Popular Astronomy: A Series of Lectures Delivered at Ipswich. George Biddell Airy

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СКАЧАТЬ avoirdupois. Occasionally I shall be prepared to go into details; but my principal business will be to show the great steps upon which those who wish to ​study Astronomy may enter, and by which they may attain a general comprehension of the rules which will lead them from one step to another.

      I shall now proceed with my subject.

      We will consider what are the general phenomena of the motion of the stars which are to be observed on any fine night. I must observe in the first place, that I shall use the term east to denote the whole of the horizon extending from the north point, through the east point, to the south point; the term west to denote the whole of the horizon extending from the south point, through the west point, to the north. Now, if we look out on any fine night, the first general fact that we observe is this—by watching that eastern horizon from time to time, through the whole extent from north to south, we see stars are rising; and by watching that western horizon from time to time, through the whole extent from north to south, we see that stars are setting. By looking out at different times in the course of an evening, you will see these things as I have pointed out. The next general fact which you will observe is this—that the stars do not rise perpendicularly. They rise obliquely; and those which rise near to the south or near to the north rise very slantingly indeed. Those nearest to the east rise at a certain slope, which is different for every different place upon the earth. Those which set near to the north or near to the south set very slopingly; those which set nearest to the west set with a sharp incline. This is the case so far as regards merely the rising and setting of the stars. But if you trace the whole path of any one of these stars, it describes such a course as the following. It rises somewhere in the east, in the sloping direction I have described; it continues to rise with a ​path, becoming more and more horizontal, till it reaches a certain height in the south, where its course is exactly horizontal; and then it declines by similar degrees, and sets at a place in the west, just as far from the north point as the place where it rose in the east, If you select a star that has risen near to the north, it takes a long time in rising to its greatest height, it rises to a higher place in the south, and sets by the same degrees. Lastly, if you look to the north, and give your attention to those stars which are fairly above the horizon, you find the stars going round and describing a complete circle: these stars are called circumpolar.

      Here I would remind my auditors that it is necessary, in order to understand a Lecture upon Astronomy, that they should have a little previous knowledge of the science that they should know the names and situations of some of the more conspicuous stars, otherwise it will be impossible for them to proceed. I therefore assume that a portion of my audience possess this requisite knowledge. I presume you know which is the Polar Star; I presume also that you know which is the Great Bear. Now, these are objects of such importance, that nobody ought to think of entering an astronomical lecture-room who is not acquainted with them. There is another star remarkable for its brilliancy, which is in this country circumpolar, called Capella; and there is another star, which is also nearly circumpolar, it is the bright star in the constellation Lyra.

      Now I will call your attention to each of these in succession, The Polar Star is one which, roughly speaking, does not change its place during the whole night. Whenever you look out you find it in the same place. But speaking a little more accurately, ​it does change its place and move in a small circle. If you examine the stars of the Great Bear, you will observe that they turn in a circle considerably larger than that of the Polar Star, but they are still visible in the whole extent of the circle, and they turn completely round in it, without descending below the horizon. If you examine the next bright star Capella, which you will find on the globe in the constellation Auriga, you will find it describes a circle also, of which the Pole Star is apparently the centre. It goes very near the horizon when lowest in the north, and almost over our heads when highest in the south. If you examine the movement of the last of the stars I have mentioned, namely, the bright star in the constellation Lyra, you will find it moves in such a circle that it as nearly as possible touches the horizon. In the south of England it just descends below the north horizon; here (at Ipswich) it does not, but it passes so near the horizon that it can rarely be seen in the north.

      Thus, if we fix a straight rod in a certain standard direction, pointing nearly, but not exactly, to the Polar Star, we find that the stars which are close in the direction of this rod, as seen by viewing along it, describe a very small circle; the stars further from it describe a larger circle; others just touch the northern horizon; whilst, in regard to others, if they do describe a whole circle at all, part of that circle is below the horizon; they are seen to come up in the east, to pass the south, and to go down in the west, and they are lost below the horizon from that place till they rise again in the east. These are the fundamental phenomena of the stars. It is important that any person, who wishes to understand Astronomy, should look into the matter, and see with his ​own eyes that the. stars really do partake of these motions; that the Polar Star does nearly stand still; that the stars at various distances from the Polar Star, do move round in the way I say, one in a circle of one size, and another in a circle of another size: that others do move round in circles still larger, so that at their lowest points they just touch the north horizon; that others move round in circles so large that the lower part of these circles is lost, whilst the higher part rises above the horizon. 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; the first idea that naturally occurs is, that the starry heavens, as we see them (I do not affix any precise meaning to that term), or a shell in which the stars seem to be fixed, do turn round an axis. It is necessary to show that this is supported by accurate means of observation. Now there is one instrument in use in the best Astronomical Observatories, which is specially intended for the elucidation of this phenomenon—it is the instrument called the Equatoreal. I should be glad if some of the wealthy manufacturers in this town would set up an Equatoreal instrument. The Equatoreal is an instrument, which, in one form, is represented in Figure 1. It turns round an axis ​AB, and the axis is placed in that direction which

      leads to the point of the sky around which the stars ​appear to turn, and which is not far from the Polar Star. The axis being adjusted with great accuracy in that direction, the instrument turns round that axis, and it carries the telescope CD, which, of course, so long as you give it no other motion, retains the same inclination to that axis; but to which you may give another motion, so as to place it in different positions, as C'D' or C"D", directed to stars in different parts of the heavens. The instrument, then, is employed for the purpose of giving evidence as to the motions of the stars. It is used in this manner. The telescope is directed to any one star, and then by turning the instrument round the axis, it is found, that without any alteration in the position of the telescope in relation to the axis, the telescope will follow the star from its rising to its setting. And it is the same wherever the star may be, whether near the Pole, (in which case the telescope is in such a position as C'D', very little inclined to the axis,) or far from the Pole, (in which case the telescope would be much inclined to the axis, as in the position of C"D",) upon turning the instrument round its axis, the telescope still follows the star. This is a fact of accurate observation, for the confirmation of which this kind of astronomical instrument is peculiarly adapted. In this way it is established as a general fact, that all the stars move accurately in circles round one centre.

      But there is another important thing to discover—with what rapidity do the stars turn? Do some travel quicker than others? Do some go quickly in one part, and slowly in another? Now, we have most accurate means of determining whether the speed be irregular or uniform, as regards the speed of any one star in any part of its motion—whether СКАЧАТЬ