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INTRODUCTION.

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In conversing with persons who are not officially attached to Observatories, or in other ways professionally cognizant of the technicalities of practical Astronomy, but who, nevertheless, display great interest in the science of Astronomy generally, I have frequently been struck with two remarks. The first is, that these persons appear to regard the determination of measures, like those of the distance of the Sun and Moon, as mysteries beyond ordinary comprehension, based perhaps, upon principles which it is impossible to present to common minds with the smallest probability that they will be understood; if they accept these measures at all, they adopt them only upon loose personal credit; in any case, the impression which the statement makes on the mind is very different from that created by a record of the distance in miles between two towns, or of the number of acres in a field. The second remark is, that when persons well acquainted with the general facts of Astronomy are introduced into an Observatory, they are, for the most part, utterly unable to understand anything which they see; they are impressed perhaps with the apparent complexity of subsidiary parts of the Astronomical instruments, and they imagine that the fundamental ​principles of their construction are complicated, and too obscure for the understanding of ordinary men; and they leave the Observatory without having derived from it any clear idea whatever.

In both cases, however, the difficulties are very much over-estimated: or rather, difficulties are assumed, which, in reality, do not exist at all. The measure of the Moon's distance involves no principle more abstruse than the measure of the distance of a tree on the opposite bank of a river. The principles of construction of the best Astronomical instruments are as simple and as closely referred to matters of common school-education and familiar experience, as are those of the common globes, the steam-engine, or the turning-lathe; the details are usually less complicated.

In the application of the ordinary principles of geometry and trigonometry to such Astronomical measures as those to which I have alluded, it may sometimes be expedient to resolve the process into several successive steps, and these steps may perhaps require different kinds of treatment. But the remark which I have made applies to every individual step; all are simple and within ordinary comprehension, and the only complexity arises from the circumstance that the student may find it necessary to have a clear view of several such steps at once, in order to perceive the connexion between the first standard of length and the numerical measure last obtained. With these impressions on my mind, I had long wished for some opportunity of endeavouring to explain to intelligent persons the principles on which the instruments of an Observatory are constructed, (omitting all details, so far as they are ​merely subsidiary,) and the principles on which the observations made with these instruments are treated for deduction of the distances and weights of the bodies of the Solar System, and of a few stars, (omitting all minutiæ of formulæ and all troublesome details of calculation). To attempt to go further than this would, in fact, amount to undertaking a complete work on Astronomy, which was far beyond my intentions.

Such an opportunity appeared to present itself in the course of Lectures which I engaged to give to the Members of the Ipswich Museum and their friends.^[1] And the ideas which I have enounced above have been carefully kept in view, in the object and in the details of every Lecture.

I have endeavoured in the first place, to point out how much of the fundamentals of Astronomy may be obtained with the coarsest observation with the unaided eye. And here I should remark, that the science which is thus obtained by personal observations is vastly superior (as far as it goes) to that which is obtained by any other method. The knowledge derived from Lectures is exceedingly imperfect: that derived from careful reading is admirable for its accuracy and fulness, but occupies the mind rather as a train of internal ideas than as a series of consequences deduced from the observations of nature: but that inferred from actual personal observation carries with it a degree of reality and certainty, as the veritable science of external objects, which nothing else can give.

I have endeavoured, in the next place, to show ​that the instrumental conceptions derived from the use of a common globe are sufficient, in almost every case, for the understanding of the instruments in an Observatory; that the elements which are the subjects of observation with Astronomical instruments, are the same as those with which we are most familiar in the ordinary globe-problems; and that a person who understands the latter can proceed at once with the former.

Afterwards, I have endeavoured to explain that the methods used for measuring Astronomical distances are in some applications absolutely the same as the methods of ordinary theodolite-surveying, and are in other applications equivalent to them; and that in fact there is nothing in their principles which will present the smallest difficulty to a person who has attempted the common operation of plotting from angular measures.

The elucidation of the theory of centripetal and disturbing forces is necessarily less complete. Still it appears probable that a general conception of the nature of the action of those forces, perfectly accurate as far as it goes, and sufficient to preserve the student from the gross errors into which many persons have fallen, may be obtained from explanations like those here offered.

The methods of ascertaining the weight of the Earth and other bodies, are still more difficult of explanation; yet it is hoped that something may be done even in these.

The first conclusion, then, which I would desire to impress upon the student of Astronomy, who enters upon the science with a tolerable understanding of ordinary practical matters, is, that nothing is totally beyond his reach. Complete ​knowledge of every theoretical and instrumental detail can only be obtained by those who will devote to them a large portion of their lives; but sound knowledge of the principles of nearly every part, can be obtained by the reasonable efforts of persons possessing common opportunities for general knowledge.

The second conclusion, however, is that, easy of comprehension as are the successive steps of Astronomical investigation, the united succession of all is not to be seized lightly. Let no one think that the problem, for instance, of the measure of the Earth's distance from the Sun is to be mastered by one reading. Again and again must the student return to it; again and again must he doubt and become convinced; again and again must he trace his own diagrams, and express the reasoning in his own language, before all the troubles (I will not call them difficulties) are overcome. And perhaps one of the most valuable results to be derived from a truly intellectual study of Astronomy is, the habit of keeping up a sustained attention to all the successive steps of a long series of reasonings. Power, and with it dignity, are gained to the mind by this noble exercise.

To those who will venture upon the study of the science in this connected way, I can promise an ample and immediate reward. It is not simply that a clear understanding is acquired of the movements of the great bodies which we regard as the system of the world, but it is that we are introduced to a perception of laws governing the motion of all matter, from the finest particle of dust to the largest planet or sun, with a degree of uniformity and constancy, which otherwise we could hardly have ​conceived. Astronomy is pre-eminently the science of order.

The immediate object of my Lectures would be obtained, if they should be found to offer some facilities to those who, acting under the inducements to which I have alluded, may endeavour to obtain a connected and accurate view of the principles of Astronomy. But I should think myself highly rewarded if I could believe that the insight into principles thus obtained, would induce any one to enter carefully into its details.

G. B. AIRY.

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Royal Observatory, Greenwich.

1 ↑ These Lectures were originally delivered at Ipswich, on Monday evening, March 13th, 1848, and the five following evenings.