Abstract
SIR HAROLD SPENCER JONES opens his presidential address to Section A (Mathematics and Physics) by pointing out that, until far into the eighteenth century, astronomical observations were concerned almost entirely with the determination of the positions and movements of the heavenly bodies. In the seventeenth century the transit instrument became the standard instrument for the determination of right ascensions, while the mural circle was generally used for the measurement of declinations. Eventually the two instruments were combined to form the transit circle, which, with improvements in detail, has continued up to the present day to be used for the fundamental determination of positions. As requirements for precision have increased, its limitations have become more apparent. The errors of adjustment, continually changing with the ambient temperature conditions, are difficult to control accurately ; complicated flexure effects, the error of figure of the pivots, and minute mechanical displacements in a telescope which is swung into different positions, introduce errors which are partly of a systematic nature and limit the accuracy with which a fundamental reference system of star places can be provided. Consideration is, therefore, being given to other designs of meridian instrument, likely to be less affected by systematic errors. In one of these the position of the instrument for each observation is determined by optical methods ; in another the moving parts are reduced to a mirror, which can be turned to reflect light into one or other of two horizontal telescopes in the meridian, to the north and south of the mirror. For the determination of time, the development of the principle of Airy's reflex zenith tube to permit of photographic observations provides a much higher accuracy than is attainable with the transit circle.
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Summaries of Addresses of Presidents of Sections. Some Developments in Astronomical Instruments. Nature 164, 387–388 (1949). https://doi.org/10.1038/164387a0
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DOI: https://doi.org/10.1038/164387a0