The Sun

Some observations recently made by Prof. W. H. Pickering in Jamaica, make the value of sunlight 540,000 times that of moonlight. This makes the sun’s “stellar magnitude” minus 26·83, and that of moonlight minus 12·5. Prof. Pickering finds that the light of the full moon is equal to 100,000 stars of zero magnitude. He finds that the moon’s “albedo” is about 0·0909; or in other words, the moon reflects about one-tenth of the light which falls on it from the sun. He also finds that the light of the full moon is about twelve times the light of the half moon: a curious and rather unexpected result.

M. C. Fabry found that during the total eclipse of the sun on August 30, 1905, the light of the corona at a distance of five minutes of arc from the sun’s limit, and in the vicinity of the sun’s equator, was about 720 candle-power. Comparing this with the intrinsic light of the full moon (2600 candle-power) we have the ratio of 0·28 to 1. He finds that the light of the sun in the zenith, and at its mean distance from the earth, is 100,000 times greater than the light of a “decimal candle” placed at a distance of one metre from the eye.[1] He also finds that sunlight is equal to 60,000 million times the light of Vega. This would make the sun’s “stellar magnitude” minus 26·7, which does not differ much from Prof. Pickering’s result, given above, and is probably not far from the truth.

From experiments made in 1906 at Moscow, Prof. Ceraski found that the light of the sun’s limb is only 31·4 to 38·4 times brighter than the illumination of the earth’s atmosphere very near the limb. This is a very unexpected result; and considering the comparative faintness of the sun’s corona during a total eclipse, it is not surprising that all attempts to photograph it without an eclipse have hitherto failed.[2]

From Paschen’s investigations on the heat of the sun’s surface, he finds a result of 5961° (absolute), “assuming that the sun is a perfectly black body.”[3] Schuster finds that “There is a stratum near the sun’s surface having an average temperature of approximately 5500° C., to which about 0·3 of the sun’s radiation is due. The remaining portion of the radiation has an intensity equal to that due to a black body having a temperature of about 6700° C.” The above results agree fairly well with those found by the late Dr. W. E. Wilson.[4] The assumption of the sun being “a black body” seems a curious paradox; but the simple meaning of the statement is that the sun is assumed to act as a radiator as if it were a perfectly black body heated to the high temperature given above.

According to Prof. Langley, the sun’s photosphere is 5000 times brighter than the molten metal in a “Bessemer convertor.”[5]

Observations of the sun even with small telescopes and protected by dark glasses are very dangerous to the eyesight. Galileo blinded himself in this way; Sir William Herschel lost one of his eyes; and some modern observers have also suffered. The present writer had a narrow escape from permanent injury while observing the transit of Venus, in 1874, in India, the dark screen before the eyepiece of a 3-inch telescope having blistered—that is, partially fused during the observation. Mr. Cooper, Markree Castle, Ireland, in observing the sun, used a “drum” of alum water and dark spectacles, and found this sufficient protection against the glare in using his large refracting telescope of 13·3-inches aperture.Prof. Mitchell, of Columbia University (U.S.A.), finds that lines due to the recently discovered atmospherical gases argon and neon are present in the spectrum of the sun’s chromosphere. The evidence for the existence of krypton and xenon is, however, inconclusive. Prof. Mitchell suggests that these gases may possibly have reached the earth’s atmosphere from the sun. This would agree with the theory advanced by Arrhenius that “ionised particles are constantly being repulsed by the pressure of light, and thus journey from one sun to another.”[6]

Prof. Young in 1870, and Dr. Kreusler in June, 1904, observed the helium line D₃ as a dark line “in the spectrum of the region about a sun-spot.”[7] This famous line, from which helium was originally discovered in the sun, and by which it was long afterwards detected in terrestrial minerals, usually appears as a bright line in the spectrum of the solar chromosphere and “prominences.” It has also been seen dark by Mr. Buss in sun-spot regions.[8]

The discovery of sun-spots was claimed by Hariotte, in 1610, and by Galileo, Fabricius, and Scheiner, in 1611. The latter wrote 800 pages on them, and thought they were small planets revolving round the sun! This idea was also held by TardÈ, who called them Astra Borbonia, and by C. Malapert, who termed them Sydera Austricea. But they seem to have been noticed by the ancients.

