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THE GREEK ALPHABET. *** The small letters of this alphabet are so frequently employed in Astronomy that a tabular view of them, together with their pronunciation, will be useful to many unacquainted with the Greek language.

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BOOK I.

THE SUN AND PLANETS.

CHAPTER I.

THE SUN. O

“Oye Sun and Moon, bless ye the LORD: praise Him, and magnify

Him for ever."-Benedicite.

Astronomical importance of the Sun.-Solar parallax.The means of determining

it.-By observations of Mars.-By Transits of Venus.--- Numerical data.-Light and Heat of the Sun.-Gravity at the Sun's surface.-Spots.-Description of their appearance.How distributed.-Their duration.-Period of the Sun's Rotation.Effect of the varying position of the Earth with respect to the Sun.-Their size.Instances of large Spots visible to the naked eye.The Great Spot of October 1865.Their periodicity.-Discovered by Schwabe.Table of his results.--Table of Wolf's results.-Curious connexion between the periodicity of sun-spots and that of other physical phenomena.The Diurnal variation of the Magnetic Needle.-Singular occurrence in September 1859.- Wolf's researches. Spots and Terrestrial Temperatures and Weather.- Ballot's inquiry into Ter. restrial Temperatures. The Physical Nature of Spots.- The Wilson-Herschel Theory.-Luminosity of the Sun.-Historical Notices.-Scheiner.-Faculæ. Luculi.- Nasmyth's observations on the character of the Sun's Surface. Huggins's conclusions.-Present state of our knowledge of the Sun's constitution.Tacchini's conclusions.

TF there is one celestial object more than another which may 1 be regarded as occupying the foremost place in the mind of the astronomer, it is the Sun: for, speaking generally, there is scarcely any branch of astronomical inquiry with which, directly or indirectly, the Sun is not in some way associated. It will be only appropriate therefore to deal with this important body at the very commencement of a treatise on Descriptive Astronomy

By common consent, the mean distance of the centre of the Earth from the centre of the Sun is taken as the chief unit of astronomical measurement.

The most approved method of determining the value of this was at one time believed to be by the aid of observations of transits of the planet Venus across the Sun" (as was first pointed out by Halley). The problem is, for various reasons, an intricate one in practice, but when solved places us in possession of the amount of the Sun's equatorial horizontal parallax ; in other words, gives us the angular measure of the Earth's equatorial semi-diameter as seen from the Sun's centre, the Earth being at its mean distance from the Sun. With this element given, it is not difficult to determine, by trigonometry, the Sun's distance, expressed in radii of the Earth ; reducible thereafter to miles.

Encke, of Berlin, executed an able discussion of the observations of the transits of Venus in 1761 and 1769, and deduced 8.571" as the amount of the angle in question. From this it was found that the mean distance of the Earth from the Sun is 24065.1 times the equatorial radius of the former (3963 miles), equal to 95,370,000 miles; but these results, excellent as they were once thought to be, have long ceased to command the acceptance of astronomers, the fact being that modern experience has discredited Halley's method.

At a meeting of the Royal Astronomical Society, on May 8, 1857, Sir G. B. Airy proposed to adopt a suggestion of Flamsteed’sd for determining the absolute dimensions of the solar system, founded upon observations of the displacement of Mars in Right Ascension, when it is far E. of the meridian and far W. of the meridian, as seen from a single observatory; such

A Every one who wishes thoroughly to "get up” the Sun should read Young's Sun. Secchi’s magnificent work Le Soleil, of which a second and much enlarged edition was published in 1875, must not be forgotten.

b See Book II. post.

c Der Venusdurchgang von 1769, p. 108. Gotha, 1824. Followed by later and better results in the Berlin Abhandlungen für 1835, p. 295.

d Baily, Life of Flams'eed, p. 32.

observations to commence a fortnight before and to terminate a fortnight after the Opposition of the planet. In consequence of the great eccentricity of the orbit of Mars, this method is only applicable to those Oppositions during which the planet is nearly at its least possible distance from the Earth. Airy pointed out the several advantages of this method, viz. :—that Mars may then be compared with stars throughout the night; that it has 2 observable limbs, both 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 offers advantages over Venus for observations of displacement in Right Ascension. Airy also entered into some considerations relative to certain of the forthcoming Oppositions, and named those of 1860, 1862, and 1877, as favourable for determining the parallax in the manner he suggestedo.

Le Verrier announced in 1861that 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 Encke's value of 8.571". He fixed 8.95" as its probable value, though, as Stone pointed out, this conclusion taken by itself rests on a not very solid foundation 8.

The importance of a re-determination was thus rendered more and more obvious, and Ellery, of Williamstown, Victoria, 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 were reduced by Stone, and the mean resulth was a value of 8.932" for the solar parallax, with a probable error of only 0-032". This result was singularly in accord with Le Verrier's theoretical deduction. Winnecke's comparison of the Pulkova and Cape observations of Mars yielded 8.964".

Month. Not., vol. xvii., pp. 208–21. vol. iv., p. 101. Paris, 1861. May, 1857. Some practical hints on the & Month. Not., vol. xxvii., p. 241. April conduct of observations are given by A. 1867. Hall in Ast. Nach., vol. lxviii., No. 1623, Month. Not., vol. xxij., p. 185, April Jan. 16, 1867.

1863. Annales de l'Obserratoire Impérial,

The Opposition of 1877 was observed under favourable circumstances by Gill at the Island of Ascension, and his observations yielded as their final result a parallax of 8.78”, with a probable error of o‘012". This implies a mean distance of the Earth from the Sun of 93,080,000 milesi

Thus, though there may be some uncertainty in the amount of the correction, there is no doubt that the Sun is nearer than was formerly considered to be the case.

The distance amended to accord with a parallax of 8.8" is about 92,890,000 miles, with an error not likely much to exceed 150,000 milesk.

Hansen contributed something towards the elucidation of the matter. As far back as 1854 that distinguished mathematician expressed his belief that the received value of the solar parallax was too small, and in 1863 he communicated to Sir G. B. Airy a new evaluation, derived from his Lupar theory by the agency of the co-efficient of the parallactic inequality. The result was 8.9159", a quantity fairly in accord with the other values set forth above?

Such is a brief statement of the circumstances which caused such special interest to attach to the transits of Venus which were to happen on December 8, 1874, and December 6, 1882: for it was supposed, that, all things considered, transits of Venus were most to be relied on for the purpose of ascertaining the amount of the Sun's parallax. The particular circumstances of the transits in question will come under notice hereafter. Meanwhile it may be stated that Stone has deduced 8.823" as the general result of all the British observations of the

| Mem., R. A. S. xlvi., p. I, 1881: Month. Not., vol. xli., p. 323. April 1881.

* C. A. Young in Sid. Mess., vol. vi., p. 11, Jan. 1887.

1 Month. Not., vol .xxiv., p. 8. Nov. 1863. The amount of the correction to Encke's determination is about equal to the apparent breadth of a human hair seen from a distance of 125't, or that of a sovereign at a distance of 8 miles. The whole amount of the parallax has been

put as the measurement of a ball one foot in diameter seen from a station nearly 4:4 miles distant from the ball. Unless the observer can “determine the diameter of the ball so that he shall not be uncertain in his measure to the amount of 0:03 of an inch, his work will not add anything useful to present knowledge." (Sid. Mess., vol. vij., p. 101, March 1888).

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