98 CELESTIAL LATITUDE AND LONGITUDE. is observed by the most inattentive spectator; but the knowledge of the latter must be the result of patient observation. The other primary planets, when seen from the sun, do not describe exactly the same circle among the stars, that the earth does; but are sometimes on one side of the ecliptic and sometimes on the other. But none of them, except Juno, Pallas, and Ceres, are ever farther distant from the ecliptic than eight degrees. So that within a zone or belt of sixteen degrees, that is, eight degrees on each side of the ecliptic, the planets, except those just named, are always to be found. This zone, or broad belt, is called the Zodiac. The ecliptic then is an imaginary circle in the heavens passing through the middle of the zodiac, and situated in the plane of the earth's orbit. A plane is an even level surface. If you suppose a smooth thin solid plane cutting the sun through the centre, extending out as far as the fixed stars, and terminating in a circle which passes through the middle of the zodiac; in this plane the earth would move in its revolution round the sun; it is therefore called the plane of the earth's orbit. The points, where the orbit of any heavenly body cuts the plane of the ecliptic, are called the nodes of that body. The point, where the body passes from the north side of the plane of the ecliptic to the south, is called its descending node; where it passes from the south to the north, its ascending node. The ecliptic, as well as every other circle, great or small, is divided into three hundred and sixty degrees; but it has also another division into twelve signs, of thirty degrees each, called the twelve signs of the zodiac. These signs derive their names from clusters of stars, or constellations, which, as the ancients imagined, resembled certain animals. They are most commonly represented by characters, and the names given them should be made familiar; for the sun, as he appears to move round in the ecliptic, seems to enter these clusters of stars, and is therefore said to be in this or that sign. If the axis of the earth be supposed to extend both ways to the starry heavens, its places or points among the stars are the celestial poles, one north and the other south, directly over or beyond the poles of the earth of the same name. If the plane of the earth's equator were extended every way to the starry heavens, the circle it would make among the CELESTIAL LATITUDE AND LONGITUDE. 99 stars is called the celestial equator. Now the celestial equator does not coincide with the ecliptic, but makes an angle with it of twenty-three degrees and twenty-eight minutes, that is, the axis of the earth is not perpendicular to the plane of the ecliptic, but is inclined twenty-three degrees and twenty-eight minutes. Thus we have two great circles, the ecliptic and equator, passing through the heavens eastwardly and westwardly, from either of which the latitude of the heavenly bodies might be estimated. But astronomers have selected the ecliptic for this purpose, and have supposed lines or circles to cross it at right angles, as the meridians do the equator; which lines or circles are called secondaries to the ecliptic. The points where all the secondaries meet, are called the poles of the ecliptic; which points are twenty-three degrees twenty-eight minutes from the celestial poles. Hence the latitude of a heavenly body is its distance from the ecliptic, measured on a secondary to the ecliptic; and like latitude on the earth, it can never exceed ninety degrees. The longitude of a heavenly body is the distance of a secondary to the ecliptic, reckoned from some given uniform secondary, called the prime secondary. But the longitude of heavenly bodies, unlike longitude on the earth, is reckoned only eastward; it may extend, therefore, to three hundred and sixty degrees. It is usually stated in signs, degrees, minutes, and so forth; and the prime secondary, from which it is reckoned, cuts the ecliptic in the beginning of the sign Aries, a point where the celestial equator crosses the ecliptic. If a secondary, for instance, passing through a heavenly body, cuts the ecliptic eighteen degrees in the sign Capricorn, then, since the first point of Capricorn is nine signs eastward from the first point of Aries, the longitude of that body is nine signs, eighteen degrees. But it is often important to know the distance of a heavenly body from the celestial equator, as well as from the ecliptic. This distance is its declination, and is reckoned on a meridian, as latitude is on the earth. Its distance from the beginning of Aries, reckoned on the equator, is its right ascension; which, like celestial longitude, is reckoned through the whole circle, or three hundred and sixty degrees. Two planets are said to be in conjunction with each other, when they have the same longitude, or are in the same degree of the ecliptic on the same side of the heavens, though their latitude be different. 786961 A They are said to be in opposition, when their longitudes differ half a circle, or they are in opposite sides of the heavens. QUESTIONS.-1. What is the ecliptic?-explain. 2. What is the zodiac?-explain. 3. What is meant by the plane of the earth's orbit? 4. What are nodes? 5. What are the divisions of the ecliptic? 6. What are the celestial poles? 7. What is the celestial equator? 8. How is the axis of the earth situated with regard to the plane of the ecliptic? 9. What are the poles of the ecliptic? 10. What is the latitude of a heavenly body? 11. The longitude? 12. How is the longitude of a heavenly body reckoned and stated? 13. What exam ple is given? 14. What is the declination of a heavenly body. 15. Right ascension ? 16. When are two planets said to be in conjunction? 