Page:Popular Astronomy - Airy - 1881.djvu/37

Rh position before that star, whatever it is, comes under the meridian. If we can write down in figures (for these are the things by which alone we can preserve a satisfactory record)—if we can write down in figures how far the globe has to turn from a certain position, till any one star comes under the meridian of the globe, or under the imaginary meridian which passes over our heads; and if at the same time we can tell how far the star is from this pole, round which the whole of the sphere turns, we can fix the place of the star. These are the two co-ordinates. I pray your attention to these things, which are necessary for determining the position of a star—one, how far the globe must turn before the star is on the meridian; the other, what is the measure of the distance from the pole of the heavens to the star at the time when it does come on the meridian, or, indeed, at any other time, as that distance does not sensibly change in a day.

The thing to which I would first direct your attention is, the way in which we ascertain how far the globe must turn before the star comes into the meridian. Figure 9 represents what is called the Transit Instrument. It is an instrument in perpetual use in every observatory. You see the instrument is not adapted to gaze at all points of the heavens; in ordinary use it can be turned round the axis AB, and has no other motion whatever. Now, what we want to do with this transit instrument is, to supply the place of the brass meridian of a common celestial globe. We cannot put a brass meridian over the heavens, or over our heads; but we want to make a telescope move in such a manner that the line CDE passing along the telescope, and prolonged to the starry heavens, shall exactly describe a curve resembling