Page:Popular Astronomy - Airy - 1881.djvu/103

Rh Polar Star appears above the north horizon, at its highest and lowest positions, and thus to determine the height of the Pole above the north horizon. It is also used to determine how far the sun is above the south horizon; and from these two measures the sun's angular distance from the Pole is obtained. When we were speaking of the transit instrument, I described its use in this manner: suppose we observe the time when some well-known star passes the meridian, (for instance, the bright star of Aquila, adopted for that purpose by one of my predecessors, Dr. Maskelyne,) and also the time when the sun passes the meridian. The sun passes the meridian so many hours, minutes, and seconds after the star. We bring the place of the star on the celestial globe under the meridian, and then we turn the globe through the corresponding angle; then we know that the sun's place will be somewhere under the meridian. At the time that it passes, it has a certain elevation above the horizon, and therefore a certain distance from the Pole, expressed in degrees and minutes, which is found by the Mural Circle: we take this number of degrees and minutes along the meridian from the Pole; and thus we find the place where the sun was at the time of observation. We make a mark on the globe at that place. We repeat these observations every day of the year; we get measures of the same kind; and we find a series of places such as those shown in Figure 24 When we come to examine all these as laid down together, which may be done roughly on a globe, or more accurately by calculation, we shall find that they are all lying in a great circle. You must understand what is meant by a great circle: it is a circle dividing the sphere into equal parts. Its plane therefore passes through