harkness made an exhaustive study of terrestrial magnetism and the influence of iron armor on the behavior of the compass. His report was published by the Smithsonian Institution in 1871.
Harkness was attached to the Hydrographic Office of the United States Coast Survey in Washington, DC, from October 1866 until October 1867, when he was transferred to the Naval Observatory. There, he pursued a lengthy career in astronomy. He was appointed astronomical director of the Naval Observatory in 1894 and director of the Nautical Almanac Office in 1897. Harkness held both posts until his retirement in 1899. He was elected vice president of the American Association for the Advancement of Science in 1881 and 1885, and its president in 1893.
In 1871, Harkness was appointed one of the original members of the United States Transit of Venus Commission, charged with planning and coordinating American observations of the 9 December 1874 and 6 December 1882 transits. By timing the passage of Venus across the Sun’s face, astronomers hoped to better determine the solar parallax, and from that to calculate an improved value of the astronomical unit, the average distance between the Earth and Sun. Harkness developed most of the instruments, plus the observation and reduction techniques used by the transit parties. For the 1874 transit, Harkness employed the relatively new technology of wet-plate photography.
A disagreement arose between Harkness and influential commission members Simon Newcomb and Edward Pickering , as to the accuracy of the photographically determined astronomical unit. Nevertheless, Harkness vigorously defended photographic observations of transits even after the German and English teams abandoned them in favor of visual observations. Harkness’s 1881 paper, “On the Relative Accuracy of Different Methods of Determining the Solar Parallax,” was instrumental in convincing US and French astronomers to continue the use of photography – now the dry-plate process – for the 1882 transit. From the photographic data on both transits, Harkness published what is arguably his most significant contribution to astronomy, The Solar Parallax and Its Related Constants. There, he reported a solar parallax of 8.842 ± 0.0118″, equivalent to an astronomical unit of 92,455,000 miles with a probable error of 123,400 miles. He later refined the parallax to 8.809 ± 0.0059″ and the astronomical unit to 92,797,000 miles with a probable error of 59,700 miles.
Among Harkness’s other scientific contributions were the discovery of the coronal spectral line K 1474 during observations of the total solar eclipse of 7 August 1869, the invention in 1877 of the spherometer caliper, which was the most accurate device known at the time to determine the figure of the pivots of astronomical instruments, an 1879 paper on the theory of the focal curve of achromatic telescopes, extensive experimentation in the 1880s to improve photographic recording of both the ordinary solar spectrum and the coronal spectrum during eclipses, and improvements to Naval Observatory facilities and observing procedures in the 1890s.
Harkness’s correspondence and writings are archived at the USNO, Washington, DC, and the University of Rochester.