CHAPTER I. General Phenomena of the Solar System, History, &c., 66 II. The Sun-His Distance, Magnitude, &c., INTRODUCTION 1. ASTRONOMY is the science of the heavenly bodies-the Sun, Moon, Planets, Comets, and Fixed Stars. 2. In entering upon this study, the phenomena of the heavens, as they appear on a clear evening, are the first objects that demand our attention. Our first step is to learn the names and positions of the heavenly bodies, so that we can identify, and distinguish them from each other. In this manner they were observed and studied ages before books were written, and it was only after many careful and repeated observations, that systems and theories of Astronomy were formed. To the visible heavens, then, the attention of the pupil should be first directed, for it is only when he shall have become, in some measure, familiar with them, that he will be able to locate his Astronomical knowledge, or fully comprehend the terms of the science. 3. For the sake of convenient reference, the heavens were early divided into constellations, and particular names assigned to the constellations and to the stars which they contain. A constellation may be defined to be a cluster or group of stars embraced in the outline of some figure. These figures are, in many cases, creations of the imagination; but in others, the stars are in reality so arranged as to form figures which have some resemblance to the objects whose names have been assigned to them. These divisions of the celestial sphere bear a striking analogy to the civil divisions of the globe. The constellations answer to states and kingdoms, the most brilliant clusters to towns and cities, and the number of stars in each, to their respective population. The pupil can trace the boundaries of any constellation, and name all its stars, one by one, as readily as he can trace the boundaries of a state, or name the towns and cities from a map of New England. In this sense, there may be truly said to be a Geography of the Heavens. 4. The stars are considered as forming, with reference to 1. What is Astronomy? 2. What first studied? First step? 3. How are the heavens divided, and why? What is a constellation? What of these figures? In what sense may there really be a "Geography of the heavens?" 4. How are the stars classified, as respects their magnitudes? What expedient for designating their places In the heavens? their magnitudes, sixteen classes; the brightest being called stars of the first magnitude, the next brightest, stars of the second magnitude, and so on to the sixth class, which consists of the smallest stars visible to the naked eye. The next ten classes are seen only through telescopes. In order to be able to designate with precision their situa. tions, imaginary circles have been considered as drawn in the heavens, most of which correspond to, and are in the same plane with, similar circles, supposed for similar purposes, to be drawn on the surface of the Earth. 5. In order to facilitate the study of Astronomy, artificial representations of the heavens, similar to those of the surface of the Earth, have been made. Thus, a Celestial Atlas, composed of several maps, accompanies this work. Before, however, proceeding to explain its use, it is necessary to make the pupil acquainted with the imaginary circles alluded to, called the Circles of the Sphere. CIRCLES OF THE SPHERE. 6. The Axis of the Earth is an imaginary line, passing through its centre, north and south, about which its diurnal revolution is performed. The Poles of the Earth are the extremities of its axis. The Axis of the Heavens is the axis of the Earth produced both ways to the concave surface of the heavens. The Poles of the Heavens are the extremities of their axis. The Equator of the Earth is an imaginary great circle passing round the Earth, east and west, everywhere equally distant from the poles, and dividing it into northern and southern hemispheres. The Equator of the Heavens, or Equinoctial, is the great circle formed on the concave surface of the heavens, by producing the plane of the Earth's equator. A plane is that which has surface but not thickness. The plane of a circle is that ima. ginary superficies which is bounded by the circle. 7. The Rational Horizon is an imaginary great circle, whose plane, passing through the centre of the Earth, divides the heavens into two hemispheres, of which the upper one is called the 5. What helps to facilitate the study of the heavens? Circles? Cailed what f 6. Axis of the Earth? Poles? Axis of the heavens? Poles of the heavens? Equator of the Earth? Equator of the heavens, or Equ noctial? 7. Rational horizon? Sensible or apparent ? visible hemisphere, and the lower one, the invisible hemisphere. It is the plane of this circle which determines the rising and setting of the heavenly bodies. The Sensible or Apparent Horizon, is the circle which termi nates our view, where the Earth and sky appear to meet. To a person standing on a plain, this circle is but a few miles in diameter. If the eye be elevated five feet, the radius of the sensible horizon will be less than two miles and three quarters; if the eye be elevated six feet, it will be just three miles. The observer being always in the centre of the sensible horizon, it will move as he moves, and enlarge or contract, as his station is elevated or depressed. 