Popular Astronomy: A Series of Lectures Delivered at Ipswich

LECTURE II.

IN the last lecture, I endeavoured to point out to you the principal phenomena of the motions of the stars, as observed on any fine night. 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. The first of these is the Polar Star, which everybody ought to know: the second is the constellation of the Great Bear, which most people know as Charles' Wain; the third is the bright star Capella; the fourth is the bright star of Lyra. I described their motions: and I then pointed out to you that the observations were rendered more accurate by means of the instrument named the Equatoreal, which makes a telescope turn on an axis parallel to ?the direction of the axis round which the stars appear to turn; and that we find, by fixing the telescope to the axis in such a position that it is directed to any one star, and then, by continuing to turn the instrument upon its axis, the telescope will follow the star from its rising to its setting. This I mentioned as establishing an important point, that the stars undoubtedly do appear to revolve round that axis. I then described the use of the clock-work for causing the Equatoreal instrument to revolve uniformly. And I pointed out to you, as a thing of importance, that when the clock-work is in action to whatever star we may direct the telescope, however far that star may be from the Pole, or however near it may be to the Pole, the telescope does continue to revolve after it, so that the star is always kept in sight, or in the field of view. Inasmuch, therefore, as all the stars appear to revolve uniformly round one axis, it follows that the stars keep their relative places or positions, that is to say, the heavens turn as it were all of a piece. Of course there is no explanation of that, except one of these two-either that the heavens are solid and go all of a piece; or that the heavens may be assumed to be fixed or immovable, and that we and the earth are turning instead of them.

I then particularly mentioned that, taking advantage of this circumstance, instruments are contrived for daily use in every Observatory in the world, as adapted to defining the places of celestial objects. In the first place I directed your attention to the transit instrument, as one of the most important instruments used in taking our observations. This is mounted like a cannon, turning upon two pivots, and possessing no other motion; these pivots resting ?on stone piers, if the instrument is of a large size, or upon metal piers, if the instrument is of a smaller size; the telescope so adjusted, and turning in this manner, moves only in the meridian. And here it is important to remark that in all standard observations in Astronomy, the instrument is not turned to stars in any part of the heavens, but we have to wait until the stars come upon the meridian. We must so manage as not to be too late, or we lose our observation. The transit instrument must be adjusted in reference to our notion of what we want to observe. The object of all this is to define the places of the stars, in relation one to another; the places of the planets, the sun, the comets, the moon, in relation to the stars, and so on: in fact the use of all observing instruments of this class is to define the place of one object in relation to some one or other fixed objects. I then endeavoured to explain that for exact definition of the place of an object, it is necessary to use a system of what, in mathematics, are called co-ordinates; and that, when the object is or appears to be upon a surface, two co-ordinates are necessary. By this term, I mean two measures of some kind or other; as distances from two fixed lines, or distances from two fixed points, or length of the line from one point and inclination of that line to the horizon. Thus, for determining the position of the stars in reference one to another, it is a matter of importance to choose the most convenient co-ordinates. Considering the stars as they are represented on the celestial globe, if we wish to define the place of any star, the most convenient co-ordinates we can use are these: in the first place, to see how far the globe must have turned from a certain position before the star passes under the brass meridian; and in the next place to see ?when that star passes under that meridian, how far it is from the Pole round which the globe turns.

I then pointed out that the transit instrument is one of the instruments particularly adapted to this purpose. The transit instrument does by its motion on the axis I described, trace on the sky a curve exactly similar to the brass meridian of the globe, provided these conditions be observed: first, the axis must be horizontal; secondly, the telescope must be square to its axis; and thirdly, when the telescope is turned to the north, it must in its sweep pass over the centre of rotation of the stars. All this I fully explained, but I give this recapitulation that it may be kept in recollection as we proceed with our lectures. By means then of this transit instrument, the condition of representing the brazen meridian by an imaginary track of the telescope through the heavens is fulfilled. I then mentioned that we make use of a clock in all observations; that the way of using it is, having noted the time when some star or object passes the meridian, to find by the clock the interval of time until other stars or planets pass the meridian.

I may now add one subject which I omitted, and it is to state what we mean by a Sidereal Day. We observe on this day a bright star, for instance Arcturus, passing the meridian. We note the time by our clock, in hours, minutes, and seconds, and the fraction of a second. To-morrow we again observe the passage of Arcturus across the meridian. The interval between these passages is a sidereal day. A sidereal day is not quite the same as a common day. But I do not insist on that at present, because it is connected with other things, one of which is the motion of the sun. It is important to understand ?that that is what we mean by a sidereal day. I cannot tell you now what sidereal time is, and for this plain reason: I have not yet got the starting-point which marks the beginning of the sidereal day. All that I can at present say is, that the interval from the time of the passage of one star one day, to the time of the passage of the same star next day, is understood to be twenty-four hours of sidereal time.

Having proceeded so far in relation to the times of the passing of the stars, and the quantity of rotation which the globe must perform from the meridianal passage of a fixed star which we know, to that of a planet or similar object whose position we want to determine, I mentioned the use of the Mural Circle, by which we determine the altitude of the object when it is passing the meridian. And here I must observe, that one of the most important adjustments of the Mural Circle depends on reflection from the surface of quicksilver. It is not my province now to allude to optics as a science; I merely allude to it to indicate a thing important to our present purpose: the law of reflection of light from a surface of quicksilver. The surface of the quicksilver takes a position parallel to the horizon, with a degree of suddenness and certainty to which we know nothing similar. Light is reflected from the surface of the quicksilver, just as it would be from a looking glass. Now, the thing which I wished to point out as the great practical fact is this: that supposing SG and S´O, in Figure 13, to represent the direction of the light coming from the star, and OG´F´ the direction of the light reflected from the quicksilver; then the inclination of S´O or SG to the horizon is the same as the inclination of OG´ to the horizon; and if S´O or SG approach nearly to a flat with the horizon, OG´ will ?approach nearly to a flat with the horizon; and if S´O or SG approach nearly to a perpendicular to the horizon, OG´ will approach nearly to a perpendicular to the horizon. These are the facts upon which the use of observations by reflection is founded. If we place this small trough in such a position that the telescope looks into it, and if we see a star, we know that the light which comes from that star is reflected in such a manner, that the position of that telescope must be as much inclined downwards, as the position of the direct ray of light from the star is inclined upwards. From these observations we infer the position of the telescope when it is horizontal I then pointed out to you that by the use of this, we ascertain the elevation of the Polar Star at its highest and its lowest positions, and that by taking the mean of these we have the height of the Pole; and therefore, getting the elevation of the Pole on one side of the Zenith, and getting the elevation of any other star...