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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
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 suu, they are most favorably placed for observation. Tne stars at G, on the contrary, will be invisible, for the snn intervenes between them and the earth: they are on the meridian of the spectator about the same time as the snn, 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 theii 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 stare 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 stare ia not as apparent to us as his daily motion, because his superior partying 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 (t)—vernal equinox; when the sun enters the next sign, Taurus (&), the earth in fact has passed on to Scorpio 01). 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 heavens which is occupied by the sign Libra.
3. An apparent movement of the sun, north and south.—Having now spoken of the apparent diurnal and annual motions of the sun, there yet remains a third motion, which has doubtless oftentimes attracted our attention. In summer, at midday, the sun is high in the heavens; in the winter, quite low, near the southern horizon. In summer he is a long time above the horizon; in the winter, a short time. In summer he rises and sets north of the east and west points; in winter, south of the east and west points. This subject is so intimately connected with the next, that we shall understand it best when taken in connection with it.
4. Change Of The Seasons.—Variation In Length Of Day And Night.—By closely studying the accompanying illustration and imagining the various positions of the earth in its orbit, let us try to understand the several points.
I. Obliquity of the ecliptic.—The axis of the earth is inclined 23^° from a perpendicular to its orbit. This angle is called the obliquity of the ecliptic.
II. Parallelism of tlie axis.—In all parts of the orbit, the axis of the earth is parallel to itself and constantly points toward the North Star.* This is only an instance of what is very familiar to us all. Nature reveals to us nothing more permanent than the axis of rotation in anything that is rapidly turned. It is its rotation which keeps a boy's hoop
* There is a slight variation from this, which we shall soon notice.