THE POPULAR EDUCATOR 



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 F G, and thus we can learn the exact length of the 

 line M L, 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 

 i to an observer in the sun by the semi-diameter of the earth 

 is found to be about 8'82", 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 tho 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 pro- 

 blem 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 maadce, are found at times 

 to exist upon its surface. The centre of these ia 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 per- 

 manent 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, an'd 

 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 tele- 

 scope. 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 

 diminiBh 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 inti- 

 mate 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 faculce. 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 con- 

 nection 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 cease- 

 less 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 somo 

 further light might be thrown on the matter. It must be re- 

 membered, however, that Mercury itself can only be seen at 

 occasional intervals, and with difficulty, owing to its proximity 

 to tho sun ; and that therefore a planet much nearer to the sun 

 would never be far enough removed from that body to be clearly 

 seen. Almost the only opportunity, then, of observing it would 

 be when it was in transit. 



As soon as Le Verrier had made his statement, a French phy- 

 sician 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 observa- 

 tions 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 ac- 

 count of Mercury, the next planet in order of distance. 



