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Page:Popular Science Monthly Volume 20.djvu/318

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MANY of the discoveries of science which at the time are regarded merely as refinements—very interesting, but without practical value—sooner or later find their special uses in supplying wants before unfelt. It is but one of the evidences of the advance of civilization that exact methods of dividing and measuring time are now in demand, not only by scientists and professional men as formerly, but by persons in the most ordinary pursuits of life. To railroad-men and watch-makers as a matter of necessity, to manufacturers and business-men as a matter of economy, and to individuals as a matter of convenience, it has come to be highly important to know what is the exact time of day to the second, in circumstances where half a century ago it would have quite sufficed to know the minute or even the hour. This may be due to the increased value of time when measured by the number of events or the magnitude of operations which modern ingenuity is capable of crowding into a given interval; there can be no doubt that a second to-day records a greater stride in the world's progress than did many hours in the days of our ancestors. Of so great importance, for many evident reasons, has the knowledge of the exact time become, that much thought of some of the best heads has been devoted to methods of ascertaining it and making it available by distribution for public use.

The methods of obtaining the exact time by astronomical observations have long been well established, and are, except in minor details, the same in all parts of the world. It will here be sufficient to say, in explanation of the usual method, that time is determined by observing the transit, over the meridian, of stars—or other heavenly bodies—whose position is known by previous calculation verified by repeated observation. The difference between the time of the calculated meridian passage and the time indicated by the clock when the star was observed to pass the meridian is the error of the clock. The face-reading of the clock at the instant of transit corrected for this error is the exact time at that instant.[1]

  1. It is, perhaps, needless to say that the operation of taking time by the transit instrument is really far more complicated than would appear from the description above; but the difficulties arise only from mechanical or physical imperfections, or from uncertain or changing conditions. Thus, corrections must always be made in nice work for errors in the instrument or its setting—such as the level, azimuth, and collimation corrections—for personal equation of the observer, and for aberration; these corrections, however, only aid the observer in ascertaining the exact instant when the star actually crossed his meridian and do not in any way affect the principle already given. For a full account of the methods of making these corrections, the reader is referred to Chauvenet's "Astronomy."