can you say of its mass or weight? about 750 times the mass of all the planets. did the ancient astronomers consider the sun to be? rge globe of fire. do astronomers at the present day consider it to be? opake body like the earth, surrounded by a atmosphere. t motion has the sun? as three motions-1st, on its axis; 2d, around re of gravity of the solar system; 3d, around re of the universe. iverse is used by astronomers, though perhaps improperly, to designate the great ament of stars in which our sun is situated.-(SEE PAGES 45 and 46.) This cluster e single stars, that can be seen with the naked eye, and all those composing the y way. The number of stars or suns in the cluster is estimated at many millions; our sun, are supposed to revolve around the common centre of gravity of the whole al thousand other distinct clusters or nebulæ, situated without our firmament, can best telescopes, nearly all of which are invisible to the unassisted eye.] er. About 71 degrees. what time does it revolve on its axis? about 25 days and a half. ow is the revolution of the sun on its axis determined? y spots on its surface, which first appear on the hey are supposed to be openings in the luminous n. What occasions these openings in the luminous atmosphere? causes. Do these spots undergo any changes? They are constantly changing, and sometimes very y. Some have appeared, others disappeared sud On what part of the sun do they appear? Is the surface of the sun, in the region of the spots, tranquil or. 12 A violent agitation. 13 Q. What reasons have we to suppose that the luminous part of the sun is intensely hot? A. 1st, the heat of its rays, when collected into a focus, is very great. 2d, its rays pass through glass with the greatest facility, (a property belonging to artificial heat in direct proportion to its intensity.) 3d, the brightness of the sun is greater than the most vivid flames, or the most intensely ignited solids. LESSON XIII. TRANSIT OF MERCURY AND VENUS. Q. What is generally meant by the transit of Mercury and Venus? Q. What is the disc of the sun or a planet? A. It is the circular illuminated surface visible to us. Q. How do Mercury and Venus appear, when passing across the sun's disc? A. They appear like black spots moving across the sun. Q. What proof have we that Mercury and Venus are not luminous bodies? A. When viewed with the telescope they sometimes appear horned like the moon. Q. On which side of the sun does a transit begin? A. On the east side, and terminates on the west side. THE SPOTS ON THE SUN. From the facts Astronomers do not agree, in all respects, as to the cause of the spots on the sun. already known, the following appears to be the most rational view of the subject. The body of the sun, which is opake, is surrounded by a transparent atmosphere, in which float two strata of luminous clouds; the lower stratum being more dense and opake, and less luminous than the upper; while the latter, by its brilliancy, furnishes the greater portion of the intense light of the sun. Above the upper stratum, the transparent atmosphere extends to a great height. The agency by which the light and heat of the sun are generated, is not known. The only agent of which we know, that presents analogous phenomena, is electricity. The northern lights are supposed to exhibit, in a feeble manner, an action similar to the luminous strata of the sun. The polar regions of the sun are tranquil, and the equatorial comparatively so; but the surface on each side of the equator, from 15 to 25 degrees therefrom, is in a state of constant and violent agitation. It is in this disturbed region that the spots are seen; no spot ever occurring farther than about 30 degrees from the equator. The spots, besides revolving with the sun, are found to have a motion from the equator towards the poles, and when they arrive at the comparatively calm region, they gradually disSometimes they close up with great rapidity, at others they appear to be suddenly broken into fragments and dispersed. Bright spots and streaks, called faculæ, apparently caused by waves in the luminous portion of the atmosphere, also appear on various parts of the disc, but are seen most distinctly near the margin. In the places where spots appear, faculæ are usually seen on the day previous to their breaking out. appear. But what causes the agitation of the sun's atmosphere, which is so great as frequently to burst open the luminous strata? Astronomers, at different times, have suggested various causes for the sun's spots, such as jets of gas issuing from the sun and decomposing the luminous clouds; high mountains, extending through the luminous strata; volcanoes, sending forth ashes, smoke, &c.; to say nothing of exploded theories of an older date, such as ashes, scoriæ, &c., on the surface of On the earth the the melted, burning mass; or bodies very near the sun, revolving round it. But if we are permitted to reason from what takes place on the earth, we would say, that a close analogy exists between the phenomena observed in our own atmosphere and in that of the sun. heat of the torrid zone causes the air to expand and rise, causing currents in the lower part of the atmosphere towards the equator, and in the upper part of the atmosphere currents towards the poles. The turning of the earth on its axis causes the under currents to take a westerly direction, while the upper currents sweep in a curve, westerly first, then towards the poles, and finally eastward. The principal disturbance of the atmosphere caused by the trade wind is in the vicinity of the tropics. Storms commencing in the torrid zone, are carried in the direction of the upper currents of air. For instance, a storm started in the West Indies, by the heating of the air over one of its islands, thus causing an upward and circular movement of the air, usually sweeps to the west and