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the pole never appear to rise or set, but describe complete circles above the horizon— that tho stars near the pole, such as those in the Great Bear, appear in one part of thencourse to move from west to east, and in another part of it from east to west—and that the revolutions of the whole, however different the circles they apparently describe, are completed in exactly the same period of time. These positions may afterwards be more particularly illustrated by means of a large celestial globe, by which it will be seen that all these appearances are the result of one general apparent motion, which, at first view, will appear to exist in the celestial sphere. An idea of the general motion of the stars may be acquired by a simpler process than what we have now described. Let any observer bring a star, in any position between the zenith and southern horizon, into an apparent contact with a tree, spire, or chimney-top, and, in the course of fifteen or twenty minutes, he will perceive that that star and others adjacent to it have moved a little space from east to west But the observations alluded to aliove are calculated to give a more satisfactory idea of this motion, and to make a deeper impression on the minds of the young.
The next series of observations might be those which demonstrate the apparent annual motion of I he sun. For the purpose of exhibiting this motion, the Pleiades, or seven stars, along with Aldebaran, might be selected as fixed points in the heavens to indicate the progressive motion of the solar orb towards the east. About the middle of January, at eight o'clock in the evening, the Pleiades will be seen on the meridian; which observation should be noted down, for the purpose of being compared with a future observation. On the 1st March, at the tame hour, these stars will be seen nearly halfway between the meridian and the western horizon, while all the other stars, at the same declination, will be found to have made a similar progress. About the 15th April, they will be seen, at the same hour, very near the western horizon; and every day after this, they will appear to make a nearer approach to that part of the heavens in which the sun appears, till, being overpowered by the splendour of his rays, they cease to be visible. From these and similar observations, it will be easy to make the young perceive, that the sun has an apparent motion from west to east, through the circle of the heavens, and that the revolution is completed in the course of a year.
They may next be taught to acquire a definite idea of the measures by which the apparent distances of objects in tho heavens are expressed. To talk to the young, as some are in the practice of doing, of two stars being a foot, a yard, or two yards asunder, is
altogether vague and indefinite, unless we an told, at the same time, at what distance the yard or foot is supposed to be placed frofl our eye. As astronomers divide the circumference of the celestial sphe re into 360 parts or degrees, they may be told, that from any point of the horizon to the zenith are 90 degrees, and, consequently, that from the eastern to the western, or from the northern to the southern points of the horizon, are 180 degrees. And, in order that they may have a definite idea, oi something approximating to it, of the extent of a degree, they may be told that the breadth of the moon is about half a degree—that the space occupied by the three stars in a straight line in the belt of Orion—sometimes distinguished by the name of the Three Kings, or the Ell and Yard—is exactly 3 degrees in length, and, consequently, tho distance between any two of them is a degree and a half —that the distance between Castor and Pollux is nearly 5 degrees—between Dub be and Afo rah, the two Pointers, in the Great Bear, is 5J degrees—and that the space between Dubbe, or the northermost pointer, and the polestar, is about 29 degrees. By familiarizing the mind with such measures, the young will soon acquire a tolerable idea of the distance of any two objects in the heavens, when the number of degrees is mentioned.
All the observations above stated may be made, in the way of an amusement, previous to the time when the pupils are expected to enter on the regular study of astronomy. They may be completed in the course of ten or twelve observations, made at different times, within the space of seven or eight months. They are intended for the purpose of stimulating the young to habits of observation and attention to the appearances of nature around them; so that, in every clear sky, they may learn to make similar observations by themselves, for confirming and amplifying their fbrmer views of the motions and aspects of the heavens. Such observations form the groundwork of astronomy, and of ail the instructions they may afterwards receive in relation to this science, although they are generally neglected. When problems on the celestial globe arc prescribed, and vague descriptions of the planetary system given, previous to having made these observations, the subject is seldom understood, and no clear nor expansive conceptions formed by the young, of the motions, phenomena, and
relations of the great bodies of the universe
It may not be necessary, in the first instance, while making these observations, to attempt any explanation of the phenomena, but merely to impress upon the mind a clear conception of the apparent motions and relativt aspertt, of the celestial orbs, as they present themselves to an attentive spectator j leaving the pupil to ruminate upon them till it shall be judged proper to direct his attention to the investigaaon of the true causes of celestial phenomena.
