TIME

Time is measured by means of regularly recurring phenomena. The celestial sphere apparently revolves once a day, and the interval between two succe ive returns of a fixed point of the sphere is called a sidereal day. Sidereal time is reckoned from the time when the first point of Aries pa es the meridian. A solar day is the interval between two succe ive transits of the sun’s center over the meridian. But owing to the varying speed of the earth in its orbit, and also to the inclination of the orbit to the equator, the sun’s apparent motion is not uniform. Hence arises the nece ity of introducing an imaginary mean sun, which may be supposed to move in the equator with a uniform velocity. The ‘equation of time’ is the difference between apparent | (or true) solar time and mean solar time at any instant. It varies from day to day. A solar day is about four minutes longer than a sidereal day, for the sun | moves eastward among the stars at the | rate of about 1° a day, hence the earth has to turn nearly 361° about its axis to complete a solar day. About March 21 a solar clock agrees with a sidereal clock, but the sidereal clock gains nearly four minutes every day; in a year it thus gains just one day. a] Standard Time—Green wich time is log;|the standard time throughout Great | Britain, and in France, Belgium, Hol- | land, and Spain. It is also, with certain | modifications the standard of the United States. In countries of great extent it is impo ible to adopt one standard time, meridians 15° apart have been chosen, all places within 714° of these meridians having the same standard. Thus at Quebec, Montreal, Toronto, Boston, New York, and Washington the time of 75° W.—five hours earlier than Green wich—is the standard. Because of the fact that a person traveling eastward from Green wich meets the sun earlier each morning (by Green wich mean time at the rate of one hour for each 15° of longitude), it follows that at 180° his local time is 12 hours ahead of Green wich; and similarly a person traveling westward has a local time {2 hours late at 180°. On returning to Green wich the one has apparently lost, the other gained a day.