MENSURATION (continued): Relatives 286 POPULAR EDUCATOR. LESSONS IN ASTRONOMY.-IX. THE SOLAR SYSTEM-COMPARATIVE SIZES AND DISTANCESTHE SUN-VULCAN. THE student will by this time have become acquainted with many of the more important phenomena of the heavens: we will, therefore, proceed now to notice in detail the principal facts relating to those of the heavenly bodies which are our nearest neighbours in space, and which belong to the same system, or family group, as does the earth. The following is a list of the principal bodies in this group :-The Sun, which is the common centre round which they all revolve; Vulcan, Mercury, and Venus, which are distinguished as the inferior planets, their orbits being included within that of the earth; the Earth; and the superior planets, Mars, the minor planets, or asteroids, Jupiter, Saturn, Uranus, a mile, will stand for Saturn; a full-sized cherry, three-quarter of a mile distant, for Uranus; and a plum, a mile and a quarter off, for Neptune. On this scale the distance of the nearest fixed star would be about 7,500 miles. As the sun is by far the largest of these bodies, we will treat of it first, and the question that immediately occurs to us is, What is the distance of this body? The accurate solution of this question is one of the most important problems in Fig. 15. and Neptune. Several of these have satellites, or secondary planets, revolving around them; and there are also several comets which are included as regular members of our system. These will be enumerated hereafter. As we inquire more particularly into the movements of these bodies, we see many striking points of similarity. They all move round the sun in the same direction, and in elliptical paths of no great eccentricity. They are all likewise opaque bodies like the earth, shining only by reflected light; and all rotate on their axes, so as to produce the changes of day and night. Their orbits, too, are all inclined o to the plane of the ecliptic. Orreries are frequently constructed, in which the different planets are represented by different-sized balls moving at various distances round a central one. It is, however, quite impossible to make these on a scale at all true to nature. Fig. 15 illustrates roughly their comparative sizes. The following, however, is a plan for obtaining a tolerably correct idea of their comparative distances and magnitudes, and the relative dimensions of their orbits: C Fig, 16. Select a large clear space, and place at one side a ball about two feet in diameter to represent the sun; Vulcan will then be represented by a small pin's head 27 feet from the globe; Mercury by a mustard-seed 82 feet distant; Venus by a pea at a distance of 142 feet; the Earth by a slightly larger pea at a distance of 215 feet; Mars by a large pin's head at a distance of 327 feet; the minor planets by grains of sand between 500 and 600 feet distant. An orange, about 2 inches in diameter, and 1,120 feet distant, will then represent Jupiter; one about two inches in diameter, distant two-fifths of 105-N.E. astronomy, as this distance is taken as a measure for determining the distances and magnitudes of most other heavenly bodies. The principle of the problem can easily be understood, though, of course, there are many difficulties in the carrying of it out. Suppose an observer, situated on the line BC (Fig. 16), wishes to ascertain the distance of an inaccessible object A; let AC be the visual ray by which it is seen at c; at right angles to this lay off another line, BC, and from в measure accurately the angle CBA. We know then the distance в C, and the measure of the angles at B and c; it is easy, therefore, to calculate the angle BAC and the length of A C. As will at once be seen, the longer BC is, the larger will the angle B A C be, and therefore the less the risk of error in measuring it. When this angle is very small, an exceedingly minute error produces a great difference in the calculated length of c A. Now, in the practical application of this principle, the utmost base-line that can be obtained is the earth's diameter; and this is so small in comparison with the distance of the sun that the angle BAC becomes too minute to be measured directly with a sufficient degree of accuracy. We are enabled, however, in an indirect way, to measure it, and thus solve the problem. The planet Venus travels round the sun in an orbit within that of the earth, and hence, at certain intervals, passes between the earth and the sun, and produces what is called a transit of the planet. On these occasions it is seen as a black spot on the bright disc of the sun. and by means of observations taken at that moment the required angle may be measured. Fig. 17 will render the mode of proceeding more clear. AB represents a base-line on the earth's surface, and CD the sun, E being the planet Venus when passing between the two. To an observer at B it will appear to travel across the sun's disc along the line HK, while to one situated at A it will pass along FG. Now if Venus were midway between the earth and the sun, no advantage would be gained, as the angle MB L would then be equal at AL B. The planet's distance from the sun is however, about two and a-half times as great as its distance from the earth, and hence the angle to be measured is so much greater. Now by accurately observing the times of the planet first coming into contact with the sun's disc at H, and of again emerging from it at K, we shall know exactly the length of H K; in a similar way, another observer can ascertain the length of the chord FG, and thus we can learn the exact length of the line ML, and the measure of the angle M B L. Of course, many of the details are omitted here, but the student should make himself master of the principle of the calculation, as being the key to all celestial measurements. So great is the importance attached to this problem that at the most recent transits several expeditions have been sent out by Government to take observations at different stations. As a result of the most accurate observations, the sun's horizontal parallax-that is, the angle that would be subtended to an observer in the sun by the semi-diameter of the earthis found to be about 882", and the mean distance of the sun is therefore about 92,700,000 miles. Until quite recently the parallax was taken at 8'6", and the sun's distance set down at 95,000,000 miles, but subsequent investigations have shown an error in these measurements. The numbers given above must, however, be taken as approximations only, subject to future correction, as every successive transit of Venus is anxiously awaited, and the most careful observations are made in various parts of the world. It must be remembered that the distance given above is the mean, the difference between the minimum and the maximum being about 3,000,000 miles. Having ascertained the distance of the sun, and knowing its apparent diameter to be about 32', it becomes a simple problem to ascertain its real magnitude; and in this way we find that its diameter is about 853,000 miles, or more than 108 times as great as that of the