The pupil's attention might be next directed to the motions of the planets, and the general phenomena of the solar system. When any of the planets are visible in the heavens, their positions in relation to the neighbouring stars should be particularly noted, so that their apparent motions, whether direct or retrograde, may be clearly perceived, which, in most cases, will be quite perceptible in the course of a few weeks or months. The direct, stationary, and retrograde movements of Mars and Venus should be particularly attended to, for the purpose of afterwards demonstrating that the annual motion of the earth accounts for the apparently irregular and complicated motions of the planetary orbs. Large diagrams, representing the apparent motions of Mars, Mercury, and Venus, as seen from the earth during the course of several revolutions, with all the apparently irregular loops and curves they appear to describe*—should be laiJ before the pupil for his particular inspection, in order that he may perceive the improbability that such motions are real, or that an Infinitely Wise Being, who is the Perfection of Order, would introduce such inextricable confusion into the motions of the most splendid of his works—A common planetarium, which shows by wheclwork, tho relative motions of Mercury, Venus, the Earth, and Mars, may be easily made to illustrate these motions, and to *olve all their phenomena. Let a circle, two or three inches broad, and of such a diameter as to surround the planets, with a few stars marked on its inside to represent the Zodiac, be suspended on three pillars, so as to inclose the Earth, Mercury, and Venus. Let a wire be fixed by a socket, on the top of the pillar which supports tho ball representing the Earth, and let this wire rest on a slit or fork fixed to the top of the pillar which supports the ball representing Mercury. When the machine is set in motion, the wire will point out on the Zodiac the apparent motions of Mercury as seen from the earth. When he passes from his greatest elongation westward to the superior conjunction and to his greatest elongation eastward, the wire will move eastward, according to the order of the signs. About its greatest elongation, it will appear stationary, and immediately afterwards will move irestward, or contrary to the order of the signs, till it arrive at the western elongation, when it will again appear stationary;— so that the pupil will plainly perceive that the direct and retrograde motions of the planets,
* Ptpeeimrne of such diagrams may be seen In 'Long's Astronomy," vol. I., and in plate 3 of "' Ferguson's Astronomy," Brewster's edition.
as seen from the earth, are m perfect accordance with a regular circular motion around the sun as a centre; and that such apparently irregular movements arise from the motion of the earth, and the different velocities of the planets, when compared with it,—just as tho objects around us appear to move in different directions, and with different velocities, when we are sailing along a serpentine river in a steamboat.
The arguments or considerations which prove that the Karth is a moving body, should next be presented to the attention, and illustrated in the most simple and familiar manner of which the subject will admit. The pupil will easily be made to perceive, that, if tho earth is at rest, the whole frame of the material universe must move round it every twentyfour hours; not only the fixed stars, but tho sun and moon, the planets and their satellites, and every comet which traverses the firmament, must participate in this motion, while, at the same time, they are moving in another and an opposite course peculiar to themselves. He will perceive, that, in proportion as these bodies are distant from the earth, in a similar proportion will be the velocity with which they perform their diurnal revolutions—that the sun behoved to move five hundred and wiitc'yteven millions of miles every day, the nearest fixed star 125,000,000,000,000 of miles in tho same time, or at the rate of fourteen hundred' millions of miles every second, and the most distant stars with a velocity which neither words can express, nor imagination conceive, —and that such motions, if they actually existed, would, in all probability, shatter the whole material frame of the universe to atoms. He may be directed to consider, that such rapid velocities (if they could be supposed to exist) arc not the motions of mere points or small luminous balls, but the motions of immense globes, many thousands of times larger than the earth—that a hundred millions of such globes are visible from our abode, besides the myriads that may be hid from human view in the unexplored regions of space—and that it is impossible to conceive how all these innumerable globes, of different magnitudes, at different distances, and moving with different velocities, could be so adjusted as to finish their diurnal revolutions at the same moment, while many of them are at the same time impelled by other forces in a contrary direction. He may be reminded that the Creator, who formed the universe, is possessed of Infinite Wisdom—that wisdom consists in proportionating means to ends, or in selecting the most appropriate arrangements in order to accomplish an important purpose—that to make the whole frame of Universal Nature move round the earth every day, merely to produce the alternate succession of day and night, is repugnant to every idea we ought to entertain of the Wisdom and Intelligence of the Divine Mind, since the same effect can be produced by a simple rotation of the earth in twentyfour hours; and since we find that Jupiter and Saturn, and other globes much larger than ours, move round their axes in a shorter period —that in all the other works of Omnipotence, means apparently the most simple are selected to accomplish the most grand and magnificent designs—and that there is no example known to us, throughout the universe, of a larger body revolving around a smaller. When such considerations are fully and familiarly illustrated, the pupil will soon be made clearly to perceive, that the rotation of the earth must necessarily be admitted, and that it will fully account for all the tliversity of diumal motion which appears in the sun and moon, the planets and the stars.
