Astronomy for Amateurs

the plane of the ecliptic, and that they disappear at the western border fourteen days after their arrival at the eastern edge. Sometimes the same spot, after being invisible for fourteen days, reappears upon the eastern edge, where it was observed twenty-eight days previously. It progresses toward the center of the Sun, which is reached in seven days, disappears anew in the west, and continues its journey on the hemisphere opposed to us, to reappear under observation two weeks later, if it has not meantime been extinguished. This observation proves that the Sun revolves upon itself. The reappearance of the spots occurs in about twenty-seven days, because the Earth is not stationary, and in its movement round the burning focus, a motion effected in the same direction as the solar rotation, the spots are still visible two and a half days after they disappeared from the point at which they had been twenty-five days previously. In reality, the rotation of the Sun occupies twenty-five and a half days, but strangely enough this globe does not rotate in one uniform period, like the Earth; the rotation periods, or movements of the different parts of the solar surface, diminish from the Sun's equator toward its poles. The period is twenty-five days at the equator, twenty-six at[Pg 100] the twenty-fourth degree of latitude, north or south, twenty-seven at the thirty-seventh degree, twenty-eight at the forty-eighth. The spots are usually formed between the equator and this latitude, more especially between the tenth and thirtieth degrees. They have never been seen round the poles. Toward the edges of the Sun, again, are very brilliant and highly luminous regions, which generally surround the spots, and have been termed faculæ (facula, a little torch). These faculæ, which frequently occupy a very extensive surface, seem to be the seat of formidable commotions that incessantly revolutionize the face of our monarch, often, as we said, preceding the spots. They can be detected right up to the poles. Our Sun, that appears so calm and majestic, is in reality the seat of fierce conflagrations. Volcanic eruptions, the most appalling storms, the worst cataclysms that sometimes disturb our little world, are gentle zephyrs compared with the solar tempests that engender clouds of fire capable at one burst of engulfing globes of the dimensions of our planet. To compare terrestrial volcanoes with solar eruptions is like comparing the modest night-light that consumes a midge with the flames of the fire that destroys a town. The solar spots vary in a fairly regular period of[Pg 101] eleven to twelve years. In certain years, e.g., 1893, they are vast, numerous and frequent; in other years, e.g., 1901, they are few and insignificant. The statistics are very carefully preserved. Here, for instance, is the surface showing sun-spots expressed in millionths of the extent of the visible solar surface: 188978 189099 1891569 18921,214 18931,464 1895974 1896543 1897514 1898375 1899111 190075 190129 190262 The years 1889 and 1901 were minima; the year 1893 a maximum. It is a curious fact that terrestrial magnetism and the boreal auroras exhibit an oscillation parallel to that of the solar spots, and apparently the same occurs with regard to temperature. We must regard our sun as a globe of gas in a state of combustion, burning at high temperature, and giving off a prodigious amount of heat and light. The dazzling surface of this globe is called a photosphere (light sphere). It is in perpetual motion, like the waves of an ocean of fire, whose roseate and transparent flames measure[Pg 102] some 15,000 kilometers (9,300 miles) in height. This stratum of rose-colored flames has received the name of chromosphere (color sphere). It is transparent; it is not directly visible, but is seen only during the total eclipses of the Sun, when the dazzling disk of that luminary is entirely concealed by the Moon; or with the aid of the spectroscope. The part of the Sun that we see is its luminous surface, or photosphere. From this agitated surface there is a constant ejection of gigantic eruptions, immense jets of flame, geysers of fire, projected at a terrific speed to prodigious heights. For years astronomers were greatly perplexed as to the nature of these incandescent masses, known as prominences, which shot out like fireworks, and were only visible during the total eclipses of the Sun. But now, thanks to an ingenious invention of Janssen and Lockyer, these eruptions can be observed every day in the spectroscope, and have been registered since 1868, more particularly in Rome and in Catania, where the Society of Spectroscopists was founded with this especial object, and publishes monthly bulletins in statistics of the health of the Sun. These prominences assume all imaginable forms, and often resemble our own storm-clouds; they rise above the chromosphere with incredible velocity, often [Pg 103]exceeding 200 kilometers (124 miles) per second, and are carried up to the amazing height of 300,000 kilometers (186,000 miles). Fig. 31.—Rose-colored solar flames 228,000 kilometers (141,500 miles) in height, i.e., 18 times the diameter of the Earth. The Sun is surrounded with these enormous flames on every side; sometimes they shoot out into space like splendid curving roseate plumes; at others they rear their luminous heads in the Heavens, like the[Pg 104] cleft and waving leaves of giant palm-trees. Having illustrated a remarkable type of solar spot, it is interesting to submit to the reader a precise observation of these curious solar flames. That reproduced here was observed in Rome, January 30, 1885. It measured 228,000 kilometers (141,500 miles) in height, eighteen times the diameter of the earth (represented alongside in its relative magnitude). (Fig. 31.) Solar eruptions have been seen to reach, in a few minutes, a height of more than 100,000 kilometers (62,000 miles), and then to fall back in a flaming torrent into that burning and inextinguishable ocean. Observation, in conjunction with spectral analysis, shows these prominences to be due to formidable explosions produced within the actual substance of the Sun, and