as the instruments of the Edinburgh observatory. With the help of Professor David Thomson [q.v.] he re-established a disused ‘observatory’ of King’s College, Aberdeen. A small portable transit instrument was unearthed and mounted; a mean solar clock was added, with arrangements for control to within a fraction of a second of Greenwich time; to this contact springs were fitted, establishing electric control of the turret clock of King’s College and other clocks of the town. Gill next procured a small Dallmeyer refracting telescope with which he made observations of double stars. He also purchased from the Rev. Henry Cooper-Key a 12-inch speculum, the equatorial mounting for which was of his own design, and the driving-clock made by his own hands on the general plan of Airy’s chronograph at Greenwich. He said, in later life, that he never found a clock which worked better. With this instrument he observed double stars and nebulae and took some photographs of the moon.
In 1870 Gill married Isobel, daughter of John Black, a farmer of Linhead, Aberdeenshire, and shortly afterwards the opportunity came to him of devoting his time exclusively to scientific pursuits. This arose out of the friendship which he had formed with Lord Lindsay (afterwards twenty-sixth Earl of Crawford, q.v.), an amateur astronomer who was attracted by Gill’s enthusiasm and skill. Gill was offered in 1872 the charge of the private observatory erected by Lord Lindsay at Dunecht. He accepted at once, although it involved a considerable sacrifice of income. The years 1873–1874 were busily occupied in the equipment of this fine observatory. The instruments comprised a 15-inch refractor by Grubb, an 8-inch reversible transit circle by Troughton and Simms, Gill’s 12-inch speculum from Aberdeen, and a 4-inch heliometer by Repsold. Visits to many European observatories were made and Gill met the leading astronomers of the day. Preparations were next made for Lindsay and Gill to take part in the observations of the transit of Venus in 1874. It was thought desirable that the longitude of their station at Mauritius should be determined with all possible accuracy. As the electric telegraph only went as far as Aden, it was necessary to transport chronometers, of which no less than fifty were hired by Gill from the best makers. These had to be carried with great care, regularly wound, and compared. Gill took his chronometers to Greenwich, checked them, and then drove off with them cheerfully to his steamer in the docks, leaving Airy and the staff at Greenwich amazed at his temerity. By incessant watchfulness, however, the journey to Mauritius and back was made without mishap. The main interest in the expedition to Mauritius centres round the observations of the minor planet Juno, made with the heliometer for the determination of the solar parallax and thus of the sun’s distance from the earth. From observations on twelve evenings and eleven mornings a very good result was obtained by Gill, who was profoundly convinced of the possibilities of the heliometer for astronomical measurements of the highest precision. As is well known, the observations of the transit of Venus, carefully planned and executed at great expense of time and money by expeditions all over the world, gave a disappointing result. But the Mauritius expedition proved to be the inauguration of a successful method of determining the sun’s distance, a measurement of fundamental importance in astronomy.
While at Mauritius Gill was invited by General Stone, chief of the military staff of the khedive, to return via Egypt and measure a base-line for a projected survey of the country. With the assistance of the American astronomer, Professor James Watson, a base-line was laid down near the Sphinx. This was Gill’s first practical experience of geodetic work.
Gill left Dunecht in 1876. He obtained from Lord Lindsay the loan of the heliometer, and with funds obtained from the Royal Society and the Royal Astronomical Society made an expedition to the island of Ascension in order to measure the distance of Mars when it came exceptionally near the earth in the year 1877, and from the result to derive the sun’s distance. In this difficult and adventurous expedition he was accompanied by his wife, who published in 1878 an interesting account of their experiences—Six Months in Ascension: an Unscientific Account of a Scientific Expedition. The observatory was first set up at Garrison, but, owing to interference from clouds, Gill heroically moved it in the course of five days to a new site, ‘Mars Bay’, just in time to secure favourable observations. The expedition was crowned with success, the sun’s distance being determined with much greater accuracy than had previously been attained.
In 1879 Gill was appointed H.M. astronomer at the Cape of Good Hope.
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