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and south poles, Z the zenith, Z' the nadir; and GICK the celestial equator. Now PB, it will be seen, is the elevation of the north pole above the horizon, or the latitude of the place. Suppose we should see a star at A, on the meridian below the pole. The earth revolves in the direction GIC; the star will therefore move along A L to Z, when it is on the meridian above the pole. It continues its course along the dotted line around to A again, when it is on the meridian below the pole, having made a complete circuit around the pole, but not having descended below our horizon. A star rising at B would just touch the horizon; one at I would move on the celestial equator, and would be above the horizon as long time as it is below—twelve hours in each case; a star rising at M, would just come above the horizon and set again at N.
Unequal diurnal velocities of the stars.—The stars appear to us to be set in a concave shell which rotates daily about the earth. As different parts of the earth really revolve with varying velocities, so the stars appear to revolve at different rates of speed. Those near the pole, having a small orbit, revolve very slowly, while those near the celestial equator move at the greatest speed.
Appearance of the stars at different places on the earth.—Were we placed at the north pole, Polaris would be directly overhead, and the stars would seem to pass around us in circles parallel to the horizon, and increasing in diameter from the upper to lower ones. Were we placed at the equator, the pole-star would be at the horizon, and the stars would move in circles exactly perpendicular to the horizon, and decreasing in diameter, north and south from those in the zenith, while we could see one half of the path of each star. Were we placed in the southern hemisphere, the circumpolar stars would rotate about the south pole, and the others in circles resembling those in our sky, only the points of direction would be reversed to correspond with the pole. Were we placed at the south pole, the appearance would be the same as at the north pole, except that there is no star to mark the direction of the earth's axis.
Motion Of The Earth In Space About The Sun.— The earth revolves in an elliptical path about the sun at a mean distance of 91£ million of miles. This path is called the ecliptic; its eccentricity, which is greater than that of the orbit of Venus, changes about i-eTo"*oTo per century, so that in time the orbit would become circular, were it not that after the lapse of some thousands of years, the eccentricity will begin to increase again, and will thus vary through all ages within definite, although yet undetermined limits. Its entire circumference is nearly 600,000,000 miles, and the earth pursues this wonderful journey at the rate of 18 miles per second. This revolution of the earth about the sun gives rise to various phenomena, of which we shall now proceed to speak.
1. Change in the appearance of the heavens in different months.—This is the natural result of the revolution of the earth about the sun. In Fig. 30, suppose
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APPEARANCE OF THE HEAVENS IN DIFFERENT SEASONS.
A B C D to be the orbit of the earth, and E F G H the sphere of the fixed stars, surrounding the sun in every direction. When our globe is at A, the stars about E are on the meridian at midnight. Being seen from the earth in the opposite quarter to the sun, they are most favorably placed for observation. The stars at G, on the contrary, will be invisible, for the sun intervenes between them and the earth: they are on the meridian of the spectator about the same time as the sun, and are always hidden in his rays. In three months the earth has passed over one-fourth of her orbit, and has arrived at B. Stars about F now appear on the meridian at midnight, while those at E, which previously occupied their places, have descended toward the west and are becoming lost in the sun's refulgence, while those about G are just coming into sight in the east. In three months more the earth is situated at C, and stars about G shine in the midnight sky, those at F having, in their turn, vanished in the west. Stars at E are on the meridian at noon, and consequently hidden in daylight; and those about H are just escaping from the sun's rays, and commencing their appearance in the east. One revolution of the earth brings the same stars again on the meridian at midnight. Thus it is that the earth's motion round the sun as a centre explains the varied aspect of the heavens in the summer and winter skies. (Hind.)
2. Yearly path of the sun through the heavens.—We have spoken of the diurnal motion of the sun. We now speak of its second apparent motion—its yearly path among the stars* If we look at the accom
* This yearly movement of the sun among the fixed stars is not as apparent to us as his daily motion, because his superior panying plate (Fig. 31), we can see how the motion of the earth in its orbit is also transferred to the sun, and causes him to appear to us to travel in a fixed path through the heavens. "When the earth is in any part of the ecliptic, the sun seems to us to be in the point directly opposite. For example, when the earth is in Libra (—)*—autumnal equinox—the sun is in Aries(f)—vernal equinox; when the sun enters the next sign, Taurus (»), the earth in fact has passed on to Scorpio ("l). Thus as the earth moves through her orbit, the sun seems to pass through the same path along the opposite side of the ecliptic, making the entire circuit of the heavens in the year, and returning at the end of that time to the same place among the stars. If the earth could leave a shining line as it passes through its orbit about the sun, we should see the sun apparently moving along this same line on the opposite side of the circle. We therefore define the ecliptic as the real orbit of the earth about the sun, or the apparent path of the sun through the heavens. The ecliptic crosses the celestial equator at two points. These are called the equinoxes.
light blots out the stars. But if we notice a star at the western horizon just at sunset, we can tell what constellation the sun is then in: now wait two or three nights, and we shall find that star is set, and another has taken its place. Thus we can trace the sun through the year in his path among the fixed stars.
* When we say " the earth is in Libra," we mean that a spectator placed at the sun would see the earth in that part of the heavons which is occupied by the sign Libra.