The History of Astrometry

The history of astrometry, the branch of astronomy dealing with the positions of celestial objects, is a lengthy and complex chronicle, having its origins in the earliest records of astronomical observations more than two thousand years ago, and extending to the high accuracy observations being made from space today. Improved star positions progressively opened up and advanced fundamental fields of scientific enquiry, including our understanding of the scale of the solar system, the details of the Earth's motion through space, and the comprehension and acceptance of Newtonianism. They also proved crucial to the practical task of maritime navigation. Over the past 400 years, during which positional accuracy has improved roughly logarithmically with time, the distances to the nearest stars were triangulated, making use of the extended measurement baseline given by the Earth's orbit around the Sun. This led to quantifying the extravagantly vast scale of the Universe, to a determination of the physical properties of stars, and to the resulting characterisation of the structure, dynamics and origin of our Galaxy. After a period in the middle years of the twentieth century in which accuracy improvements were greatly hampered by the perturbing effects of the Earth's atmosphere, ultra-high accuracies of star positions from space platforms have led to a renewed advance in this fundamental science over the past few years.Comment: 52 pages, 14 figures. To appear in The European Physical Journal: Historical Perspectives on Contemporary Physic

A seismic study of crustal structure in the region of the western isles of Scotland

In November 1979, the Department of Geological Sciences, University of Durham, in conjunction with the Department of Geology, University of Glasgow, carried out a marine seismic refraction survey between Barra In the Outer Hebrides and Girvan on the Ayrshire coast. In August/September 1981, a shorter profile, between Mull and Kintyre, was undertaken. Temporary recording stations were set up on land and explosives and alrguns used as sources at sea. In addition, data were obtained from the permanent recording networks in Scotland. The application of digital filtering techniques to the alrgun lines Is presented and reviewed. The explosive shot data were interpreted using time-term analysis, the plus- minus method and ray-tracing. Gravity and magnetic studies were used as a control on the interpretation. The depth to the basin has been examined, indicating sedimentary cover of between 1 and 3 km along the line. The variation in basement velocity has been determined. A velocity model for the upper crust has been developed indicating large lateral changes. A steep gradient Is thought to exist between Golonsay and Jura and a more gentle gradient beneath Mull. There does not seem to be evidence for a mid-crustal refractor with a sharp velocity transition across the boundary. Large changes In Pn time-terms across the Minch and Inner Hebrides basin are thought to be largely due to the varying velocity structure of the basement. The basin formation and deformation of the lower crust are thought to Involve movement within the mantle