« PreviousContinue »
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.
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.
LT. Parallelism of the 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.
from falling. For the same reason a quoit retains its direction when whirled, and it will keep 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 simply whirls it, and as it descends it will strike on the edge without breaking. As long as a top spins there is no danger of its falling, since its tendency to preserve parallel its axis of rotation 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.
IH. The rays of the sun strike the various -portions of the earth, when in any position, at different angles.—Example. When the earth is in Libra, and also when in Aries, the 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.—Example. Take the earth as it enters Capricornus (yj) and the sun in Cancer (o) He is now overhead, 23£° 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, 23j° 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 accounts for the difference in temperature between summer and winter.
V. The Equinoxes.—At the equinoxes one half of each hemisphere is illuminated: hence the name Equinox (cequus, equal, and nox, night). At these points of the orbit the days and nights are equal over the entire earth,* each being twelve hours in length.
VI. Northern and southern hemispheres unequally illuminated.—While one half of the earth is constantly illuminated, at all points in the orbit except the equinoxes the proportion of the northern or southern hemisphere which is in daylight or darkness varies. When more than half of a hemisphere is in the light, its days are longer than the nights, and vice versa.
VLT. The seasons and the comparative length of days and nights in the South Temperate Zone, at any specified time, are the reverse of those in the North Temperate Zone, except at the Equinoxes, where the days and nights are of equal length.
* Except a small space at each pole.