8. The Poles of the Horizon are two points, of which the one is directly overhead, and is called the Zenith; the other is directly underfoot, and is called the Nadir. Vertical Circles are circles drawn through the Zenith and Nadir of any place, cutting the horizon at right angles. The Prime Vertical is that which passes through the east and west points of the horizon. 9. The Ecliptic is the plane of the Earth's orbit; or the great circle which the Sun appears to describe annually among the stars. It crosses the Equinoctial, a little obliquely, in two opposite points, which are called the Equinoxes. The Sun rises in one of these points on the 21st of March; this point is called the Vernal Equinox. It sets in the opposite point on the 23d of September; this point is called the Autumnal Equinox. One half of the Ecliptic lies on the north side of the Equinoctial, the other half on the south side, making an angle with it of 2340. This angle is called the obliquity of the Ecliptic. The axis of the Ecliptic makes the same angle with the axis of the heavens; so that the poles of each are 234° apart. This angle is perpetually decreasing. At the commencement of the Christian era, it was about 23° 45'. At the beginning of 1836, it was only 23′ 27′ 38′′, showing an annual diminution of about half a second, or 45.70 in a hundred years. A time will arrive, however, when this angle, having reached its minimum, will again increase in the same ratio that it had before diminished, and thus it will continue to oscillate at long periods, between certain limits, which are said to be comprised within the space of 20° 42'. 10. The Ecliptic, like every other circle, contains 360°, and it is divided into 12 equal arcs of 30° each, called signs, which the ancients distinguished by particular names. This division commences at the vernal equinox, and is continued eastwardly round to the same point again in the following order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capri 8. Poles of the horizon? Vertical circles? Prime Vertical? 9. Ecliptic? Equinoxes? How is the Ecliptic situated with respect to the Equinoctial? Obliquity of Ecriptic? Is this angle permanent? 10. How is the Ecliptic divided? Where com menced, and how reckoned? Name sig.3 in order? How does the Sun proceed thugh th, signs? cornus, Aquarius, Pisces. The Sun, commencing at the first degree of Aries, about the 21st of March, passes, at a mean rate, through one sign every month. 11. The Zodiac is a zone or girdle, about 16 degrees in breadth, extending quite round the heavens, and including all the heavenly bodies within 8° on each side of the ecliptic. It includes, also, the orbits of all the planets, except some of the asteroids, since they are never seen beyond 8° either north or south of the ecliptic, 12. Parallels of Latitude are small circles imagined to be drawn on the Earth's surface, north and south of the equator, and parallel to it. Parallels of Declination are small circles, imagined to be drawn on the concave surface of the heavens, north and south of the equinoctial, and parallel to it; or they may be considered as circles formed by producing the parallels of latitude to the heavens. 13. The Tropic of Cancer is a small circle, which lies 231° north of the Equinoctial, and parallel to it. The Tropic of Capricorn is a small circle, which lies 23° south of the Equi noctial, and parallel to it. On the celestial sphere, these two circles mark the limits of the Sun's farthest declination, north and south. On the terrestrial sphere, they divide the torrid from the two temperate zones. That point in the ecliptic which touches the tropic of Cancer, is called the Summer Solstice; and that point in the ecliptic which touches the tropic of Capricorn, is called the Winter Solstice. The distance of these two points from the equinoctial, is always equal to the obliquity of the ecliptic, which, in round numbers, is 28c; but, as we have seen, the obliquity of the ecliptic is continually changing; therefore the position of the tropics must make a correspondent change. 14. The Colures are two great circles which pass through the poles of the heavens, dividing the ecliptic into four equal parts, and mark the seasons of the year. One of them passes through the equinoxes at Aries and Libra, and is thence called the Equinoctial Colure; the other passes through the solstitial points or the points of the Sun's greatest declination north and south, and is thence called the Solstitial Colure. The Sun is in the equinoctial points the 21st of March and the 23d of September. He in the solstitial points the 22d of June and the 22d of December. 15. The Polar Circles are two small circles, each about 661° 11. What is the Zodiac? 12. Parallels of latitude? Of declination? 18. The tropics? Cancer? Capricorn? What do these circles mark in the celestial sphere O terrestrial? 14. The Colures? Where situated? When is the Sun at the equiLoctia. ts? The solsticial? 15. What are the Polar Circles? |