George Biddell Airy

“ Now, the statement that the sun appears to move in a great circle on the globe amounts to this: that the sun appears to move in a plane which passes through the earth. For all globes represent the stars as they would appear if the observer were at the centre of the globe; and a great circle is one whose plane passes through the centre of the globe. If we found that the sun was describing one of the smaller circles on the globe, then we should say the sun was not moving in a plane round the earth. ”

“ 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. ”

“ Now, remember how it has been explained that the curvature of a planet's orbit, or the curvature of the path of a cannon ball, depends upon two circumstances; one is the velocity with which it is going, and the other is the force which acts in such a direction as to bend its path. The greater its speed, the less its path is curved; consequently, as the planet is going so exceedingly quick in the neighbourhood of K, its orbit may be very little curved there, even though the sun is there pulling it with a very great force. ”

“ It has been wondered by some persons that, when the planets approach to the sun, they are not compelled by its attractive force to fall into the sun; and when they go away from the sun, it has been wondered that they do not go quite away from the sun's influence. I endeavoured to give you a notion of the mechanical causes which produced that alteration in the velocity of the planets, which is in fact embodied in that law of Kepler's, which states that a planet describes, by the lines connecting it with the sun, equal areas in equal times. ”

“ It is indeed the great and fundamental principle of Astronomy, that the stars do move as if they were attached to a shell, or in other words, that they move all in a piece. As to the explanation of that, I shall not trouble you at present. I simply call attention to the fact, that the stars move all in a piece—either that they are connected with some one thing turning upon an axis, or that they stand still while the earth turns round an axis of its own ”

“ We must suppose that the planet has been put in motion; we cannot tell how the planets have been put in motion, but they are in motion; that is sufficient for our purpose. The planets, if there were nothing to pull them aside, would go on in a straight course, without altering their velocity. The supposition which Newton made, and on which is founded the theory of gravitation, and which is perfectly conformable with every result of observation, is, that all the planets are attracted towards the sun; that the force is different in different parts, but still always directed towards the sun ”

“ Therefore, we must make the following supposition: that if the earth does turn round the sun, its axis of rotation remains parallel to itself, having no reference whatever to the sun, whilst the earth is going round the sun; or, to express it in other words—that the earth has the power of preserving its axis of rotation in the same position all the time. Now, it is remarkable, as a mechanical fact, that nothing is so permanent in nature as the axis of rotation of anything which is rapidly whirled. We have examples of this in every-day practice. The first is the motion of a boy's hoop. ”

“ Now, the thing I wish you to understand is this: suppose Venus and the sun are at equal distances from the earth, then the earth pulls the sun out of its way just as much as it pulls Venus out of the way. The enormous difference of magnitude of the attracted bodies makes no difference in the movement which the action of the attracting body produces on them. If there are two bodies a great one and a little one, and if something else attracts them, the great body is pulled through as many feet or miles in an hour as the little one. ”

“ 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. ”

“ But there is this difference: we can see the moon, and we can see small stars close to the moon, and at the time that we observe the moon and the stars the air is not disturbed by the heat of the sun; everything is steady and is seen well; but in the heat of the day, objects are unsteady and are never seen well: we cannot see a faint star at all; and we can only see bright stars at a distance from the sun; therefore we are cut off entirely from observing the parallax of the sun in that way. ”

“ We say, therefore, that the earth's attraction would draw a planet through the same space as the moon, provided the planet were at the moon's distance; and for the greater distance of the planet, we must, on the law of gravitation, diminish that space in the inverse proportion of the square of the distance. Now I have already shown you how to compute the space through which the sun draws a planet in one hour ”

“ And I called your attention to the fact, that these motions are performed in such a way, as to give us the idea of rotation round an axis inclined to the horizon; that some of the stars move very little; that others describe larger circles; that others just touch the horizon and descend below it; that others descend on one side and rise on the other side. I mentioned the names of two or three stars admitting of easy observation, as I am desirous that you should observe a little for yourselves, because you will acquire more knowledge from personal observation than from my lectures. ”

