Fig. 499.—The Zodiac.
4. The Zodiac is a girdle extending 8° on each side of the ecliptic, in which space of 16° the planets move. The zodiac is divided into twelve parts of 30° each, called the “Signs.” These names are as under written:—
| Northern Signs. | |
|---|---|
| Spring. | Summer. |
| Aries, the Ram, March. Taurus, the Bull, April. Gemini, the Twins, May. | Cancer, the Crab, June. Leo, the Lion, July. Virgo, the Virgin, August. |
| Southern Signs. | |
| Autumn. | Winter. |
| Libra, the Balance, September. Scorpio, the Scorpion, October. Sagittarius, the Archer, November. | Capricornus, the Goat, December. Aquarius, the Waterbearer, January. Pisces, the Fishes, February. |
5. Colures are two circles dividing the ecliptic into four equal parts, and making the seasons.
6. The Horizon is the boundary line of our vision, and is called the sensible (apparent) horizon. The true horizon is the circle—as on a globe—dividing the heavens into two hemispheres. The sensible horizon is enlarged according as the eye is elevated above the ground. A man six feet high can see a distance of three miles when standing on a plain. We can always find the distance visible when we know the height at which we stand, or, inversely, we can tell the height of an object if we know the distance. We have only to increase the height one half in feet, and extract the square root for the distance in miles. On giving the distance in miles reverse the operation.
Fig. 500.—Right ascension.
For instance, for the man six feet high, as supposed, add three feet, being half his height; that makes nine feet. The square root (or number multiplied by itself to give nine) is three, which is the number of miles the man can see on a plain. Or, again, suppose we can see a tower on the level, and we know we are twelve miles away from it. The square of twelve is one hundred and forty-four feet, one-third of that is forty-eight feet, which represents the half of the original height added to the whole tower in feet; so the whole tower is ninety-six feet high. Reversing, as in the former case, we can prove this by taking the tower at ninety-six feet high and trying to find the distance we can see from its summit = 96 + 48 = 144; the square root of 144 = 12, the distance required.
7. The Nadir and the Zenith are the poles of the horizon. The zenith is exactly overhead, the nadir exactly under foot. Circles drawn through these points are azimuth circles.