LONGITUDE AND TIME Longitude is distance east or west from a given meridian. It is meas ured by degrees, minutes, and sec onds. Thus, 15° 24' 40" east longi tude denotes a position 15° 94' 40" east of the meridian from which lon gitude is reckoned.
A meridian is an imaginary line passing, on the earth's surface from pole to pole.
Each nation selects some meridian of longitude as its standard from which to reckon, but the two in prin cipal use throughout the English speaking 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 com monly 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 twenty four equal parts, called hours. Since the earth rotates on its aXiS from west to east, the sun appears to re volve around the earth from east to west, and its rays move westward at the same rate over the earth's sur face. Hence when it is noon, or twelve o'clock, at any place, it is past noon at all places east of its merid ian, and before noon at all places west of its meridian. When, for ex ample, 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 im portance of the accurate measure ment of time. Hence an outline of 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 deter mined 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 defmed as the mean interval between two returns of the sun to the vernal equinox. This takes place about March 2Ist. 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 lon ger than the solar year. Since, how ever, 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 be tween 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 di vision of time called the calendar month.
The most accurate and useful meas ure of all is the day, both because of the practical importance of the alter nation of day and night, and because, so far as can be ascertained, the time of the earth's revolution on its aids does not change by as much as nil" 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 me-. ridian. The solar day is the interval between two pa.ssages 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.