north over Florida, or the Gulf of Mexico, and then northeast, over the United States. Similar causes acting upon the atmosphere of the sun, would exhibit phenomena similar to those which we see. This explanation supposes the atmosphere of the sun to be warmer at the equator than at the poles; but as the sun does not, like the earth, receive its heat from any extraneous body, its difference of temperature must be sought for in the escape of its heat. It could attain this condition either by a more free radiation of heat at the poles than at the equator, or by its absorption as latent heat, in the evaporation from large bodies of water in the polar regions. As the sun turns on its axis, its equatorial diameter must be greater than its polar, and the stratum of atmosphere above the luminous clouds must be thicker over the equatorial region than over the polar. This must render the radiation less free at the equator than at the poles, and cause that part of the sun to be of a higher temperature. An excess of heat at the sun's equator, with its rotation on its axis, is sufflcient to cause currents in its atmosphere similar to our trade winds, and thus disturb its equatorial regions; and if the spots are caused by storms bursting open the luminous strata, their receding from the equator towards the poles is undoubtedly the effect of the same physical causes that give a similar motion to storms upon the earth. Some have supposed the body of the sun to be protected by the lower opake portion of the inner stratum of clouds, from the intense heat of the luminous strata, and thus rendered inhabitable; but several objections will at once arise to this theory. First, the body of the sun being surrounded by dense and opake clouds, could not send off its heat into space by radiation, and therefore the heat received from the clouds would accumulate and cause a high temperature. Second, the force of gravity being about thirty times as great as that of the earth, a common sized man would weigh some two or three tons; rendering it necessary to have an entirely different muscular organization. Third, it is improbable that living beings would be shut up within an impenetrable veil, and cut off from a knowledge of the planets, the stars, and the countless wonders existing in the boundless realms of space. These and other considerations render it probable that the sun is not inhabited. ign is divided into 30 degrees; each degree utes; each minute into 60 seconds, &c. reat circle is in the middle of the zodiac? cliptic, or orbit of the earth. re the names of the constellations of the zodiac and the eliptic? Taurus, Gemini, Cancer, Leo, Virgo, Libra, gittarius, Capricornus, Aquarius, and Pisces. constellations of the zodiac and the signs of the ecliptic me places in the heavens ? do not: the signs in the ecliptic have fallen e constellations about 31 degrees. e constellations of the zodiac and signs of the ecliptic nd? corresponded to each other about 22 centu is the cause of the falling back of the signs of the ecliptic onstellations? caused by the retrograde motion of the equiNOTE.) what does the length of the seasons depend? depend upon the revolution of the earth equinox to the same, again. the earth revolve around the sun in exactly the same moves from one equinox to the same equinox again? moves from either equinox to the same again, minutes sooner, than around the sun. LESSON XV. 7. DOES the sun appear to move in the heavens among the er. It has an apparent motion in the ecliptic, - around the heavens, during the year. w is this appearance caused, as the sun is in the centre, and ove? is caused by the earth's moving around the sun. he earth is in the sign Aries, where does the sun appear to be? appears to be in the opposite sign, Libra. the earth moves around in the ecliptic, where does the sun move? appears to move in the opposite part of the , and in the opposite direction from the motion earth. hich sign does the sun enter, when the north pole leans exards the sun? ncer. (21st June.) Q. Which sign does the earth enter at this time? Q. Which signs does the sun enter, when the north pole leans sideways to the sun? A. Aries and Libra. Q. Which sign does the sun enter, when the north pole leans exactly from the sun? A. Capricornus. (22d December.) Q. Which are the equinoctial signs? A. Aries, 21st of March Libra, 23d of September. Q. Which are the solstitial signs? A. Cancer, 21st of June-Capricornus, 22d of December. LESSON XVI. Question. How are the signs of the ecliptic divided? Answer. They are divided into four divisions, corresponding to the seasons. Q. Which are the spring signs? Q. Which are the autumnal signs? A. Capricornus, Aquarius, Pisces. Q. In what time do the equinoxes fall back through the whole circle of the Zodiac? A. 25,800 years. Q. What is this time called? A. The Platonic, or great year. Q. How is this motion caused? A. It is caused by a slow annual motion of the earth's axis. (NOTE.) Q. What is longitude in the heavens ? A. It is the distance from the first degree of the sign Aries, reckoned eastward on the ecliptic, the whole circumference of the heavens. Q. When the sun enters Aries, what is its longitude? Q. What is the longitude of the earth at that time? Q. When the sun enters Cancer, what is its longitude? A. 90 degrees the earth's longitude at the same time 270 degrees. Q. When the sun enters Libra, what is its longitude? A. 180 degrees-the earth's longitude 0 degrees. Q. When the sun enters Capricornus, what is the longitude? 4. 