A popular history of astronomy during the nineteenth century / [Agnes Mary Clerke].

  • Clerke, Agnes M. (Agnes Mary), 1842-1907.
Date:
1885
Catalogue details

Licence: Public Domain Mark

Credit: A popular history of astronomy during the nineteenth century / [Agnes Mary Clerke]. Source: Wellcome Collection.

    24/492 (page 4)
    his keenest attention; and he never relaxed his mental grip of a subject until it had yielded to his persistent inquisition. It was to these qualities that he owed his discoveries of the aber- ration of light and the nutation of the earth’s axis. The first was announced in 1729. It means that, owing to the circum- stance of light not being instantaneously transmitted, the heavenly bodies appear shifted from their true places by an amount depending upon the ratio which the velocity of light bears to the speed of the earth in its orbit. Because light travels with enormous rapidity, the shifting is very slight; and each star returns to its original position at the end of a year. Bradley’s second great discovery was finally ascertained in 1748. Nutation is a real “nodding” of the terrestrial axis produced by the dragging of the moon at the terrestrial equatorial protuberance. From it results an apparent dis- placement of the stars, each of them describing a little ellipse about its true, or “mean” position, in a period of eighteen years and about seven months. Now an acquaintance with the fact and the laws of each of these minute irregularities is vital to the progress of obser- vational astronomy; for without it the places of the heavenly bodies could never be accurately known or compared. So that Bradley, by their detection, at once raised the science to a higher grade of precision. Nor was this the whole of his work. Appointed Astronomer Royal in 1742, he executed during the years 1750-62 a series of observations which formed the real beginning of exact astronomy. Part of their superiority must, indeed, be attributed to the co-operation of John Bird, who provided Bradley in 1750 with a measuring instrument of till then unequalled excellence. For not only was the art of observing in the eighteenth century a peculiarly English art, but the means of observing were furnished almost exclusively by British artists. John Dollond, the son of a Spitalfields weaver, invented the achromatic lens in 1748, removing thereby the chief obstacle to the development of the powers of refracting telescopes; James Short, of Edinburgh, was