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oblique to the horizon; but forming angles with it less as they are farther from the equator; till, at the poles, they become parallel to the horizon, or coincide with that circle. To a person passing from the equator towards either pole, the pole-star of his hemisphere appears to rise, and, at a distance from the equator, the stars, the same distance from his elevated celestial pole, do not set; but appear to revolve in circles greater as they are farther distant from the pole.

At either pole, a person would be presented with a parallel sphere; all the visible stars, the pole-star excepted,* would appear to revolve in circles parallel to the horizon. The Sun, also the Moon, and other planets, would seem to revolve in circles very nearly parallel to the horizon. When first appearing in view, they would seem to skim the horizon round. Ascending gradually by spiral circles and by scarcely perceptible advances, they would move to their extreme altitude. The greatest elevation of the Sun at this time, 1831, would be 23° 27′ 38′′; of the Moon at times, 28° 36′ 41". Mercury and some of the asteroids would rise. still higher. They would descend by the same gradual spiral movement, till they again sink below the horizon.

The orbit of the Earth is formed by its annual motion round the Sun. It is an ellipse, with the Sun in one of the foci. The Sun and Earth always appear in opposite signs, the apparent motion of the Sun in the ecliptic being caused by the revolution of the Earth in its orbit.

The irregularity of the Earth's motion in its orbit was unknown to the ancients till the time of Hipparchus. About one hundred forty years before the Christian era, he first discovered that this motion was not uniform. Succeeding astronomers, long perplexed, invented many cycles and epicycles to explain the observed irregularity. It remained, however, a mystery till the true cause was discovered by Kepler. He assigned to the orbit its

* The northern pole-star, not being exactly at the pole, would appear to make a very small circuit. (See article Longitude.)

true elliptical figure, and ascertained that the annual motion of the Earth was subject to the curious law, before named, as subsisting between the planets and their principal; that if a line be drawn from the Earth to the Sun, (Plate vi. Fig. 1,) such a line would pass over equal areas in equal times; so that the nearer the Earth is to the Sun, the swifter is its motion; and the farther it is distant, the slower is its motion.

The Earth's axis, in different parts of its orbit, has nearly a similar position; for, if straight lines be drawn, representing this position in different points, such lines would be parallel to each other, except the very small variation arising from the precession of the equinoxes. The axis is not perpendicular to the plane of the Earth's orbit, but is inclined to a perpendicular line, in an angle of about 23° 28'; or it is inclined to the plane of the orbit in an angle of about 66° 32'. From the parallel position of the axis and its inclination, important effects are produced upon the Earth's surface. For to this parallel position and inclination we are indebted for the inequality of day and night, and the variety of seasons. A fit representation may be made of these by a common terrestrial globe. Let a candle be suspended in the middle of a large room or hall, a few feet from the floor; let the globe, taken from the frame, be holden east of the candle on a level with it, the north pole so elevated that the axis may form an angle with the floor of about 23° 28′; but perpendicular to a line drawn from the candle, or the poles equally distant from the candle. In this position very nearly one half of the globe will be illuminated. Thus situated, let the globe be turned gently round from west to east. Every part of the surface will pass through light and darkness in nearly equal proportions. (Plate vi. Fig. 2, Spring.) This will represent the situation of the Earth at the time of the vernal equinox, when the regions near the poles, like those at the equator, have their days and nights very nearly equal. With the axis parallel to its former position, let the globe be held under

