and stars about G shine in the midnight sky, those at F having, in their turn, vanished in the west;

Appearance of the Heavens in Different Seasons (Hind).

stars at E are on the meridian at noon, and consequently hidden in daylight; and those about H are just making their appearance in the east. One revolution of the earth will bring the same stars again on the meridian at midnight.

Thus the earth's motion round the sun as a center explains the varied aspect of the heavens in the summer and winter skies.

2. YEARLY PATH OF THE SUN THROUGH THE HEAVENS.-We have spoken of the diurnal motion of the sun. We shall now speak of its second apparent motion, its yearly path among the stars,-the ecliptic.* If we look at Fig. 37, we can see how the motion of the earth in its orbit is transferred to the sun, and causes him to appear to travel in a fixed path through the heavens. When the earth is in any part of its orbit, 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 (~)—vernal equinox; when the sun enters the next sign, Taurus (8), the earth has passed on to Scorpio (m). Thus, as the earth moves through her orbit, the sun seems to pass along the opposite side of the ecliptic, making the circuit of the heavens in a year, and returning, at the end of that time, to the same place among the stars. The ecliptic crosses the celestial equator at two points, called the equinoxes. (See page 30).

This yearly movement of the sun among the fixed stars is not so apparent to us as his daily motion, because his superior 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 in: then wait two or three nights, and we shall find that this star has set, and others have taken its place. Thus we can trace the sun through the year in his path among the fixed stars in the horizon.

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, while a spectator on the earth would see the sun, at the same time, in that part of the heavens which is occupied by the sign Aries. Just so, on June 21st, the earth enters Capricorn, and the sun, Cancer. It is customary, however, having reference solely to the sun's place, to locate the vernal equinox in Aries, and the autumnal equinox in Libra; the summer solstice in Cancer, and the winter solstice in Capricorn. In figure 37, the terms "summer solstice," "autumnal equinox," etc., refer to the season upon the earth, and to the location of the sun in the ecliptic, but are not the names of those points on the earth's orbit. The zodiacal signs are inserted for convenience of illustration, to show where the earth would be located by a solar spectator; the pupil should remember, however, that the signs belong to the ecliptic-which is the projection of the plane of the earth's orbit upon the celestial sphere, and not to the earth's path.

3. 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. In summer, at midday, the sun is high in the heavens; in winter, he is low, near the southern horizon. In summer, he is a long time above the horizon; in 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 that topic.

4. CHANGE OF THE SEASONS. VARIATION IN LENGTH OF DAY AND NIGHT.-By studying Fig. 37, and imagining the various positions of the earth in its orbit, let us try to understand the following points:

I. Obliquity of the ecliptic.-The axis of the earth is inclined 231° from a perpendicular to its orbit. This angle is called the obliquity of the ecliptic.

II. Parallelism of the axis.-In all parts of the orbit, the axis of the earth is parallel to itself, and points almost exactly toward the North Star (p. 217).

Nature reveals to us nothing more permanent than the axis of rotation in anything that is rapidly turned. It is its rotation that keeps a boy's hoop from falling. For the same reason, a quoit retains. its direction when whirled, and stays in the same plane at whatever angle it may be thrown. A man slating a roof wishes to throw a slate to the ground; he whirls it perpendicularly, and it will strike on the edge without breaking. So long as a top spins there

is no danger of its falling, since its tendency to keep its axis of rotation parallel is greater than the attraction of the earth. This wonderful law would lead us to think that the axis of the earth always points in the same direction, even if we did not know it from direct observation.

III. The rays of the sun strike the various portions of the earth, when in any position, at different angles. When the earth is in Libra, and also when in Aries, the sun's rays strike vertically at the equator, and more and more obliquely in the northern and southern hemispheres, as the distance from the equator increases, until at the poles they strike almost horizontally.

This variation in the direction of the rays produces a corresponding variation in the intensity of the sun's heat and light at different places, and accounts for the difference between the torrid and polar regions.

IV. As the earth changes its position the angle at which the rays strike any portion is varied.-Take the earth when it is in Capricornus (9) and the sun in Cancer (). He is now overhead, 231° north of the equator. His rays strike less obliquely in the northern hemisphere than when the earth was in Libra. Let six months elapse: the earth is now in Cancer and the sun in Capricornus; and he is overhead, 231° south of the equator. His rays strike less obliquely in the southern hemisphere than before, but in the northern hemisphere more obliquely. These six months have changed the direction of the sun's rays on every part of the earth's surface. This