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Page:Popular Science Monthly Volume 29.djvu/815

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UNIVERSAL TIME.

UNIVERSAL TIME.[1]
By W. H. M. CHRISTIE, F. R. S.,

ASTRONOMER ROYAL.

CONSIDERING the natural conservatism of mankind in the matter of time-reckoning, it may seem rather a bold thing to propose such a radical change as is involved in the title of my discourse. But, in the course of the hour allotted to me this evening, I hope to bring forward some arguments which may serve to show that the proposal is not by any means so revolutionary as might be imagined at the first blush.

A great change in the habits of the civilized world has taken place since the old days when the most rapid means of conveyance from place to place was the stage-coach, and minutes were of little importance. Each town or village then naturally kept its own time, which was regulated by the position of the sun in the sky. Sufficient accuracy for the ordinary purposes of village life could be obtained by means of the rather rude sun-dials which are still to be seen on country churches, and which served to keep the village clock in tolerable agreement with the sun. So long as the members of a community can be considered as stationary, the sun would naturally regulate, though in a rather imperfect way, the hours of labor and of sleep and the times for meals, which constitute the most important epochs in village life. But the sun does not really hold a very despotic sway over ordinary life, and his own movements are characterized by sundry irregularities to which a well-ordered clock refuses to conform.

Without entering into detailed explanation of the so-called "equation of time," it will be sufficient here to state that, through the varying velocity of the earth in her orbit, and the inclination of that orbit to the ecliptic, the time of apparent noon as indicated by the sun is at certain times of the year fast and at other times slow, as compared with twelve o'clock, or noon by the clock. [The clock is supposed to be an ideally perfect clock going uniformly throughout the year, the uniformity of its rate being tested by reference to the fixed stars.] In other words, the solar day, or the interval from one noon to the next by the sun, is at certain seasons of the year shorter than the average, and at others longer, and thus it comes about that, by the accumulation of this error of going, the sun is at the beginning of November more than sixteen minutes fast, and by the middle of February fourteen and a half minutes slow, having lost thirty-one minutes, or more than half an hour, in the interval. In passing, it may be mentioned as a result of this that the afternoons in November are about half an hour shorter than the mornings, while in February the mornings are

  1. Address delivered at the Royal Institution of Great Britain, March 19, 1886.