Page:Popular Astronomy - Airy - 1881.djvu/39

the telescope CD should be square to its axis AB. The astronomer ascertains whether it be truly square or not, by looking at a distant mark, first with the pivots A,B, of the instrument resting on the piers a,b, and then with the axis turned over, so that the pivots A,B, rest on the piers b,a. If the telescope points equally well to the mark in both positions of the axis, the telescope is truly square to its axis. The third condition is, that when the axis is level, and the telescope is square to its axis—on turning the instrument round its axis, the line CDE shall pass through the pole of motion of the celestial sphere. Now, the way of obtaining this condition is as follows:—We take advantage of that admirable Polar Star, which is a blessing to astronomers of the northern hemisphere. The Polar Star, as I have said, turns round like the rest, although in a small circle. Let FGHIKL represent the circle in the sky in which the Polar Star turns round in the order FGHIKLF. Suppose that in turning the transit instrument round its axis AB, the line CDE prolonged will trace on the sky the line GI or FK, as the case may be. The Polar Star in its revolution passes that line twice. Now, what we want is, that that line CDE, carried on to the sky, should be so directed, that in the motion of the telescope it should pass exactly through the centre of the circle which the Polar Star describes, and therefore that it should divide into two equal halves the circle which the Polar Star describes. We ascertain it in this manner. We can measure the description of the parts of the circle of the Polar Star by time. One of the most important parts of the apparatus by which that astronomical observation is made, is a clock. The clock should go well, and should beat loudly and distinctly. The astronomer