“ It seems perfectly absurd at the present time that anybody should start with a proposition like that to work out a physical theory. Yet it is equally absurd to assume at once that the earth is in motion, and for that reason I have, been anxious to convey to you the evidence by which it is proved generally that the earth is in motion. ”

“ We apply the same laws which relate to water or glass, to the computation of refraction by atmospheric air. We find that air will alter the course of the light in such a manner, that the beams of light enter our eye more vertically than they otherwise would do; that all objects will necessarily appear to be higher than they are in reality; and applying then a proper correction by the law of refraction, based upon experiments with water or glass, we find everything properly adjusted, and that the stars revolve round one axis most accurately in the manner we have represented. ”

“ Just before the star begins to pass, he looks to the clock face for the hours and minutes, and he then listens to the clock, which beats seconds—in that manner he gets the hour, the minute, the second, and the fraction of a second, at which that bright star passes every wire, and by taking the mean or average of these, he finds the time at which the star passes the meridian. He looks again, and he sees a planet coming into the field of view. ”

“ It is of importance that anybody, who wishes to understand Astronomy thoroughly, should look out, and see for himself, that these things do happen in the way I have attempted to describe; by the observations so made, he will acquire a conviction of the truth far deeper and more lasting than from anything that can be pointed out in a course of Lectures.
From observing the way in which these motions take place, that we may assume one point of the sky as a centre, and that the movements of the stars are of such a nature that they will appear to turn round that one centre ”

“ You will remark that all these conclusions are derived from observations of the stars; that observations of the Polar Star are used to determine the Pole round which the heavens appear to revolve; or on the more rational supposition, to determine the point of the heavens towards which the axis of the rotation of the earth is supposed to be directed; summer or winter the Polar Star is the Polar Star still, and guides our observations; summer or winter the axis of the earth is directed towards that same part of the heavens where the Polar Star is seen. ”

“ In all other cases we were able to plant Observatories at different parts of the earth, and from these different parts of the earth to make observations at the same instant on the subject in question, whether it was the distance of the sun, or of the moon, or of Mars, that was to be measured. But we cannot do so with respect to the stars, and for this reason: the mere observation of the stars from two points of the earth does not present any sensible difference whatever. ”

“ The greatest mathematician of the last century, the celebrated Euler, has written a whole "book on the motion of a top, and his Latin Treatise "De motu Turbinis" is one of the most remarkable books on mechanics, I ever read in my life. The motion of a top, I repeat, is a matter of the greatest importance; it is applicable to the elucidation of some of the greatest phenomena of nature. In all these instances there is this wonderful tendency in rotation to preserve the axis of rotation unaltered. ”

“ I shall explain to you the evidence which is within your own reach, and which proves that the sun apparently moves through the stars. I have not yet specially alluded to the sun in speaking of revolution; my remarks on that point referred only to the stars. I am now, however, going to speak of the sun a little, though it is necessary to have the stars to begin with, as fixed points in the heavens.
Everybody knows the leading difference between summer and winter; we know that the days are longer in the summer than in the winter. ”

“ For the perturbations produced by Mercury, Venus, and Mars, which have no satellites, we have no similar foundation for our computations; and here we can only go on a method of trial and error. For instance, assuming for calculation that one of these planets has as great a disturbing power as the earth, we can compute how much it will disturb another planet's motion in every position; and if the disturbing power be altered in any proportion, we know that the disturbance of the other planets motion will be altered in that proportion. ”

“ It is obvious that we cannot put a body in motion so that it shall go on in one unvaried direction, and that it shall go on for ever, for we cannot put it in motion in a place where no force will act on it ”

“ The moon, therefore, is not a small body, and the sun (which in spite of its great distance appears so large) must be a very large body. The stars are either connected in one system, or are so very far off that their relative movements are insensible.
Now, is it more likely that this large frame of things is turning one way, or that this small earth is turning the other way? Anybody must see at once, from the magnitude of things, that it is most probable the earth is turning round. ”

“ 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. ”