The annual revolution of the earth, and its position in the solar system, might be proved and illustrated by such considerations as the following:—that if this motion did not exist, the motions of all the planets would present a scene of inextricable confusion, consisting of direct and retrograde motions, and looped curves, so anomalous and irregular, as to be inconsistent with every thing like harmony, order, or intelligence—that Mercury and Venus are observed to have two conjunctions with the sun, but no opposition; which could not happen unless the orbits of these planets lay teithin the orbit of the earth—that Mars, Jupiter, and the other superior planets, have each their conjunctions with and oppositions to the sun, which could not be unless they were exterior to the orbit of the earth—that the greatest elongation of Mercury from the sun is only about 20 degrees, and that of Venus 47; but if the earth were the centre of their motions, as the Ptolemaic system supposes, they might sometimes be seer* 180 degrees from the sun, which never happens— that some of the planets appear much larger and brighter at one time than at another, on account of their different distances from the earth; but, on the other hypothesis, their brilliancy should bo always the same—that Mercury and Venus, in their superior conjunctions with the sun, are sometimes hid behind his body, and in their inferior conjunctions sometimes appear to pass across the sun's disk, like round black spots which would be impossible according to the Ptolemaic system; —and, in short, that the times in which tho conjunctions, oppositions, stations, and rclrogradationt happen, arc not such as they would be if the earth were at rest, but precisely such as would happen, if the earth move along with all the other planets, in the stations and pe
riods assigned them in the system which have the sun for its centre. From such considerations, when properly explained, the annual motion of the earth, and its relative position in the system, may be clearly demonstrated, and the pupil made to perceive the beauty and harmony of the celestial motions, and the necessity of having the great source of light and heat placed in the centre of the system. For as the sun is intended to cheer and irradiate surrounding worlds, it is from the centre alone that these agencies can be communicated, in a uniform and equable manner, to the planets in every part of their orbits. Were the earth the centre, and the sun and planets revolving around it, the planets when nearest the sun, would be scorched with excessive heat, and when farthest distant would be frozen with excessive cold.
There is another consideration by which the earth's annual revolution and its position in the system arc demonstrated ;—and that is, that the planets Mercury and Venus, when viewed through good telescopes, are found to assume different phases, in different parts of their orbits; sometimes appearing gibbous, sometimes like a half-moon, and at other times like a crescent, and a full enlightened hemisphere, which could never happen if they revolved round the earth as their centre, and if the earth was not placed in an orbit exterior to that of Venus. I have sometimes illustrated this argument, with peculiar effect, by means of an equatorial telescope and a common planetarium. By the equatorial telescope, with a power of 60 or 80 times, most of the stars of the first magnitude, and some of those of the second, may be seen even at noonday. Venus may be seen by this instrument, in the daytime, during the space of nineteen months, with the interruption of only about thirteen days at the time of her superior conjunction, and three days at the time of her inferior^ so that the phase she exhibits may be seen almost every clear day. Having placed the Earth and Venus in their true positions on the planetarium, by means of an Ephemeris or the Nautical Almanac, I desire the pupil to place his eye in a line with the balls representing these planets, and tomark the phase of Venus as seen from the earth—whether a crescent, a halfmoon, or a gibbous phase. I then adjust the equatorial telescope for Venus, if she is within the range of our view, and shmc hint the planet with the fame phase in the heavens. This exhibition never fails to gratify every observer, and to produce conviction. But it can seldom be made, if we must wait till the planet bo visible to the naked eye, and capable of being viewed by a common telescope; for it is sometimes invisible to the naked eye, for nearly one half of its course from one conjunction to a u other. Besides, the phases of this planet are more distinctly marked in the daytime, when near the meridian, than either in the morning or evening, when at a low altitude, in which case it appears glaring and undefined, on