Although in modern times there has been no extraordinary development of sun-spots at the epoch of maximum, it is not altogether impossible that in former times these spots may have occasionally increased to such an extent, both in number and size, as to have perceptibly darkened the sun’s light. A more probable explanation of recorded sun-darkenings seems, however, to be the passing of a meteoric or nebulous cloud between the sun and the earth. A remarkable instance of sun-darkening recorded in Europe occurred on May 22, 1870, when the sun’s light was observed to be considerably reduced in a cloudless sky in the west of Ireland, by the late John Birmingham; at Greenwich on the 23rd; and on the same date, but at a later hour, in North-Eastern France—“a progressive manifestation,” Mr. Birmingham says, “that seems to accord well with the hypothesis of moving nebulous matter.” A similar phenomenon was observed in New England (U.S.A.), on September 6, 1881.

One of the largest spots ever seen on the sun was observed in June, 1843. It remained visible for seven or eight days. According to Schwabe—the discoverer of the sun-spot period—its diameter was 74,000 miles, so that its area was many times that of the earth’s surface. The most curious thing about this spot was that it appeared near a minimum of the sun-spot cycle! and was therefore rather an anomalous phenomenon. It was suggested by the late Daniel Kirkwood that this great spot was caused by the fall of meteoric matter into the sun; and that it had possibly some connection with the great comet of 1843, which approached the sun nearer than any other recorded comet, its distance from the sun at perihelion being about 65,000 miles, or less than one-third of the moon’s distance from the earth. This near approach of the comet to the sun occurred about three months before the appearance of the great sun-spot; and it seems probable that the spot was caused by the downfall of a large meteorite travelling in the wake of the comet.[9] The connection between comets and meteors is well known.

The so-called blackness of sun-spots is merely relative. They are really very bright. The most brilliant light which can be produced artificially looks like a black spot when projected on the sun’s disc.

According to Sir Robert Ball a pound of coal striking a body with a velocity of five miles a second would develop as much heat as it would produce by its combustion. A body falling into the sun from infinity would have a velocity of 450 miles a second when it reached the sun’s surface. Now as the momentum varies as the square of the velocity we have a pound of coal developing 90² (= ⁴⁵⁰/5)², or 8,100 times as much heat as would be produced by its combustion. If the sun were formed of coal it would be consumed in about 3000 years. Hence it follows that the contraction of the sun’s substance from infinity would produce a supply of heat for 3000 × 8100, or 24,300,000 years.

The late Mr. Proctor and Prof. Young believed “that the contraction theory of the sun’s heat is the true and only available theory.” The theory is, of course, a sound one; but it may now be supplemented by supposing the sun to contain a certain small amount of radium. This would bring physics and geology into harmony. Proctor thought the “sun’s real globe is very much smaller than the globe we see. In other words the process of contraction has gone on further than, judging from the sun’s apparent size, we should suppose it to have done, and therefore represents more sun work” done in past ages.

With reference to the suggestion, recently made, that a portion, at least, of the sun’s heat may be due to radium, and the experiments which have been made with negative results, Mr. R. T. Strutt—the eminent physicist—has made some calculations on the subject and says, “even if all the sun’s heat were due to radium, there does not appear to be the smallest possibility that the Becquerel radiation from it could ever be detected at the earth’s surface.”[10]

The eminent Swedish physicist Arrhenius, while admitting that a large proportion of the sun’s heat is due to contraction, considers that it is probably the chemical processes going on in the sun, and not the contraction which constitute the chief source of the solar heat.[11]

As the centre of gravity of the sun and Jupiter lies at a distance of about 460,000 miles from the sun’s centre, and the sun’s radius is only 433,000 miles, it follows that the centre of gravity of the sun and planet is about 27,000 miles outside the sun’s surface. The attractions of the other planets perpetually change the position of the centre of gravity of the solar system; but in some books on astronomy it is erroneously stated that the centre of gravity of the system is always within the sun’s surface. If all the planets lay on the same side of the sun at the same time (as might possibly happen), then the centre of gravity of the whole system would lie considerably more than 27,000 miles outside the sun’s surface.

With reference to the sun’s great size, Carl Snyder has well said, “It was as if in Vulcan’s smithy the gods had moulded one giant ball, and the planets were but bits and small shot which had spattered off as the glowing ingot was cast and set in space. Little man on a little part of a little earth—a minor planet, a million of which might be tumbled into the shell of the central sun—was growing very small; his wars, the convulsions of a state, were losing consequence. Human endeavour, human ambitions could now scarce possess the significance they had when men could regard the earth as the central fact of the universe.”[12]

With reference to the late Prof. C. A. Young (U.S.A.)—a great authority on the sun—an American writer has written the following lines:—

“The destined course of whirling worlds to trace,
To plot the highways of the universe,
And hear the morning stars their song rehearse,
And find the wandering comet in his place;
This is the triumph written in his face,
And in the gleaming eye that read the sun
Like open book, and from the spectrum won
The secrets of immeasurable space.”[13]