Page:Popular Science Monthly Volume 11.djvu/186

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OR the ordinary purposes of life in a state of society which is not yet complex, a very simple system of recording the lapse of time is sufficient. Sunrise and sunset are local phenomena, which from the earliest times forced themselves upon the attention of every one, and which throughout the early centuries sufficed for the division of time. A further division of the duration of the day (as defined by the continuance of sunlight) was obtained by noting the time of noon, and there is no historic period known in which the method of obtaining a rough approximation to this instant by means of the shadow of a vertical rod or pillar was not understood. Probably the observation of such a gnomon or style constituted the first step in astronomy of precision as distinguished from that astronomy in which numbers do not play the most important part. The instant so determined is technically called the instant of apparent noon at any place, and it marks the moment when the sun is highest above the horizon and on the meridian.

Until within a hundred years this apparent time, that is the time marked by the angular distance of the sun from the meridian of any place, was the system universally adopted. A watch should mark 12h 0m 0s when the sun was highest. But the lengths of apparent solar days, or the time elapsed between two successive apparent noons, are not equal at different parts of the year, since the true sun does not move in a plane perpendicular to the earth's rotation axis (the equator), but in the ecliptic, a plane greatly inclined to the equator, and since the sun's motion in the ecliptic is not uniform. Hence arises an inequality in these apparent solar days, and a capital advance was made by the adoption of mean solar time, which is now universal. Local mean noon is the time when an imaginary sun supposed to move uniformly in the equator is on the meridian of any place, as New York, and a mean solar day is the interval between two successive mean noons. This is divided into twenty-four hours, and these again into minutes and seconds, and the length of these units is practically invariable.

The time of mean noon differs from the instant of apparent noon no less than sixteen minutes at certain times in the year, being