siderable, in fact, is the influence of this cause, that it is completely counteracted by the effect which arises from the inclination of the earth's axis. In winter the northern pole being turned from the sun, its rays then fall so obliquely upon us, as not to impart the full benefit of their heat; and its visits, moreover, are so short, that the effect of his presence is counteracted by that of his longer absence. In summer, on the contrary, the sun's rays fall more directly upon us, and therefore in greater quantity and with greater power; and as the days are then long, and the nights short, the earth and air are heated in the day-time more than they are cooled in the night, so that the heat continually accumulates.

Though the motion of the earth in its orbit is not uniform, being, as we have said, slower in summer than in winter, yet it is regulated by a certain law, which is, that a line drawn from the centre of the sun to the centre of the earth describes equal areas in equal times. Thus, if AE (Fig. 10.) represent the earth's elliptical orbit, and lines be drawn from S, the sun's place or focus, to the points B, C, D, E, F, &c. so as to make the spaces ASB, BSC, CSD, DSE, ESF, &c. equal to one another, then the portions of the orbit intercepted by these lines, namely, AB, BC, CD, DE, EF, &c. though unequal in length, will be described by the earth in equal times. This remarkable law holds with respect to all the planets, and is one of the laws known by the name of Kepler's laws, from their discoverer.

The following terms and particulars relating to the earth's orbit should here be carefully impressed on the mind.

1. The point of the earth's nearest approach to the sun, is called its Perihelion or Higher Apsis.

2. The point of its greatest distance is called its Aphelion or Lower Apsis.

3. The longer diameter of the ellipse which joins these two points, is called the line of the Apsides.

4. The place of the earth's aphelion is the 9th degree of Capricorn.

5. The eccentricity of the earth's orbit is the distance of the sun from the central point, which is estimated at 1,597,000 miles.

6. The distance from the sun to the extremity of the shorter diameter of the earth's orbit, (LS, Fig. 9.) is the earth's mean distance from the sun, estimated at 95,000,000 miles. How this is ascertained will be shown in a subsequent chapter.

CHAPTER XII.

OF THE SOLAR AND SIDEREAL DAY, AND THE EQUATION OF TIME.

THE succession of day and night is caused, as we have seen, by the rotation of the earth upon its axis. But by reason of the earth's motion in its orbit round the sun, a day is somewhat longer than the time which the earth takes to perform one complete revolution upon its axis. For, as the earth's motion in its orbit causes the sun to appear every day a little further eastward in the ecliptic, it will require something more than one complete rotation to bring round the same meridian from the sun to the sun again. Between the noon of one day and the noon of the next, the earth must perform one complete rotation, and just so much of another as is equivalent to the sun's daily advance eastward. The interval between one noon and the next is called a Solar day, the mean or average length of which is 24 hours. But the earth's rotation upon its axis is completed in 23h. 56′ 4′′. This is called a Sidereal day, because it is measured by means of the fixed stars; for the stars are situated at such an immense distance from us, that the diameter of the earth's orbit is but a point in comparison, and therefore any meridian will revolve from a fixed star to that star again, in exactly the same time as if the

portions of his progress in that circle do not make equal differences in his situation as referred to the equator; and it is only by referring his place to the equator, or by finding what is called his right ascension,* that we can determine with accuracy the amount of his daily removal eastward, and consequently the amount of that additional time by which the solar exceeds the sidereal day. While passing through the signs Pisces and Aries, where his path is most inclined to the equator, his daily advances in right ascension being smallest, he will come proportionably sooner to the meridian, and consequently render the solar day shorter than the mean. But while passing through Gemini and Cancer, where his path lies nearly parallel to the equator, and crosses the meridians after they have in some degree converged, his daily advances in right ascension being the greatest, he will come accordingly later to the meridian, and render the solar day longer than the mean.

There are, then, two causes which combine to render the solar days of unequal length; first, the varying velocity of the sun's apparent motion in the ecliptic, occasioned by the earth's unequal motion in its orbit; and next, the obliquity of the ecliptic to the equator. Hence arises the distinction between mean time and apparent time: the former being that measurement of time which is made by a well regulated clock or watch, and which reckons the duration of the day uniformly at 24 hours; the latter being the time shown by the

* See above, p, 57.

interval between noon and noon be protracted. Whereas, when it advances a shorter distance than usual in its eastward course, it will come to the meridian sooner, and thus shorten the solar day.

This, however, is not the only cause of the difference in the length of the solar days. For were the earth's motion in its orbit, and the consequent apparent motion of the sun in the ecliptic, always uniform, still a difference in the length of the solar days would arise from the obliquity of the sun's apparent path in the heavens. This can only be illustrated by means of a globe. Let equal portions be taken on the ecliptic and the equator of a celestial globe, by placing marks at every tenth degree, and then, the globe being turned westward, it will be found that every successive mark upon the ecliptic, between the first of Aries and the first of Cancer, will come sooner to the meridian than the corresponding mark upon the equator, the difference increasing from the first of Aries to the 15th of Taurus, and diminishing from that point to the first of Cancer, where the two corresponding marks come to the meridian together. But between the first of Cancer and the first of Libra, the successive marks upon the ecliptic will come to the meridian later than the corresponding marks upon the equator, the difference increasing to the 15th of Leo, and decreasing from that point till the marks again coincide at the first of Libra. The same will be found in the southern portion of the ecliptic. Thus it appears, that though the sun should move uniformly in the ecliptic, yet equal