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The consequence of this is, that the pole of the earth is irregularly shifted, so that it travels in a slightly curved line, giving it a kind of "wabbling" or "nodding" motion, as shown—though greatly exaggerated—in Fig. 35. The obliquity of the ecliptic, which we consider 23° 28', is the mean

PATH OF THIS NORTn POLB

of the irregularly curved line »TM» Hkavenb. and is represented by the dotted circle.

Periodical change in the obliquity of the ecliptic.— Although it is sufficiently near for all general purposes to consider the obliquity of the ecliptic invariable, yet this is not strictly the case. It is subject to a small but appreciable variation of about 46" per century. This is caused by a slow change of the position of the earth's orbit, due to the attraction of the planets. The effect of this movement is to gradually diminish the inclination of the earth's equator to the ecliptic (the obliquity of the ecliptic). This will continue for a time, when the angle will as gradually increase; the extreme limit of change being only 1° 21'. The orbit of the earth thus vibrates backward and forward, each oscillation requiring a period of 10,000 years. This change is so intimately blended, in its effect upon the obliquity of the ecliptic, with that caused by precession and nutation, that they are only separable in theory; in point of fact, they all combine to produce the waving motion we have already described. As a consequence of this variation in the obliquity of the ecliptic, the sun does not come as far north nor decline as far south as at the Creation, while the position of all the terrestrial circles— Tropic of Cancer, Capricorn, Arctic, etc.—is constantly but slowly changing. Besides this, it tends to vary slightly the comparative length of the days and nights, and, as the obliquity is now diminishing, to equalize them. As the result of this variation in the position of the orbit, some stars which were formerly just south of the ecliptic are now north of it, and others that were just north are now a little further north; thus the latitude of these stars is gradually changing.

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Change in the major axis (line of apsides) of the earth's orbit.—Besides all the changes in the position of the earth in its orbit due to precession, the line connecting the aphelion and perihelion points of the orbit itself is slowly moving. The consequence of this is a variation in the length of the seasons at different periods of time. In the year 4089 b. c, about the supposed epoch of the creation, the earth was in perihelion at the autumnal equinox, so that the summer and autumn seasons were of equal length, but shorter than the winter and spring seasons, which were also equal.* In the

* There is much discrepancy in the views held concerning the Great Year of the astronomers, as it is often called. (See 14 Weeks in Geology, pp. 273-3, note.) The statement given in the text is that held by Lockyer, Hind and others. The terms, it year 1250 A. D., the earth was in perihelion when it was at the winter solstice, December 21, instead of January 1st, as now; the spring quarter was therefore equal to the summer one, and the autumn quarter to the winter one, the former being the longer. In the year 6589 A. D., the earth will be in perihelion when it is at the vernal equinox; summer will then be equal to autumn and winter to spring, the former seasons being the longer. In the year 11928 A. D., the earth will be in perihelion whpn it is at the summer solstice: finally, in 17267 A. D., the cycle will be completed, and for the first time since the creation of man the autumnal equinox will coincide with the earth's perihelion.

Permanence In The Midst Of Change.—We thus see that the ecliptic is constantly modifying its elliptical shape; that the orbit of the earth slowly oscillates upward and downward; that the north pole steadily turns its long index-finger over a dial that marks 26,000 years; that the earth, accurately poised in space, yet gently nods and bows to the attraction of sun and moon. Thus changes are continually taking place that would ultimately entirely reverse the order of nature. But each of these has its bounds, beyond which it cannot pass. The promise made to man after the Deluge, is that "while the earth remaineth, seed-time and harvest, and cold and heat, and summer and winter, and

should be noticed, are applied to the real position of the earth and not the apparent position of the sun. The dates are those given by Chambers in his Descriptive Astronomy.

day and night shall not cease." The modern discoveries of astronomy prove conclusively that the seasons are to be permanent; that the Creator, amid all these transitions, has ordained the means of carrying out His promise through all time.

Refraction.—The atmosphere extends above the earth about 500 miles. Near the surface it is dense, while in the upper regions it is exceedingly rare. The rays of light from the heavenly bodies

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passing through these different layers are turned downward toward a perpendicular more and more as the density increases. According to a wellknown law of optics, if the ray of light from a star were bent in fifty directions before entering the eye, the star would nevertheless appear to be in the line of the one nearest the eye. The effect of this is, that the apparent place of a heavenly body is higher than the true place. This is illustrated in Fig. 36. The sun at S, were it not for the atmosphere, would send a direct ray to L. Instead, the ray at A is refracted downward, and would then enter the eye at N ; passing, however, through a layer of a different density, at B it is again bent, and meets the eye of the observer at C. He sees the sun, not in the direction of the curved line C B A S, but that of the straight line CBS.

The amount of refraction varies with the temperature, moisture, and other conditions of the atmosphere. It is zero for a body in the zenith, and increases gradually toward the horizon (as the thickness of the intervening atmosphere increases), where it is about 33'.

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Change of place and appearance of the sun and moon. —The sun may be really below the horizon, and yet

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