account of the brilliancy of its light, and the undulating vapours near the horizon, through which it is seen. As actual observations on the planets in the heavens make a deeper and more convincing impression on the mind of a young person, than mere diagrams or verbal explanations, I consider an equatorial telescope in conjunction with a celestial globe and an orrery, as essentially necessary to every teacher of astronomy; as. independently of its use, now hinted at, it is the test and most comprehensive instrument for conveying an idea of the practical operations of this science. It may be made to serve the general purposes of a transit instrument, a quadrant, an equal altitude instrument, a theodolite, an azimuth instrument, a level, and an accurate universal sundial. It serves for taking the right ascensions and declinations of the heavenly bodies, and for conveying a clear idea of these operations. It may be made to point to any phenomena in the heavens whose declination and right ascension are known; and, in this way, the planets Mercury, Hcrschel, Ceres, Pallas, Juno, and Vesta, a small comet, or any other body not easily distinguished by the naked eye, may be readily pointed out.*
The cause of the variety of tentont may next be explained and illustrated. It is difficult, if not impossible, by mere diagrams and verbal explanations, to convey a clear idea on the subject; and therefore some appropriate machinery must be resorted to, in order to assist the mind in forming its conceptions on this point. The difficulty is, to conceive how the sun can enlighten the North Pole without intermission, during one half of the year, and the South Pole during the other, while the poles of the earth never shift their position, but are directed invariably to the same points of the heavens. This is frequently attempted to be illustrated by means of a brass hoop with a candle placed in its centre, and a small terrestrial globe carried round it having its axis inclined to the brass circle, which is intended to represent the orbit of the earth. But this exhibition requires some dexterity to conduct it aright, and after all is not quite satisfactory. An orrery, having all the requisite movements by wheel-work, and where the Earth moves with its axis parallel to itself and inclined to
* A small Equatorial, having the Horizontal, Declination, and Equatorial circles about six Inches diameter, surmounted with a twenty-inch achrotnilM' telescope, with magnifying powers of from 30 to 80 times, may be procured for about fifteen or sixteen guineas, which will serve every general purpose in teaching astronomy.
the plane of the ecliptic, is the best instrument for illustrating all the variety of the seasons. When such a machine cannot be procured for this purpose, its place may be supplied by a neat little instrument, called a Tellurium, which has been manufactured for many years past by Messrs. Jones, Holburn. London, and may be purchased for about thirty shillings. This instrument consists of a brass ball representing the sun—which may be occasionally screwed off, and a lamp substituted in its place —an ivory ball representing the earth, having the circles of the sphere drawn upon it, a small ball representing the moon, and about eight wheels, pinions, and circles. It exhibits the annual motion of the earth, and the moon revolving around it, with its different phases, the causes of eclipses, the retrograde motion of the moon's nodes, and the inclination of its orbit to the plane of the ecliptic. The earth is movable on an axis inclined 23^ degrees to the ecliptic, and its axis preserves its parallelism during its course round the sun. The seasons arc exhibited on this instrument as follows:—the index, which points out the sun's place and the day of the month, is placed at the 21st March, the time of the vernal eqinox, and the north and south poles of the earth are placed exactly under the terminator, or boundary between light and darkness. When the machinery is moved by the hand till the index points to the 21st of June, the time of the summer solstice, then the North Polar regions appear within the boundary of light, and the South Polar within the boundary of darkness. Turning the machine till the index points to September 23d, both poles again appear on the boundary of light and darkness. Moving it on to December 21st, the Arctic circle appears in darkness, and the Antartic in the light. During these motions, the earth's axis keeps parallel to itself, pointing uniformly in the same direction. This exhibition is quite satisfactory and convincing; the only objection to the instrument is, that it is small,—about eight or nine inches diameter—and, consequently, will admit only four or five individuals at a time to inspect its movements with distinctness.
