THE SUN BOOK I. SUN AND PLANETS. CHAPTER I. THE SUN. "O ye Sun and Moon, bless ye the LORD: praise Him, and magnify Astronomical importance of the Sun.—Solar parallax.-The means of determining it.— Numerical data.-Light and Heat of the Sun.-Spots.-Description of their appearance.-How distributed.-Their duration.-Effect of the varying position of the Earth with respect to the Sun.-Their size.-Instances of large Spots visible to the naked eye.-Their periodicity.- Discovered by Schwabe.-Table of his results.-Curious connexion between their periodicity and that of other physical phenomena.—Singular occurrence in September 1859.-Wolf's researches.—Spots and Terrestrial Temperatures.-Their Physical Nature.-The Wilson-Herschel Theory-Historical Notices.-Scheiner.-Faculœ.-Luculi.- Nasmyth's observations on the character of the Sun's Surface.-Huggins's ditto.-Ballot's inquiry into Terrestrial Temperatures. I as F there is one material object more than another which may be regarded as occupying the foremost place in the mind of the astronomer, it is the Sun. Speaking generally, there is scarcely any branch of astronomical inquiry with which, directly or indirectly, the Sun is not associated in some way or other. Even in sidereal astronomy, with which a connexion would at first sight scarcely seem possible, it comes before us (as we shall in due course see) in reference to the supposed proper motion of the solar system through space. Under these circumstances, and bearing in mind the position it holds in the universe, it will not appear unreasonable if we devote to it the very foremost place. B By common consent, the mean distance of the Earth from the Sun has been accepted as the usual unit of astronomical measurement. The most approved method of determining the value of this is (as was first pointed out by Halley) by the aid of observations on transits of the planet Venus across the Sun (to be dealt with generally hereafter). The problem is an intricate one for various reasons, and an examination of it does not fairly come within the scope of this work: suffice it then to say, that the observations above alluded to place us in possession of the amount of the Sun's equatorial horizontal parallax; in other words, give us the angular measure of the Earth's equatorial semi-diameter as seen from the Sun, the Earth being at its mean distance from that luminary. With this element given, it is not difficult to determine, by simple trigonometry, the Sun's distance, expressed in radii of the Earth, reducible thereafter to miles. Professor Encke, of Berlin, executed an able discussion of the observations of the transit of Venus in 1769, and deduced 8.5776′′ as the amount of the angle in questiona. From this we find the mean distance of the Earth from the Sun to be 24046-9 times the equatorial radius of the former (3962.8 miles), equal to 95,293,055 British statute miles. We shall soon see that this has ceased to be a definitive value. At a meeting of the Royal Astronomical Society, held on May 8, 1857, the Astronomer Royal proposed a method for determining the absolute dimensions of the solar system, founded upon observations of the displacement of Mars in right ascension, when it is far east of the meridian and far west of the meridian, as seen at a single observatory; such observations to be made a fortnight before and a fortnight after the opposition of the planet. In consequence of the great eccentricity of the orbit of Mars, the method is only applicable to those oppositions during which the planet is nearly at its least possible distance from the Earth. Mr. Airy pointed out the advantages of this method in the various respects that Mars may then be compared with stars throughout the night; that it has two observable limbs, both a Der Venusdurchgang von 1769, p. 108. Gotha, 1824. admitting of good observation; that it remains long in proximity to the Earth; and that the nearer it is, the more extended are the hours of observation, in all of which matters Mars is a better object upon which to found deductions than Venus likewise observed for right ascensional displacement. He also entered into some considerations relative to certain of the forthcoming oppositions, and named those of 1860, 1862, and 1877, as favourable for determining parallax in the manner he suggested". The general advantage of having observations made on this plan was explained to be, that transits of Venus would not occur till 1874 and 1882, and that, apart from the inconvenience of holding so long in suspense our accepted determination of the Sun's distance, there were terrestrial difficulties which might interfere with observations when the time actually did arrive. Another astronomer now appears on the scene. M. Le Verrier announced in 1861c that he could only reconcile discrepancies in the theories of Venus, the Earth, and Mars, by assuming the value of the solar parallax to be much greater than the usually received value of 8.5776". He fixed 8.95" as its probable value. The importance of a re-determination was thus rendered more and more obvious, and Ellery, of Williamstown, Victoria, New South Wales, succeeded in obtaining a fine series of meridian observations of Mars, at its opposition in the autumn of 1862, whilst a corresponding series was made at the Royal Observatory, Greenwich. These have been reduced by Stone, of Greenwich, and the mean result