The magnitudes of the fixed stars appear to be different from one another, which difference may arise either from a diversity in their real magnitudes, or distances; or from both these causes acting together. The difference in the apparent magnitude of the stars is such as to admit of their being divided into six classes, the largest being called stars of the first magnitude, and the least which are visible to the naked eye, stars of the sixth magnitude. Stars that cannot be seen without the help of glasses are called telescopic stars. The number of stars, visible at any one time to the naked eye, is about one thousand; but Dr. Herschel, by his skilful improvements of the reflecting telescope, has discovered that the whole number is great beyond all conception. Upon viewing the heavens during a clear night, we discover a pale irregular light, and a number of stars whose mingled rays form the luminous tract called the milky-way. The stars themselves are at too great a distance to be perceived by the naked eye; and among those which are visible with a telescope there are spaces apparently filled with others in immense numbers. Many whitish spots or tracts (called nebula) are visible in different parts of the heavens, which are supposed to be milky-ways at an inconceivable distance. The distance of these remoter bodies is so vast and measureless, that we can hardly speak of it except in relation to the inconceivable swiftness of light. The rays by which they are now made visible to the eye of the astronomer, the rapid motion of which might circle the earth while one is pronouncing a syllable, have been darting forward for thousands and ten thousands of years to reach us. All the events and revolutions, which history records, have taken place during their progress. They commenced their career, it has been computed, at a period of such remote antiquity, that, compared with it, the date of that time, when God gave the earth to man for a habitation, is but of yesterday. Dr. Herschel has calculated that the distance of the remotest nebulæ, exceeds that of the nearest fixed star at least three hundred thousand times. Upon this fact, he thus remarks; a telescope with a power of penetrating into space, like my forty feet one, has also, as it may be called, a power of penetrating into time past. To explain this we must consider that from the known velocity of light, it may be proved that when we look at the star called Sirius, the THE CONSTELLATIONS. 119 rays which enter the eye cannot have been less than six years and four months and a half coming from that star to the observer. Hence it follows that when we see an object at the calculated distance, at which one of these very remote nebula may still be perceived, the rays of light which convey its image to the eye, must have been more than nineteen hundred and ten thousand, that is, almost two millions of years on their way; and that, consequently, so many years ago, this object must already have had an existence in the sidereal heavens, in order to send out those rays by which we now perceive it. But when we have reached the utmost distance to which the power of our instruments can penetrate, who will say, that we are approaching any limits of the creation? who will say, that if the disembodied spirit should travel forward through eternity, numberless systems would not be continually spreading before it? All that part of the universe which we are able to discern, is peopled by inhabitants, who have the common want of heat and light; who will say, that there are not other parts of the material universe inhabited by beings of different natures, to whom these wants are unknown? It is only some portion, we know not how small, of the material universe which is obvious to our senses; who will attempt to define the limits of the invisible world? who will attempt to set bounds to the works of infinite power and infinite goodness? QUESTIONS.-1. What are fixed stars?-why so called? 2. How does it appear that they do not borrow their light? 3. What is said of the magnitude of the stars? 4. Number? 5. Describe the milkyway (or galaxy.) 6. What calculations did Dr. Herschel make? [NOTE. Many stars, single to the naked eye, appear double, triple, and even quadruple, through a telescope. Dr. Herschel found that in more than fifty double stars, a change of situation really takes place: it is concluded, therefore, that they describe orbits round a centre of gravity.] LESSON 55. The Constellations. THE first people who paid much attention to the fixed stars were the shepherds in the beautiful plains of Egypt 120 THE CONSTELLATIONS. and Babylon. Endowed with a lively fancy, they divided the stars into different companies or constellations, each of which they supposed to represent the image of some animal, or other terrestrial object. Of these ancient constellations there were fifty, to which the moderns have added about thirty others. Twelve of these constellations are in the zodiac, bearing the