Bryan Cookson, Master of Arts (MA), wrote to David Gill*, astronomer at the Royal Observatory, Cape of Good Hope, in September 1897 from Beauly (near Inverness), Scotland, to ask his advice about becoming an astronomer, instead of entering his father's lucrative business. Gill, who had faced the same choice in his early years, advised him to follow his inclinations and study astronomy. The two met personally in 1901, when Cookson was completing his studies at Cambridge. He published two papers that year, "Description of a floating photographic zenith telescope..." and "On the accuracy of eye observations of meteors...", both in the Monthly Notices of the Royal Astronomical Society (1901).
At that time it had already been decided that Cookson would come to the Cape observatory to learn practical astronomy, and he arrived that same year. Around the time of the opposition of Jupiter in 1901, between 24 June and 27 September, he made heliometer observations of the relative position angles and distances of Jupiter's four large satelites. (A fifth much smaller inner satelite had been discovered by Barnard in 1892). The work was carried out under Gill's direction, according to a plan followd by Gill when making similar observations in 1891. At the opposition in 1902, during 48 nights, Cookson made a second series of 570 observations. At the same time astrographic plates of Jupiter's satelites were taken for the same purpose. He also completed the reduction of the observations made the previous year and discussed the results in "The mass of Jupiter and corrections to the elements of the orbits of the satelites from heliometer observations made at the Cape during the years 1901 and 1902" (Ibid, 1904). In 1905 he was still (or again) working with the heliometer at the Cape, making a series of observations to determine whether the instrument's scale value is affected by a term depending on the square of the (angular) distance measured. Such a term would have affected his determination of the mass of Jupiter, but it turned out to be close to zero.
In 1906 Cookson completed the first of two monographs on his work at the Cape: "Determination of the mass of Jupiter and orbits of the satelites, from observations made with the Cape heliometer". It was published as Volume 12, Part 2, of the Annals of the Cape Observatory. The next year his second contribution, "Determination of the elements of the orbits of Jupiter's satelites, from photographs taken at the Cape in 1902", was published in the same series (Vol. 12, Part 4). A paper on the same topic also appeared in the Proceedings of the Royal Society (Series A, 1908). Years later his observations, combined with those made by Gill in 1891 and others made at the Cape in 1903 and 1904, were used by the Dutch astronomer W. de Sitter* to derive final inclinations and nodes of the orbital planes of the satelites.
Meanwhile Cookson also investigated variations in latitude and wrote "The effect of the lunar deflection of the vertical on latitude observations" (Proceedings of the Philosophical Society, 1906). Later he used the instrument that he had first described in 1901 in "A research on the aberration constant and the variation of latitude by means of a floating zenith telescope" (Memoirs of the Royal Astronomical Society, (1911, Vol. 60, pp. 83-139).