270 degrees-the earth's longitude at the same time 90 degrees. [NOTE. This variation is caused by the pole of the earth varying a little every year. This motion of the pole of the earth is similar to that sometimes shown by a top, as it spins around on the point. The stem of the top will have a circular motion, describing a cone with the apex or top down. This circular motion of the pole of the earth is very slow, varying only 50" every year, and requires 25.868 years to complete a revolution-which is called the Platonic or great year. The pole of the earth is now approaching the North Star, and will continue to approach it for about 240 years, when it will be less than half a degree from it; it then will begin to recede, and in 12,900 years will e more than 47° from it, when it will begin to approach it, and will continue in the same co INTRODUCTION TO ASTRONOMY. LESSON I. Question. WHAT is the body called upon which we live? Q. What idea had the ANCIENTS respecting the shape of the earth? A. They believed it was an extensive plain, rendered neven by hills and mountains. Q. Why did they think it was an extended plain? A. Because they formed their opinions from appearapces only. Q. Did they believe that the earth had any motion? A. They did not; they believed that the earth rested on a solid, immovable foundation. [They very naturally came to this conclusion, as they were entirely ignorant of the laws of ttraction or gravitation. They believed that if the earth were to turn over, that every thing oukl be precipitated from its surface.] Q. Had they any definite ideas respecting what held the earth up? A. Their views were very vague and unsatisfactory. [There have been many absurd ideas advanced, at different ages of the world, as to what suported the earth. Some supposed it to be shaped like a CANOE, and to float upon the waters; thers, that it rested upon the back of an ELEPHANT or huge TURTLE; while, according to mynology, Atlas supported it upon his shoulders: but, what kept the waters in their place, or upon hat the Elephant, Turtle, or Atlas stood-this was a mystery they COULD NEVER SOLVE.] Q. Did they believe the earth extended the same distance in all irections? A. They believed it to extend much farther from east o west than from north to south. [They observed that in going east or west, on the same parallel of latitude, no change took ace in the appearance of the heavens; but in going north or south, on the same meridian, every xty miles caused a difference of one degree in the elevation of the pole, and in the position of the cles of daily motion of the sun and other heavenly bodies; therefore they concluded that the rth was very long from east to west, but comparatively narrow from north to south. From this iginated the use of the TERMS longitude and latitude; longitude meaning length, and latitude, eadth.] Q. What ideas had they respecting the motions of the sun, moon, nd stars? A. They supposed that they revolved around the arth, from east to west, every day.. Q. What was this system called, that supposed the earth to be at st in the centre, and all the heavenly bodies to revolve around it? A. The Ptolemaic system. Ptolemy asserted, that the sun, moon, planets, and stars revolved around the earth, from east Q. We see no body at rest that does not touch some p support, but we see bodies in motion supported for different l time without resting upon any other surface; if the earth rest upon any thing, is it probably at rest? A. It is more probable that it is in motion. Q. If we throw a ball, does the same side always remain Q If a fly were on the ball, would distant objects appear be stationary? A. They would appear to revolve around the often as it turned over. Q. If the earth is moving in space, is it in accordance known motion of ordinary bodies, to suppose that the sa remains forward? A. It is not. It is more reasonable to suppose turns on its axis. Q. If the earth turns, and we are carried round on its surf appearance must the sun and distant stars necessarily present A. They must appear to move around the earth opposite direction. Question. WHAT other reason can you give for the earth's Answer. The stars are so distant, that their would be immensely swift in comparison with t tion of the earth, to produce the same effect. Q. But have we not positive proof, and that too of differer that the earth turns on its axis? A. We have.—1. The shape of the earth, elev the equator and depressed at the pôles, can counted for on no other supposition. 2. A body at the equator, dropped from a west, every 24 hours; and to account for their not falling upon the earth, when they passed height, falls eastward of the perpendicular. er it, he supposed that they were each fixed in a separate hollow crystalline globe, one within other. Taus the moon was in the first; Mercury in the second; Venus in the third; the sun the fourth; Mars in the fifth; Jupiter in the sixth; Saturn in the seventh;-(the planet rschel was not known at this time) the fixed stars in the eighth. He supposed the stars to in one sphere as they are kept in the same position with respect to each other. To permit the it of the stars to pass down to the earth, he supposed these spheres or globes were perfectly ar or transparent like glass. The power which moved these spheres, he supposed, was comnicated from above the sphere which contained the stars.] Question. EVERY one is conscious that the sun, which rises daily the east and sets in the west, is the same body; where does it go ring the night? Answer. It appears to pass round under the earth. Q. When we look out upon the stars, on successive evenings, they pear to have a definite position with respect to each other, and a stward movement like the sun; what motion do they appear to ye from their setting to their rising? 4. They appear to pass under the earth. Q. From the north to the south point of the heavens, there is a tinuous arc of stars, and in their passage under the earth they are at all disarranged, what can you infer from this fact? That 3. The trade winds and ocean currents in the cal regions are clearly traceable to the same caus Q. If the earth is moving in space, does it proceed in a line? |