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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 when 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.

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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 xhe 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 CBAS, 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'.

Fig. 37.

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

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

Fig. 86.

[graphic][merged small]

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'.

[graphic]

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

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