the candle, and turned round as before. This will represent the Earth at the time of the summer solstice. (Plate vi. Fig. 2, Summer.) The Arctic regions, or the whole northern frigid zone, will be in the light; the Antarctic regions, or the southern frigid zone, in darkness. From the Arctic circle to the equator, every part will have more light than darkness, and proportionally more as nearer to the circle, or farther distant from the equator. Every part south of the equator to the polar circle will have more darkness than light, and proportionally more as farther distant from the equator, or nearer to the Antarctic circle. The whole region from this circle to the south pole will be in entire darkness. A western position, or the globe placed west of the candle, will represent the situation of the Earth at the time of the autumnal equinox, the days and nights again equal over the whole Earth. (Plate vi. Fig. 2, Autumn.) Placed directly over the candle, the globe will exemplify the winter solstice, with the short days and long nights of the northern hemisphere, and the long days and short nights of the southern hemisphere. (Plate vi. Fig. 2, Winter.) It will be perceived that the candle, being less than the globe, shines over a fraction less than one half of it; while the Sun, being much larger than the Earth, must illumine a fraction more than one half of its surface. These differences are too small to affect the representation. It has been usual, in representing the seasons by a diagram, to draw an ellipse for the Earth's orbit, and to place the figure of the Earth in different positions. In most of these, no consistent view is given of spring and autumn. In all, so great stretch of imagination is required, that it was thought advisable to omit the ellipse in this Compendium, and give a simple view of the Earth as illuminated by the Sun at the different seasons. Let the student extend his views from the globe and the candle to the Earth and the Sun, and he may have some adequate conception of the cause of inequality in the length of days and nights, and the variety of seasons. To form

extended views of the great objects of the heavens, diagrams, and globes, and orreries, may be of great use; but care must be taken, that the mind be not confined to these auxiliaries; otherwise, instead of being helps, they may lead to embarrassment.

The Earth makes a complete revolution round the Sun, or from a star to the same star again, in 365 days 6 h. 9 m. 12 s. This is called the sideral year. From an equinox or a solstice to the same again, it revolves in 365 d. 5 h. 48 m. 51 s. 36 ds. This is usually called the tropical year; but sometimes the equinoctial or solstitial year. It is usually reckoned from the first degree of Aries, but may be computed from any other point of the ecliptic. The Earth performs a revolution, from the aphelion of its orbit to the same again, in 365 d. 6 h. 14 m. 2 s. This is denominated the anomalistic year.

On account of the elliptical form of the Earth's orbit, the Sun being in one of the foci; and on account of the present place of the aphelion, the Earth is between seven and eight days longer in passing the six northern signs than the six southern. Comparing the time from the vernal equinox to the autumnal with that from the autumnal to the vernal, will make this apparent. The inhabitants of the northern hemisphere, by this inequality, enjoy a greater share of the Sun's influence than the southern; or the northern enjoy this influence for a longer time. It would seem, however, that the southern inhabitants must have the most intense tropical Sun, as their summer is at the time when the Sun is nearest the Earth.

The aphelion of the Earth's orbit moves forward l' 2" in longitude, 11".882 in absolute space, in a year. Hence it passes from a point in the ecliptic to the same again in 20,903 years. But a complete revolution from a star to the same star again requires about 110,000 years. Considering the time of the creation, according to the received opinion from the Mosaic account, 4004 years before the Christian era, it must have been about

the time when the northern and southern hemispheres enjoyed summers and winters of equal length, the earth being in the aphelion of its orbit about the time of the vernal equinox. Probably the equality was at the very time of the creation; and the slight variation, which now appears, arises from a want of complete accuracy in the ancient computation of time; or from the great difficulty of ascertaining the exact motion of the aphelion, in which authors are not perfectly agreed.*

The aphelion moving forward increased the length of northern summer. The ascendency thus gained in the northern hemisphere continued to increase till about the year 1255. From that time to the present, the length of northern summers has decreased, and will continue to decrease till about the year 6481, when the summers in the different hemispheres will again become equal. After that, the southern will have the superiority, or the longest continuance of the Sun's influence, for more than 10,000 years.

The Earth, in the aphelion of its orbit, is more than 3,000,000 miles farther from the Sun than it is in the perihelion. It is in the aphelion on the second day of July the present year, 1831; though, on account of bissextile, it will be in that point on the first of July in the next year, 1832. It has generally been in the aphelion on the first day of July, and in the perihelion on the last day of December, since the commencement of the present century. In the latter part of the century, it will be in the aphelion on the second day of July; in the perihelion on the last day of December in some years, on the first day of January in other years, varying on account of bissextile. No doubt the whole earth is warmer at the time of winter than at the time of summer, in the

*

Though the sacred writings stand in no need of auxiliaries; and though perfect reliance should not be placed on astronomical calculations, as proof of Scripture chronology; yet the circumstance of equality of seasons, at the time the Earth commenced its being and revolutions, must be considered a forcible corroboration of the Mosaic account.

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