Sun

Sun is the centre of the system of bodies consisting of the planets and their attendant satellites. To them he radiates light and heat; he regulates their movements, and controls their distances from himself. But he was not always regarded as the centre of this system. It was true that his apparent motions were the cause of day and night, and gave the variations in the seasons, but these apparent motions were regarded in early times as actual. It was natural that so obviously important a body should receive attention, and so we find that the Egyptians as well as the Chaldees recorded their observations on his movements and eclipses. After Copernicus had asserted that the Sun, and not the Earth, was the centre of our system, and that the Earth and other planets moved round him, it began to enter into men's minds to consider him in different ways and to attempt to discover the cause of his power and the mysteries of his constitution. One of the most obvious problems to solve was to find his distance from the Earth, and this could only be done by making use of measurements concerned with other planets besides the Earth. This problem of finding the Sun's distance is of immemse importance, but, owing to its extreme difficulty, it is not quite settled even now. This distance - the radius of the Earth's orbit - is the unit of length of the universe; distances of the stars are measured in terms of it, and any error in its computation will give rise to multiplied errors throughout the whole range of astronomy. The problem, stated in its simplest form, is to find what angle is subtended at the Sun by the Earth's semi-diameter; and, if it were not for the presence of our own atmosphere, this could be found out by direct measurements on the Sun when he is on the horizon, but the disturbing effects of the atmosphere make such a solution impossible. Indirect means had, therefore, to be devised for obtaining the value of this angle or horizontal parallax. [PARALLAX,] Aristarchus had attempted to compare the distances of the Earth from both Sun and Moon, but his method was naturally imperfect. Relative distances of the different planets from the Sun have, however, been known since the time of Kepler [KEPLER'S LAWS], so that only one ahsolute was necessary in order to know all the others, provided that that measurement could be relied upon. In 1672 Richer at Cayenne and Cassini at Paris, from observations on the opposition of Mars, estimated the Sun's distance to be about 87,000,000 miles. Flamsteed gave it as 81,700,000 miles, and Picard as 41,000,000. The differences were truly enormous. The transits of Venus were now hailed as a means of solving the problem. At the rare times when Venus partially eclipsed the Sun, the planet appears as a dark spot on the Sun's disc, and it is only necessary to exactly determine, on the one hand, the moment when she enters the brightness or leaves it, or, on the other, the time she takes to complete the passage. Unfortunately for this, these transits are rare; two take place within about eight years, and then there is a gap of over a century. The transits observed in 1761 and 1769 were found to give the most discordant results: until Encke, in 1824, published the result of his interpretations of the different observations. He gave 95,250,000 miles as the probable value, and this was gladly accepted till doubts were published by Hansen in 1854 and re-echoed by other observers. An opposition of Mars in 1862 reduced the value to about 91,000,000, and supported the views of the doubters. Again Fizeau and Foucault had succeeded in measuring the velocity of llght by means of measurements on the Earth only, and in the same year (1862), by using this velocity, Foucault, too, came to the conclusion that the Sun was nearer than Encke had deduced. Astronomers looked forward to the coming transit of Venus of 1876 with confidence, believing that this would settle all doubt. Halley's method, published in 1706, was improved, and the difficulties of Delisle's (of 1760) were found to be greatly decreased owing to modern methods of fixing longitudes; to these methods photography was added and the world was breathless with expectation as the time of transit drew near. Then it was found that from the eighty different improvised observatories conflicting statements poured in. A new difficulty arose: the atmosphere of Venus introduced the effects of refraction, and the bewildered observer was unable to say when the transit actnally began or ended. The transit of 1882 left the uncertainty still nearly 2,000,000 miles. In 1877 Dr, Gill obtained a good result from an opposition of Mars, and later (in 1888) observed the minor planet Iris, following this work with similar observations on Victoria in 1889. From his work the value 92,700,000 miles is accepted as having the smallest probable error, an error regarded as less th 200,000 miles.

