The Lake County Astronomical Society

Astronomy Bio...David Gill

Jay Bitterman

Sir David Gill was born in Aberdeen Scotland on June 12, 1843; he was the eldest surviving son of a well-established clock and watchmaker. At Marischal College he attended classes for two years given by the physicist and astronomer James Clerk Maxwell, after which he studied clock making in Switzerland. There he gained experience in business practices, became expert in intricate mechanisms and fluent in French. He continued his studies in Coventry and Clerkenwell (London) after which he went back to Aberdeen to run his father's business for ten years. During this time he developed an interest in astronomy and astronomical techniques.

In 1862 he was allowed to use the small telescope at King's College Observatory after returning to Aberdeen. He also acquired a 12-inch (30.5 cm.) reflector for the College so he could try to determine stellar parallaxes.

In 1872 he was invited to become director of Lord Lindsay's new Observatory at Dun Echt, which was 12 miles from Aberdeen. Gill was given the task to equip and supervise the construction of this new Observatory, which Lindsay insisted on being the very best possible. Gill met many important European astronomers, skilled instrument-makers and gained contacts that were to be very useful in his later position at the Cape of Good Hope. He became proficient in using the heliometer, which was a rather old-fashioned instrument that required a great deal of precise hand and eye coordination. The heliometer was originally designed to measure the variation of the Sun's diameter during different seasons, and it was also used to accurately measure angular distances between stars. He was known to be the last great master of the heliometer and his parallax measurements of the Southern stars were unsurpassed for many years. He went on a six-year expedition with Lord Lindsay and others to Mauritius to measure the distance of the Sun and other related constants, particularly during the 1874 transit of Venus. Gill used a method that Edmond Halley proposed which involved combining the times of the transit of Venus across the face of the Sun as observed from as many places as possible, widely spaced around the world.

When he was on the Island of Mauritius, he used his heliometer to observe the near approach of the minor planet Juno, and was able to deduce an accurate value of the solar parallax. While on a private expedition sponsored by The Royal Astronomical Society to Ascension Island in 1877, he used the same technique to measure solar parallax by considering the near approach of Mars. He used a 4-inch (10 cm) heliometer for both of these expeditions.

In 1879 he was appointed to the Observatory at the Cape of Good Hope that had been constructed in 1820 to observe the Southern Hemisphere. The building was in poor condition and most of the instruments were in need of repair. The obtained observational results were never published. Gill was very unhappy and was especially dissatisfied with the Airy transit circle; although it was exactly like the one at Greenwich. Gill devised an improved version which, when finally built in 1900, became the standard for most transit circles. After using the 4-inch heliometer for many years, in 1887 he installed a 7-inch (18 cm) instrument. With the collaboration of many astronomers, he made extensive studies of some of the minor planets including Iris, Victoria and Sappho. In 1901 he determined the first accurate measurement of the solar parallax: 8.80". Gill's measurement was used in all almanacs until 1968, when it was recalculated as 8.794 by observations made with a Mariner space probe using the radar echo method. He also determined the distances, using the heliometer, of twenty of the brighter and nearer Southern stars.

In 1882 it was the bright comet that was of great interest to astronomers. When Gill saw a photograph of the comet he realized that it would be possible to chart and measure the positions of a star using photography. He immediately started an extensive project, with the aid of other observatories, to produce the Cape Durchmusterung, which locates the positions and brightness of more than 450,000 Southern stars. It was the first important astronomical project to use photography.

Gill was also a member of the original council for the International Astrographic Chart and Catalogue that would give the precise positions for all stars to the 11^th magnitude. Although all of the photographs had been taken by 1900, it was not completed until 1961.

On May 24, 1900 he was made Knight Commander of the Order of the Bath. In 1906 he retired for health reasons, and died of pneumonia in London on January 24, 1914.