17. In opposition? [NOTE. The points at which the ecliptic cuts the celestial equator are called the equinoctial points. Those two points of the ecliptic farthest from the equator are called solstices. Ap'ogee, that point of the orbit of the moon which is farthest from the earth. Perigee, that point which is nearest to the earth.] 18. Look at fig. 40. and point out the ecliptic, zodiac, and signs of the zodiac. LESSON 45. Day and Night. Ver'nal, belonging to the spring. Intersect', to cut, to divide each other mutually. By the diurnal motion of the earth, the same phenomena appear as if all the celestial bodies turned round it; so that in its rotation from west to east, when the sun or a star just appears on the eastern side of the horizon, it is said to be rising, and as the earth continues its revolution, it seems gradually to ascend till it has reached its meridian; here the object has its greatest elevation, and begins to decline till it set, or become invisible on the western side. In the same manner the sun appears to rise and run his course to the western horizon, where he disappears and night ensues, till he again illuminate the same part of the earth in another diurnal revolution. One half of the earth's surface is constantly illuminated, and by the regular motion of the earth on its axis, every place is successively brought into light and immersed in darkness. If the axis of the earth were always perpendicular to the plane of the ecliptic, the days would every where be of the same length, and just as long as the nights. For an inhabitant at the equator, and one on the same meridian towards the poles, would come into the light at the same time, and, on the other side, would immerge into darkness at the same time. And since the motion of the earth is uniform, they would remain in the dark hemisphere just as long as in the light; that is, their day and night would be equal;-the plane of the ecliptic would coincide with the plane of the equator. But as the ecliptic and equator make an angle with each other of twenty-three degrees and twentyeight minutes, or in other words, as the axis of the earth has such an inclination to the plane of its orbit, it is manifest that, except the earth be in that part of its orbit where the ecliptic cuts the equator, an inhabitant at the equator and one on the same meridian towards the poles, will not come into the light at the same time, nor, on the other side, immerge into darkness at the same time. And since the axis of the earth always preserves the same inclination, they will, except at the points where the two great circles intersect each other,-remain in the dark and light hemispheres different times, that is, their day and night will be unequal. The points where the equator cuts the ecliptic are at the beginning of the signs Libra and Aries. The earth is at these points of its orbit, or, as it is commonly said, the sun enters the sign Aries on the twentieth of March, and the sign Libra on the twenty-third of September. Hence at these periods, and at no others, the days and nights are equal all over the world; and on this account they are called equinoxes; the first the vernal, and the second the autumnal equinox. At these seasons, the sun rises exactly in the east at six o'clock, and sets exactly in the west at six o'clock; -the light of the sun is then terminated by the north and south poles, and as all parts of the earth turn round once in twenty-four hours, every place must receive the rays twelve hours, and be deprived of them for the same time. But at other seasons, when the rays of light are not terminated by the north and south poles, but extend over the one and do not reach the other, it must be manifest, from a moment's inspection of the circles drawn on globes, or common maps of the world, that day and night will be unequal in all places except those situated on the equator, where they will be always equal. At the poles there is but one day and one night in a year, each of six months. The sun can never 102 CHANGES OF THE SEASONS. shine beyond a pole farther than twenty-three degrees and twenty-eight minutes; for that is the extent of his declination; and when he has declination from the celestial equator either north or south, he must shine beyond one pole and not to the other; the days, therefore, will be longest in one hemisphere when they are shortest in the other. The subject of this lesson may be illustrated, by hanging any round body above or below the level of a candle so as to correspond with the sun's declination. It will be seen, that the light shines over one pole and does not reach the other. If the ball be then turned round, it will be observed, that the circles performed by any parts of the surface are unequally divided by the light; that it will be constant day or night near the north pole, as the ball is depressed or elevated, and that all the phenomena will be reversed in the other, or lower hemisphere. QUESTIONS.-1. What phenomena appear from the diurnal motion of the earth? 2. Under what circumstances would the days and nights be every where of the same length? Why? 3. Why is not the day and night always equal to an inhabitant at the equator, and to one on the same meridian towards the poles? 4. At what points does the equator cut the ecliptic? 5. When is the earth at those points of its orbit?-and what happens at these periods? 6. At other seasons? 7. What is said of day and night at the poles? 8. How may the subject of this lesson be illustrated? 9. Look at fig. 40, and illustrate the variations in the lengths of the days and nights. LESSON 46. Changes of the Seasons. Obliquity of the Ecliptic, the angle which the ecliptic makes with the equator. Look nature through, 'tis revolution all; All change, no death. Day follows night, and night Then melts into the Spring. Soft Spring, with breath |