A full and specific description should next be given of all the facts connocted with the solar system—the distances and magnitudes of the sun and planets—their annual and diurnal revolutions—the solar spots—the belts and satellites of Jupiter—the rings of Saturn —the phases of Venus—the spots of Mars, and the mountains and cavities of the Moon. After which some details might be given of the facts which have been ascertained respecting comets, variable stars, double and treblo stars, new stars, stars once visible which have disappeared, and the numerous nebula which x2 (113)
arc dispersed through different regions of tho heavens. The pupils should now be gratified with a view of some of these objects through good telescopes. A telescope, magnifying about 30 times, will show the satellites of Jupiter, the crescent of Venus, the solar spots, and tho rugged appearance of the Moon. With a magnifying power of 60 or 70, the ring of Saturn, the belts of Jupiter, the shadows of the lunar mountains and cavities, and all the phases of Venus, may be distinguished. Dut the views of these objects obtained by such magnifying powers are unsatisfactory. No telescope should be selected for this purpose less than a 3J feet Achromatic, with powers varying from 40 to 180 or 200 times.* A power of 150 is a very good medium for inspecting all the more interesting phenomena of the heavens. With this power, distinct and satisfactory views may he obtained of the solar spots, the phases of Mercury, Venus, and Mars, the belts, and sometimes the spots of Jupiter, and the shadows of his satellites, the ring and some of the moons and belts of Saturn, the spots of Mars, the minute hills and cavities of the moon, several of the double stars, and many of the most remarkable nehvlrr. To perceive distinctly the division of Saturn's ring, requires a power of at least 200 times. In exhibiting such objects to the young, especially when the lower powers arc used, some attention is requisite to adjust the instrument to distinct vision, as their eyes are generally more convex than the eyes of persons advanced in life, and those who are short sighted will require an adjustment different from that of others. Unless this circumstance be attended to, their views of celestial phenomena will frequently be unsatisfactory and obscure. In exhibiting the surface of the moon, the period of half-moon, or a day or two before or after it, should generally be selected; as it is only at such periods that the shadows of the mountains and vales, and the circular ridges, can be most distinctly perceived. At the timo of full moon, its hemisphere presents only a variegated appearance of darker and brighter streaks, and no shadows are discernible; so that, from the telescopic appearance of the full moon, wo could scarcely determine whether or not its surface were diversified with mountains and vales.
Previous to exhibiting the moon through a telescope, it may be proper to give the observers an idea of some particular objects they will sec, on which their attention should be fixed, and from which they should deduce
• An Achromatic telescope of this description, with an object-glass, 45 inches focal distance, and about three inches diameter, with 4 or 5 maenlfying powers, with a brass tube mounted on a brass tripod, may be purchased in London, for 25guineas.
certain conclusions. For, a view of the moon, for the first time, through a powerful telescope, is apt to overpower the eye, and to produce a confused and indistinct perception. As one of the peculiarities of the lunar surface consists in the numerous cavities, and plains surrounded with circular ridges of mountains, and insulated mountains rising from a level surface—an idea of the shadows and circumstances by which these objects arc indicated should be previously communicated. This may be done by means of a saucer, the top of a small circular box, or any other object which may represent a plain surrounded by a circular ridge. In the middle of any of these objects may be placed a small peg to represent a mountain. Then plating a candle at the distance of a foot or two, so as to shine obliquely upon the objects, the inside of the circular dish farthest from tho candle will be seen enlightened, while a considerable portion of the bottom will be covered by the shadow thrown upon it by the side next the candle, and the shadow of the peg will be seen verging toward the enlightened side. This previous exhibition will give them an idea of the form of some of the mountains and vales on the lunar surface, and enable them to appreciate the nature of those striking inequalities which appear near the boundary between the dark and enlightened parts of the moon. Other objects which diversify the moon's surface may be represented and illustrated in a similar manner, and sufficient time should be allowed to every observer for taking a minute inspection of all the varieties on the lunar disk. The solar spots may bo viewed with ease, by interposing a coloured glass between the eye and the image of the sun; but, in looking through the telescope in the ordinary way, they can be perceived by only ont individual at a time. In order to exhibit them to a company of 30 or 40 persons at once, the image of the sun may bo thrown on a while wall or screen. I have generally exhibited them in the following manner. To a 3$ feet Achromatic telescope, I apply a diagonal rjejrimt, which has a plain metallic speculum placed at half a right angle to the axis of the telescope. By this eye-piece, after the room has been darkened as much as possible, the image of the sun and his spots is thrown upon the roof of the apartment, which forms a beautiful circle of light, and exhibits all the spots which then happen to diversify his surface. His apparent diurnal motion is also represented, along with the motions of any thin fleeces of clouds which may happen to cross his disk. In this way, too, the proportional magnitudes of the spots may be measured, and compared with the diameter of the sun. ami. of course, their real magnitudes