CHAPTER XIV. EQUATION OF TIME. THE period required by the revolution of the Earth round the Sun to bring a given star in precisely the same direction, is 365d. 6h. 9' 12". This is called a siderial year. The period from one vernal equinox to another is 365d. 5h. 48′ 49′′. This is called a solar year. The cause of this variation, will be assigned under the head of the sion of the equinoxes. preces Were a clock so regulated, that from the 15th of June one year, to the 15th of June the next year, there should not be a second's variation with the Sun, that clock would keep mean time, but apparent time by the Sun would rarely be the same during the year. The cause of this difference is to be accounted for, under this head of EQUATION OF TIME. This difference between equal and apparent time, depends, first upon the inclination of the Earth's axis to the plane of its orbit; and secondly, upon the elliptical, or oval form of the Earth's orbit; for being an ellipse, the Earth's tion is quicker in its perihelion, than in its aphelion. mo The rotation of the Earth upon its axis, is the most equable motion in nature, and is completed in 23 hours, 56 minutes, and 4 seconds. This space is called a siderial day, because any meridian on the Earth will revolve from a fixed star to that star again in this time. What is a siderial year? How long? What is a solar year? How long? On what causes depend the difference between mean and apparent tir. ? How long is a siderial day? Hence, if the Earth had only a diurnal motion, the day vould be nearly four minutes shorter than it is. But a solar, or natural day, which our clocks are intended to measure, is the time which any meridian on the Earth will take in revolving from the Sun, to the Sun again, which is about 24 hours, sometimes a little more, sometimes less. This is occasioned by the Earth's advancing nearly a degree in its orbit, in the same time that it turns eastward on its axis; and hence the Earth must make more than a complete rotation, before it can come into the same position with the Sun, that it had the day before. Some idea of this may be formed by the hands of a clock; suppose both of them to set off together at twelve o'clock, the minute hand must travel more than a whole circle before it will overtake the hour hand; that is, before they will be in the same relative position. If the Equator coincided with the Ecliptic, and the orbit of the Earth were a perfect circle, an equal period of 24 hours would transpire between the Sun's appearing in the meridian one day and the next throughout the year. But the angle of 23° 28', made by the Ecliptic with the Equator is causing unequal portions of the Ecliptic to correspond with equal portions of the Equator. This must affect the apparent time from the Sun, and can easily be explained by the instructer on the Globe. Sup pose, for example, that the Suu and a star were to set out together from one of the equinoctial points, and to move continually through equal arcs What is a solar day? What illustration can be given from a clock ? in equal times; the star in the Equator and the Sun in the Ecliptic: then it is plain that the star, moving in the Equator, would always return to the meridian exactly at the end of every 24 hours, as measured by a well regulated clock, that keeps equal time; but the Sun, moving in the Ecliptic would come to the meridian, sometimes sooner than the star and sometimes later, according to their relative situations; and they would never be found upon that circle exactly together except on four days of the year; namely, on the 20th of March, when the Sun enters Aries; on the 21st of June, when he enters Cancer; on the 23d of September, when he enters Libra; and on the 21st of December, when he enters Capricorn. Again it must be observed, that if the Earth's motion in its orbit were uniform, as it would be if the orbit were circular, then the whole difference between equal time by the clock, and apparent time by the Sun, would arise from the inclination of the Earth's axis. But this is not the case, for the Earth travels when nearest the Sun, that is, in the winter, more than a degree in 24 hours;—and when remotest from the Sun, that is, in summer, less than a degree in the same time. From this cause the natural day would be of the greatest length, when the Earth was nearest the Sun, for it must continue turning the longest time after an entire rotation, in order to bring the meidian of any place to the Sun again; and the shortest day, when the Earth moves slowest in her orbit. What effect on this difference in mean and apparent time has the unequal velocities of the Earth in different parts of its orbit ? The above irregularities, combined with thos arising from the inclination of the Earth's axis make up that difference which is shown by the equation table, which is here subjoined. This table is adapted to the second year after Leap year, but will be found within a minute of the truth for every year, and therefore is sufficiently accurate for the regulation of common clocks and watches. From the equation table, how many times in the year will the Sun and clock be together? How widely apart? |