same names with the signs of the zodiac or ecliptic. But these constellations and signs do not coincide, for the equinoctial points are not stationary, but move backward, and the sign Aries always begins at one of them, and all the other signs each succeed Aries in order; it follows therefore that all the signs of the ecliptic or zodiac move backward with the equinoxes. The distance which they move annually is about fifty seconds of a degree; so that with respect to the fixed stars the equinoctial points fall backwards thirty degrees, in about two thousand two hundred years, whence the stars will appear to have gone forward thirty degrees, with respect to the signs of the ecliptic, which are always reckoned from the equinoctial points. This shows the importance of distinguishing between the signs of the zodiac and the constellations of the zodiac; for stars, which are in one sign at one time, will be in the succeeding one at another. Thus, the stars which were formerly in Aries, are now in Taurus, and so on. When these names were given to the signs and constellations, it is supposed that each sign coincided with the constellation of the same name; but on account of this moving of the equinoctial points, or, as it is termed, the precession of the equinoxes, there is now about one sign or thirty degrees difference. The period will be completed in about twenty-six thousand years. Among the northern constellations, none are more remarkable than that which is nearest to the north pole, and termed the little bear. The last star of its tail is but two degrees from the pole; hence it is called the polar star. It is easily distinguished from the neighbouring stars, because it scarcely appears to change its position, and is always in the same part of the heavens. By its fixed situation it becomes a guide to travellers, and particularly to mariners who are sailing on the open seas. Before the discovery of the compass sailors had no surer guide than the polar star; and even now, when the sky is serene, they repose in many cases HYMN TO THE NORTH STAR. 121 with greater certainty upon the direction of this star, than upon the magnetic needle. Hymn to the North Star. THE sad and solemn night Has yet her multitude of cheerful fires; Walk the dark hemisphere till she retires: Day, too, hath many a star To grace his gorgeous reign, as bright as they : Unseen, they follow in his flaming way. And thou dost see them rise, Star of the Pole! and thou dost see them set, Thou keep'st thy old unmoving station yet, There, at morn's rosy birth, Thou lookest meekly through the kindling air, Chases the day, beholds thee watching there; On thy unaltering blaze The half-wrecked mariner, his compass lost, Fixes his steady gaze, And steers, undoubting, to the friendly coast; And they who stray in perilous wastes, by night, Are glad when thou dost shine to guide their footsteps right. And, therefore, bards of old, Sages, and hermits of the solemn wood Did in thy beams behold A beauteous type of that unchanging good, 122 FORMS AND DIVISIONS OF TIME. That bright eternal beacon, by whose ray The voyager of time should shape his heedful way. QUESTIONS.-1. What is said of the first division of the stars into constellations? 2. Why do not the constellations and signs of the zodiac coincide? 3. What is the present difference between them? 4. At what rate does the change take place? 5. Describe the situation of the polar star. LESSON 56. Forms and Divisions of Time. As the form of the year is various among different nations, so is its beginning. The Jews, like most other nations of the East, had a civil year, which commenced with the new moon in September; and an ecclesiastical year, which commenced from the new moon in March. The Persians begin their year in the month answering to our June; the Chinese, and most of the inhabitants of India, begin it with the first moon in March; and the Greeks with the new moon that follows the longest day. In England and America, the civil or legal year formerly commenced on the twenty-fifth of March, and the historical year on the first of January. But since the alteration of the style, which took place in 1752, the civil year in both countries has likewise begun on the first of January. The principal division of the year is into parts called months, which are either astronomical or civil. An astronomical or natural month is that which is measured exactly by the motion of the Earth or Moon, and is accordingly either lunar or solar. A lunar month is the time the moon takes to revolve round the earth, which she performs in twenty-seven days, seven hours, and forty-three minutes. A solar month is that space of time in which the earth runs through one of the signs of the zodiac; as the earth constantly travels through the twelve signs in three hundred and sixty-five days five hours and forty-nine minutes, each solar month is found by dividing this number by twelve, to contain thirty days, ten hours, and twenty-nine minutes. |