Page:Popular Astronomy - Airy - 1881.djvu/36

22 measure the distance AC horizontally from one end of the wall, and CD vertically from the floor. That would define it accurately, and I could write down the measures in figures, so that a person at any distance could make a speck in a position exactly similar on another wall. I might do it in other ways. I might measure the distance AD from the corner A, and the distance ED from the corner E, and describing circles with these sweeps in my compasses from each corner in succession, I should be able to find exactly the position of that speck of dirt. I might do it in another way, too. I might say, if I go from the corner A to that speck of dirt D, the distance is so many feet, and the inclination of the line AD to the horizon is such an inclination as I can represent. That would do. But, in whatever way I do it, I must take two measures; there is no way in which it is possible, in the nature of things, that the position of that speck of dirt on the wall, or the position of a star in the sky, can be represented, except by two elements.

Now the question presents itself. What are the two elements most convenient for representing the position of a star in the heavens? There are two elements which, ever since accurate astronomical observations began, have been fixed on by all astronomers as the most advantageous. One is thus described: supposing we can fix on the imaginary pole or place of rotation of the stars, then one element is the distance of the star, as measured from that pole in degrees. I will speak in a short time of what is really meant by a degree. The other is, supposing the celestial globe, or the sphere of the heavens, to turn round an axis, as we have shown it does; then the question is, how far has it to turn from a certain