is a value of 8.932" for the solar parallax, with a probable error of only o'032", supposing the probable error of a single observation to be 0.25". This result is singularly of accord with Le Verrier's theoretical deduction. Winnecke's comparison of the Pulkova and Cape observations of Mars yields 8-964". Though there may be some uncertainty in the amount of the correction, there is no doubt that the Sun is nearer than has hitherto been considered to be the case. The distance amended to accord with a parallax of 8.94" is about 91,430,000 miles. Hansen has b Month. Not. R. A.S., vol. xvii. pp. 208-21. contributed something towards the elucidation of the matter, which must not be passed over. As far back as 1854 this distinguished mathematician expressed his belief that the received value of the solar parallax was too small, and in 1863 he communicated to the Astronomer Royal a new evaluation, derived from his Lunar theory by the agency of the co-efficient of the parallactic inequality. The result is 8.9159", a quantity fairly of accord with the other values set forth above. The acceptance of a new value for the solar parallax necessitates the recomputation of all numerical quantities involving the Sun's distance as a unit. Having ascertained the true mean distance of the Earth from the Sun, it is not difficult to determine by trigonometry the true diameter of the latter body, its apparent diameter being known from observation; and, as the most reliable results shew that the Sun at mean distance subtends an angle of 32′ 34′′, it follows that the true diameter is 852,584 miles. It is generally accepted that there is no compression. The surface of this enormous globe therefore exceeds that of the Earth 11,574 times, and the volume 1,245,130 times; since the surfaces of spheres are to each other as the squares of the diameters, and the volumes as the cubes. The lineal value of 1" of arc at the mean distance of the Sun is 448 miles. The Sun's mass, or attractive power, exceeds that of the Earth 314,760 times, and (approximately) is 674 times the masses of all the planets put together. By comparing the volumes of the Sun and the Earth and bringing in the value of the masses, we obtain the relative specific gravity or density of the two. The Sun's volume exceeds the Earth's by 1,245,130 to 1; the Sun's mass exceeds the Earth's in the lesser ratio of 314,760 to 1. Therefore the density of the Sun is to the density of the Earth as 314,760 to 1,245,130, or as 1 to 4. Then taking Baily's value of the density of the Earth (567 times that of water), the density of the Sun is 143 times that of water. e Month. Not. R.A.S., vol. xxiv. p. 8. The amount of the correction is about equal to the apparent breadth of a human hair seen from a distance of 125 ft. This consideration of the comparative lightness of matter composing the Sun has led Sir J. Herschel to think that it is "highly probable that an intense heat prevails in its interior, by which its elasticity is reinforced, and rendered capable of resisting [the] almost inconceivable pressure [due to its intrinsic gravitation] without collapsing into smaller dimensions"." We thus see that the Sun is eminently worthy of the important position it holds as the centre of our system, and thus early it will be appropriate to mention that the Sun is to be regarded as a fixed body so far as concerns ourselves; therefore when we say that the Sunrises," or the Sun "sets," or the Sun moves through the signs of the zodiac once a year, we are stating a conventional untruth; it is we that move and not the Sun, the apparent motion of the latter being an optical illusion. The Sun is a sphere, and is surrounded by an extensive and rare atmosphere, and is self-luminous, emitting light and heat which are transmitted certainly beyond the planet Neptune, and therefore more than 2700 millions of miles. Of the Sun's heat, it has been calculated that only 381000000 part reaches us, so what the whole amount of it must be passes human comprehension, like many other things in science. Our annual share would be sufficient to melt a layer of ice all over the Earth 38 yards in thickness, according to Pouillet h. Another similar calculation determines the direct light of the Sun to be equal to that afforded by 5563 wax candles of moderate size, supposed to be placed at a distance of one foot from the observer. The light of the Moon being probably equal to that of only one candle at a distance of 12 feet, it follows that the light of the Sun exceeds that of the Moon 801072 times, according to Wollaston. Zöllner's ratio is 618,000. When telescopically examined, the equatorial zones of the Sun are frequently found to be marked with dark spots or maculai, each Outlines of Ast., p. 297. Ganot, Physics, p. 331. This was calculated on the old value of the solar parallax. I have not altered it. h Cited by Ganot, as above. To shew the great power of the calorific rays of the sun, I may mention that in constructing the Plymouth Break-water, the men, working in diving bells, at a distance of 30 ft. below the surface, had their clothes burnt by coming under the focus of the convex lenses placed in the bell to let in the light. i Lat. macula, a blemish. Mr. W. R. Dawes upholds a further classification : he applies to the ordinary black central portions the term umbra (shadow), on the highly probable ground that the |