Brukkaros Solar Observatory

In 1926 the National Geographic Society, in cooperation with the Smithsonian Institution, financed an expedition to South-West Africa (now Namibia) to measure the intensity of solar radiation. The project was managed by the Smithsonian Astrophysical Observatory, which from 1906 to 1944 was directed by Dr Charles Greeley Abbot. During these years the observatory was dedicated mainly to the study of possible variations in the solar constant and the effects of such variations on the weather. For this purpose solar observatories had been established in Chile and Arizona. To check and confirm the small variations in the solar constant found at these two sites, Abbot received a grant from the National Geographic Society in March 1925 to select a site for a third solar observatory, equip it, and carry out observations for a few years.^[1] After preliminary investigations a decision was reached to site the observatory somewhere in present Namibia.^[2][3]

Abbot arrived in South Africa (which administered South West Africa under a League of Nations mandate at that time) in March 1926. After consulting several local experts and considering sites near the town of Aus and at the Spitzkoppe he decided that the best site would be on Brukkaros Mountain, about 100 km north of Keetmanshoop. He visited the mountain in the company of Mr A. Dryden, inspector of works at Keetmanshoop, who played a major role in the planning and building of the observatory. They provisionally selected a site inside the ring mountain on the south-western side, just below the rim.^[4]

The Brukkaros Solar Observatory was built at 25°52.8′S 17°46.7′E / 25.8800°S 17.7783°E / -25.8800; 17.7783, at an elevation of about 1510 meters.^[5] Its design was based on that of the observatories already operating in Chile and Arizona. A shallow tunnel, 3 m wide and 2.2 m high, was excavated by blasting and extended outwards by cemented stone walls and a concrete roof to a total length of 10.5 m. This space was divided into three small rooms by wooden partitions and a stone instrument platform built in front of it.

An Abbot silver-disk pyrheliometer was placed on the platform outside the observatory tunnel to measure the solar irradiance. The instrument was on an equatorial mount, so that the sun could be followed by adjusting a single screw. Exposures needed to be accurately timed, for which purpose a pendulum beating half seconds was used. The altitude of the sun at the time of each observation was measured with a theodolite, so that the amount of air through which the radiation had passed could be calculated.^[6] A pyranometer was used to measure irradiance from a circular region of sky around the sun, excluding the sun itself. It was on the same mount as the pyrheliometer, so that it would automatically point in the same direction. The stone platform also contained a coelostat, consisting of two flat mirrors, one of which was driven by clockwork and mounted so as to keep the sun's rays reflected continuously through an opening in the northern wall of the tunnel. The mirrors were made of stellite, a hard cobalt alloy that does not form tarnish easily. Inside the tunnel a well dispersed spectrum of the sun was slowly passed over the sensitive strip of a bolometer. The resulting bolograph displayed the relative amounts of energy at each wavelength in the visible and infrared region of the spectrum, which could be used to determine atmospheric absorption at various wavelengths. After various corrections, the value of the solar constant could be calculated.

Regular weather observations were also made at the site from 11 August 1927 and the results reported to the Weather Bureau in Windhoek.^[7][8]