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THE SUN.

INTRODUCTION.

IT WOULD BE DIFFICULT to form an idea of the length of time during which the phenomena of day and night, and of the varying seasons, transpired without attracting men's attention to the orb which governs both the day and the year. That the science now called Astronomy had its origin in the consideration of the Sun's apparent motions we can scarcely doubt. Clear indications remain, indeed, that the earliest efforts of men to determine the motions of the celestial bodies were directed to the great centre and ruler of the planetary scheme. But when those efforts were first made, what were the first conceptions of astronomers as to the nature of the solar motions, and how, in the process of time, those conceptions assumed the form described in the earliest records of astronomical research, we probably shall never know.

Nor, so far as my purpose in these pages is concerned, is it of any great importance that the truth in

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these matters should be ascertained. It would doubtless form an interesting subject of study to trace the first progress of men as they endeavoured to elucidate the secrets of the heavens. Even what we know of the early researches of astronomers is full of interest, and not wanting in instruction. Their failures as well as their successes teach us a useful lesson of patience and of perseverance. The confidence with which at times they insisted on adopting erroneous theories may serve to teach us a lesson of modesty and caution. And the gradual process by which observation, and thoughtful reasoning upon observation, led men to the successful solution of so many noble problems, is as full of interest to the thoughtful student as the most spiritstirring scenes of history. But where, as in the present instance, it is the object of the writer to exhibit the clearest possible picture of what is, it may be gravely questioned whether it is wise to present the full history of a series of researches which proceeded often on erroneous hypotheses. It is sufficiently difficult to convey in the compass of a single volume clear and accurate conceptions of a wide astronomical subject; and the task is rendered much more difficult where the history of false or imperfect theories is mixed up with the account of recognised truths.

But there is yet another reason for not undertaking to give in this work a history of the progress by which men attained their present conceptions respecting the Sun, nor even attempting the far easier task of showing how, by a series of simple observations, the position of

the Sun in the solar system, and especially with reference to our Earth, might have been ascertained. The fact is, that such an undertaking would differ but little from an attempt to combine a complete history of astronomy-or at the least a complete discussion of all known astronomical relations-with that special discussion of the Sun's nature and condition which forms the essential object of this treatise. For there is not a single chapter of a treatise on general astronomy which is not more or less associated with the relations presented by the Sun; and it may even be said that there is not a single subject dealt with by astronomers which does not owe its chief interest to such an association.

Therefore I find myself compelled to forego that mode of treating my subject which had seemed the best when I was dealing with the planet Saturn. One can, without prolixity, discuss the gradual progress of research by which the relations presented by a single planet have been, or might be, ascertained. But a preliminary research of this sort would require, in the case of the Sun, a volume-and no small one-to itself.

The course I propose, therefore, to adopt is as follows:-I shall pass over all that portion of the history of astronomy which relates to the determination of the Sun's central position in the system he governs. I shall give no account of the methods by which the nature of the Sun's diurnal motions were determined, nor shall I show how from this knowledge, combined with the gradual survey of the terrestrial globe and

the recognition of the Sun's apparent annual progress around the sidereal heavens, the ancients recognised the fact that either the Sun travels yearly around the Earth, and is carried also daily round with the heavens, or else that this Earth on which we live speeds yearly The around the Sun, rotating daily on her axis. observations on the planets by which the true interpretation of these apparent motions of the Sun was eventually obtained must also remain undiscussed.

We are to begin, then, by regarding the Sun as the recognised centre of the solar system, ruler over a scheme of worlds, on which he pours forth abundant supplies of heat and light.

So regarding the Sun, we shall first be led to inquire into the distance of the great luminary, in order that we may determine his real dimensions. The fundamental problem of astronomy-the determination of the Sun's distance-a problem which has at the present time a special interest on account of those approaching transits of Venus from the observation of which astronomers hope to obtain new and better measures-will therefore form the subject of the opening chapter of this work. This chapter will naturally include the consideration of the Sun's dimensions, and of the scale of the solar system generally.

Then, next, the question of the Sun's mass, and of the influences which he exerts by reason of his mass, will come to be considered. We shall measure the might of the giant which rules the whole family of planets, and consider the limits of his domain. We

shall inquire what motions it is in his power to control at this or that distance, and so determine the limits of his power to gather fresh materials from out the surrounding spaces, either in such sort as to recruit his own mass, or as to enlarge the crowd of relatively minute bodies which circle continually, as we know, around him.

Then we shall proceed to discuss what the spectroscope has taught respecting the actual materials which constitute the Sun's substance, placing this inquiry before those chapters which deal with the telescopic aspect of the Sun's surface, the prominences, and the corona, in order that there may be no break in the narrative in passing from the era of unaided telescopic research to the recent era of mixed telescopic, spectroscopic, photographic, and polariscopic observation. In other words, an account of the principles of spectroscopic research, including so much of the history of spectroscopic analysis as is sufficient to make the subject clear (and therefore necessarily including an account of the Sun's general structure as indicated by the solar spectrum), will precede the narrative of that long series of researches which commenced with the discovery of the solar spots. The chapter on spectroscopic analysis will close with an explanation of those special modes of spectroscopic research which have seemed unintelligible, or rather incredible, to many, but yet depend on principles of exceeding simplicity.

Then will follow chapters describing the discoveries which have been made by aid of the telescope-and its

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