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Weights and Measures: Greenwich

Each nation selects some meridian of longitude as its standard from which to reckon, but the two in principal use throughout the Englishspeaking world are that of England which passes through Greenwich and that of the United States which passes through Washington. The meridian of Greenwich is most commonly employed as an international standard.

All meridian lines run north and south, and when the rays of the sun are vertical at any point of a given meridian, it is midday or noon at all places on this meridian which are then lighted by the sun. One half of every meridian circle is in light and the other half in darkness.

The rotation of the earth on its axis gives the unit of time, called a day. The day is divided into twentyfour equal parts, called hours. Since the earth rotates on its axis from west to east, the sun appears to revolve around the earth from east to west, and its rays move westward at the same rate over the earth's surface. Hence when it is noon, or twelve o'clock, at any place, it is past noon at all places east of its meridian, and before noon at all places west of its meridian. When, for example, it is noon at Cincinnati, it is later than noon at New York, and before noon at St. Louis.

Measurement of Time.-Formerly, when traveling was slow, time could be adequately measured for practical purposes by reference to the sun and by means of sundials and hourglasses.

But the introduction of modern means of rapid communication and the invention of clocks and watches have enormously increased the importance of the accurate measurement of time. Hence an outline of 1 the present mode of measuring time is of general interest.

The standards afforded by nature are the revolution of the earth about the sun, constituting the year; the revolution of the moon about the earth, the month; and the rotation of the earth upon its axis, the day. Of these the longest which can be determined by direct observation is the year. The length of the year may be established by observation in two ways, which give rise respectively to the "solar" and the "sidereal" year.

The solar year may be defined as the mean interval between two returns of the sun to the vernal equinox. This takes place about March 21st. This is the year upon which the change of the seasons depends. The sidereal year is the mean interval between two returns of the sun to the same star. This is the true time of the earth's revolution, but is slightly longer than the solar year. Since, however, the change of seasons is what marks the length of the year for the practical purposes of life, the solar year is universally recognized as the standard.

The lunar month or the interval between two new moons is the next shorter unit of time. It does not, however, consist of an even number of days, nor is it an even fraction of the solar or sidereal year. Hence it has given place to an arbitrary division of time called the calendar month. 1 The most accurate and useful measure of all is the day, both because of the practical importance of the alternation of day and night, and because, so far as can be ascertained, the time of the earth's revolution on its axis does not change by as much as Too of a second in a century. As in the case of the year, there are two ways of determining the duration of the day, giving rise respectively to the sidereal and the solar days. The sidereal day is the interval between two passages of a star across the meridian. The solar day is the interval between two passages of the sun over the meridian. Hence astronomers have set aside the use of the sidereal year and sidereal day, and the lunar month and year, as standards, and have based the measurement of time for practical purposes on the solar day and year, which are determined by the passage of the sun over the meridian. 66 Accepting this as the basis of the measurement of time, it remains to distinguish between "apparent solar time," mean solar time," "local time," and "standard time," of which the first is determined by the order of nature and the rest by agreement; among authorities for the conven ience of civilization.

Apparent solar time is, to speak exactly, the moment at which the meridian of any place passes under the sun as the earth revolves. On account of the facts that the path of the earth about the sun is an ellipse, that the earth is accordingly farther from the sun at certain seasons of the year than others, and hence that it travels faster at one time than another (and for other reasons) the interval between two passages of the sun over the meridian at a given place, i. e., a solar day varies slightly from day to day. Hence, as a matter of convenience, an average is struck.

Mean solar time, as this average is called, is determined by the motion of an imaginary sun called the " mean sun," conceived as moving with per fect uniformity. The hours as thus determined are those measured by an accurate timepiece. Hence apparent solar time is set aside and mean solar time becomes the universal standard. 66 وو Local time is the mean solar time of any place; that is, the moment at which the place passes under the mean sun as the earth revolves.

This moment is called noon. The sun at noon passes around the earth at the rate of four minutes for each degree of longitude. It takes about three hours to pass from New York: to San Francisco. Hence at noon in New York it is later than noon on WEIGHTS AND MEASURES the side toward London, east; and earlier on the side toward San Francisco, west.

After the introduction of railroads and steamships the use of local time became a source of great confusion.

The traveler's watch would vary from the local time four minutes for every degree of longitude traversed with the added modification caused by the time occupied in the journey. Prior to the year 1883 each railway system adopted its own standard of local time. Hence the traveler could not determine the time of the arrival of trains without adjusting his watch to the local time of each railway system he patronized.