If evidence on the Sun's distance shows great laxity in agreement, much more disagreement has prevailed as to the estimate of the Sun's ternperature. Newton first investigated the amount of heat received by the Earth, and many have followed his lead. Pouillet dlscovered that the vertical rays of the Sun bestow upon every square centimetre 1.7 gram degree Centigrade units of heat per minute. then laws were suggested, connecting this measurable quantity with the temperature of the Sun, allowance being made for absorption of heat by the atmosphere. But laws which answered for moderate temperatures were shown to fail at high ones, and measurements of a different kind were introduced. It was found that the Sun's own atmosphere absorbed a considerable amount of his radiation, and Frost in 1891 showed that this absorption was about seven-seventeenths of the whole. Langley in 1880 began to measure the intensity of different rays in the spectrum by means of their effect on the electrical conductivity of platinum, and after untiring research he was led to raise Pouillet's constant from 1.7 to 3 gram degrees as the value of the Sun's heat unaffected by our atmosphere. Of this, however, hardly 60 per cent. reaches the Earth's surface. Deductions as to the actual temperature of the Sun are not reliable, as we do not know how matter behaves in the unknown condition in which it exists in the Sun. Viole in 1876 suggested 1,500° to 2,500° C. as the possible limits, but in 1881 raised the value to 3,000° C., while in 1892 Le Chatelier gave it as 7,600°. The subject is still, therefore shrouded in uncertainty.

Observations during eclipses, enormously helped since 1860 by the use of photography, have taught us much about the Sun's surface. The spectroscope has in the hands of many observers disclosed the presence of iron, nickel, magnesium, sodium, and, in fact, nearly all the known metals in the form of vapour upon the solar surface. Above these rise the vast jets of flame or prominences, which are again succeeded by the corona. Beneath the metallic vapours comes the photosphere, the limit of the solar disc. Beginning at the outside, the corona may be said to be a solar appendage, but not a solar atmosphere. It has been seen to reach a distance from the Sun equal to twelve times his own diameter. It does not seem to be affected by gravitation to the Sun. There is no increase of density nearer the luminary; neither does it rotate with the Sun. It consists of gaseous matter, among which is hydrogen and an unknown substance called "coronium," while particles in the solid or liquid state are also present. It is continually in a state of motion to and from the Sun, but the constituent atoms must be so far apart as to represent what we should regard as a high vacuum. Comets pass through it with unaffected velocity. The average appearance of the photosphere is that of a heap of sand or shot spotted over with points of blackness, but spread over this are bands and spots of less granular appearance. The more granular part has been described as the "willow-leaf" structure, and is probably due to the difference in brightness of rapidly ascending and descending currents of vapour. The uniformity of the photosphere is also interrupted, especially in certain belts, by what are known as sun spots (q.v.). Near these are often to be seen white raised tracts called faculae, regions giving evidence of great pressure. The chromosphere is a vaporous layer completely enveloping the photosphere, and consisting of hydrogen and an unknown element, "helium." It is several thousand miles thick, but varies from time to time. It has been an object of much beauty and interest during eclipses, when it is seen to be of a reddish tint with an irregular edge. While the true atmosphere constantly sends flashes of vapour (containing iron, magnesium, etc.) into it from below, it, on the other hand, shoots up into enormous prominences like clouds and flames, of which the cloud-like ones are more permanent. Although these protuberances were first noted only during eclipses, since 1869 they have been observed constantly by means of a special arrangement of the slit of the spectroscope suggested simultaneously by Lockyer and Janssen. The corona, however, has as yet only been studied during eclipses, and its constitution is unsettled, although the existence of rapidly-dispersing clouds of hydrogen above the prominences has led to the belief that the corona, too, may consist of the same gas, highly-rarefied and accompanied by more helium, the illumination being caused by magnetism. The combined spectroscope and photographic appliances have done good work in this branch of astronomy, and Professor Hale has succeeded in photographing faculae, spots, prominences, chromosphere - in fact, all the phenomena of the solar surface.