projecting masses of incandescent hydrogen into space with considerable force. Nor is this all. During an eclipse one sees around the black disk of the Moon as it passes in front of the Sun and intercepts its light, a brilliant and rosy aureole with long, luminous, branching feathers streaming out, like aigrettes, which extend a very considerable distance from the solar surface. This aureole, the nature of which is still unknown to us, has received the name of corona. It is a sort of immense atmosphere, extremely rarefied. Our superb torch, accordingly, is a brazier[Pg 105] of unparalleled activity—a globe of gas, agitated by phenomenal tempests whose flaming streamers extend afar. The smallest of these flames is so potent that it would swallow up our world at a single breath, like the bombs shot out by Vesuvius, that fall back within the crater. What now is the real heat of this incandescent focus? The most accurate researches estimate the temperature of the surface of the Sun at 7,000°C. The internal temperature must be considerably higher. A crucible of molten iron poured out upon the Sun would be as a stream of ice and snow. We can form some idea of this calorific force by making certain comparisons. Thus, the heat given out appears to be equal to that which would be emitted by a colossal globe of the same dimensions (that is, as voluminous as twelve hundred and eighty thousand terrestrial globes), entirely covered with a layer of incandescent coal 28 kilometers (18 miles) in depth, all burning at equal combustion. The heat emitted by the Sun, at each second, is equal to that which would result from the combustion of eleven quadrillions six hundred thousand milliards of tons of coal, all burning together. This same heat would bring to the boil in an hour, two trillions nine hundred milliards of cubic kilometers of water at freezing-point. [Pg 106] Our little planet, gravitating at 149,000,000 kilometers (93,000,000 miles) from the Sun, arrests on the way, and utilizes, only the half of a milliard part of this total radiation. How is this heat maintained? One of the principal causes of the heat of the Sun is its condensation. According to all probabilities, the solar globe represents for us the nucleus of a vast nebula, that extended in primitive times beyond the orbit of Neptune, and which in its contraction has finally produced this central focus. In virtue of the law of transformation of motion into heat, this condensation, which has not yet reached its limit, suffices to raise this colossal globe to its level of temperature, and to maintain it there for millions of years. In addition, a substantial number of meteors is forever falling into it. This furnace is a true pandemonium. The Sun weighs three hundred and twenty-four thousand times more than the Earth—that is to say, eighteen hundred and seventy octillions of kilograms: 1,870,000,000,000,000,000,000,000,000,000 (1,842,364,532,019,704,433,497,536,945 tons). In Chapter XI we shall explain the methods by which it has been found possible to weigh the Sun and determine its exact distance. [Pg 107] I trust these figures will convey some notion of the importance and nature of the Sun, the stupendous orb on whose rays our very existence depends. Its apparent dimension (which is only half a degree, 32′, and would be hidden from sight, like that of the full moon, which is about the same, by the tip of the little finger held out at arm's length), represents, as we have seen, a real dimension that is colossal, i.e., 1,383,000 kilometers (more than 857,000 miles), and this is owing to the enormous distance that separates us from it. This distance of 149,000,000 kilometers (93,000,000 miles) is sufficiently hard to appreciate. Let us say that 11,640 terrestrial globes would be required to throw a bridge from here to the Sun, while 30 would suffice from the Earth to the Moon. The Moon is 388 times nearer to us than the Sun. We may perhaps conceive of this distance by calculating that a train, moving at constant speed of 1 kilometer (0.6214 mile) a minute, would take 149,000,000 minutes, that is to say 103,472 days, or 283 years, to cross the distance that separates us from this orb. Given the normal duration of life, neither the travelers who set out for the Sun, nor their children, nor their grandchildren, would arrive there: only the seventh generation would reach the goal, and only the fourteenth could bring us back news of it. Children often cry for the Moon. If one of these[Pg 108] inquisitive little beings could stretch out its arms to touch the Sun, and burn its fingers there, it would not feel the burn for one hundred and sixty-seven years (when it would no longer be an infant), for the nervous impulse of sensation can only be transmitted from the ends of the fingers to the brain at a velocity of 28 meters per second. 'Tis long. A cannon-ball would reach the Sun in ten years. Light, that rapid arrow that flies through space at a velocity of 300,000 kilometers (186,000 miles per second), takes only eight minutes seventeen seconds to traverse this distance. This brilliant Sun is not only sovereign of the Earth; he is also the head of a vast planetary system. The orbs that circle round the Sun are opaque bodies, spherical in shape, receiving their light and heat from the central star, on which they absolutely depend. The name of planets given to them signifies "wandering" stars. If you observe the Heavens on a fine starry night, and are sufficiently acquainted with the principal stars of the Zodiac as described in a preceding chapter, you may be surprised on certain evenings to see the figure of some zodiacal constellation slightly modified by the temporary presence of a brilliant orb perhaps surpassing in its luminosity the finest stars of the first magnitude. [Pg 109] If you watch this apparition for some weeks, and examine its position carefully in regard to the adjacent stars, you will observe that it changes its position more or less slowly in the Heavens. These wandering orbs, or planets, do not shine with intrinsic light; they are illuminated by the Sun. The planets, in effect, are bodies as opaque as the Earth, traveling round the God of Day