earth. The best idea we can give of this immense size, is to state that if the sun were hollow, and the earth were placed in its centre, there would be room enough for the moon to continue to revolve round it without touching the sun's surface, even if the moon's distance were increased to nearly double what it is now. The sun's volume is so great that it would require 1,300,000 globes of the size of the earth to be rolled into one to equal it, and it is 450 times as large as all the planets that revolve around taken together. Some idea can be formed of its light and heat when we remember the enormous distance we are from its surface, and the degree to which, notwithstanding this, we feel its power. Its light is computed to be equal to that of 5,500 standard candles, placed at a distance of a foot from the surface to be illuminated. We naturally want to know something more of the physical properties of this wonderful and stupendous orb, but we are to a great extent baffled in this inquiry, though many great and important discoveries have been recently effected by means of spectrum analysis. In this way it has been ascertained that many of the metals present in the earth are also present in the sun. When pieces of very dark glass are placed in front of the eye-piece of a telescope, so as to screen the eye from the intense glare of the sun, its surface may be carefully examined, and is found to present an appearance by no means uniform. Many dark spots (Fig. 18), termed macula, are found at times to exist upon its surface. The centre of these is usually of a very dark colour, and is surrounded by a ring much lighter in appearance, which is known as the penumbra. These spots are very irregular in shape, and frequently change in size or disappear altogether. At times, however, they remain permanent sufficiently long to be traced disappearing at one edge of the disc, and after an interval appearing again at the other. We thus learn that the sun, like the other members of our system, is in constant rotation on its axis, and the period of this rotation is found to be 25 days, 7 hours, and 48 minutes. These spots are occasionally so large as to be distinctly observed by the naked eye. Some of them have been measured, and their breadth found to be more than 100,000 miles. Most of them, however, are only visible with the aid of the telescope. Careful records have been kept of the appearance of these spots, and it is found that during some years there is scarcely a day elapses without some being visible. They then diminish in frequency for about five or five and a-half years, when the number is at a minimum; the surface being then free from them on more than half the days of observation. They then increase again in number for the next five and a-half years; and thus their period appears to be about eleven years. A most remarkable fact has been noted in connection with this, and that is, that the daily variation in the magnetic needle is found to have a precisely similar period, and to increase or diminish with the increase or diminution in the numbers of spots. Other phenomena seem further to show that there is an intimate relationship between the movements of the magnetic needle and the sun. Whether or not future observations may reveal to us more of the nature of this bond, we cannot say, but fresh discoveries on the subject are frequently being made. These spots are usually accounted for by supposing the sun to be a dark opaque body surrounded by two atmospheres, the outer one highly luminous, and the inner one more dense. It is supposed then that some powerful internal convulsion breaks through these layers, and thus reveals the dark surface of the sun beneath. In addition to these dark spots, others of unusual brilliancy are frequently observed. These are termed facula. Telescopic investigations show that, besides these markings, the whole surface has somewhat of a mottled appearance. According to Nasmyth, it presents an appearance as if it were covered over with scattered filaments shaped like willow-leaves. The whole question of the physical constitution of the sun is, however, engaging the attention of many astronomers. A total eclipse of the sun presents good opportunities for the observation of many points, and among the most remarkable features in connection with these phenomena is the appearance of red flames or protuberances surrounding the dark body of the moon at the moment of total obscuration. These have recently been seen at other times also, and are believed to be connected with the solar atmosphere. The symbol is frequently employed in almanacks to denote the sun. We now pass on to notice the planets which revolve in ceaseless courses around this grand central luminary. Till recently, Mercury was supposed to be the nearest planet to the sun. About ten years ago, however, the celebrated French astronomer, Le Verrier, having very carefully examined the movements of this planet, found in it a slight variation, which he could only account for by supposing that the mass of the planet Venus was incorrectly ascertained, or else that there was a planet revolving round the sun in an orbit within that of Mercury. These statements of his were published in the hope that some further light might be thrown on the matter. It must be remembered, however, that Mercury itself can only be seen at occasional intervals, and with difficulty, owing to its proximity to the sun; and that therefore a planet much nearer to the sun would never be far enough removed from that body to be clearly Almost the only opportunity, then, of observing it would be when it was in transit. seen. As soon as Le Verrier had made his statement, a French physician named Lescarbault stated that on the 26th of March, 1859, he had seen a small body pass across the sun, but had not liked to announce the fact, no other observer having called attention to it. Le Verrier at once saw him, and carefully inquired into the matter. At first he thought the whole affair was a delusion; but after questioning the physician, and inquiring about the apparatus he used, he became convinced that he had indeed discovered a new planet, which was then called Vulcan. From this one observation no very decisive details could be drawn, so as to calculate its orbit accurately; its distance from the sun was, however, set down at about 14,000,000 miles, and its time of revolution in its orbit at a little under twenty days. It was conjectured that a second transit might be observed in March, 1860; but though a careful watch was kept, it was not seen, nor has it been seen again up to the present time. Many, on this account, treat the whole affair as a mistake. Instances have, however, been previously recorded of spots resembling planets being seen on the sun, and it may be that future observations will show that the planet really exists, and that these have been transits. We cannot, however, give a definite statement either way, so we merely thus briefly state the fact of the alleged discovery, and shall commence our next lesson with some account of Mercury, the next planet in order of distance. |