Page:Popular Astronomy - Airy - 1881.djvu/49

Rh the stars, or the planets, in reference one to another. What we want to know is, the interval of the successive times at which they pass the meridian. Assuming that the starry heavens turn uniformly, this interval (which in the instance above we have supposed to be one hour), enables us, if we wish, for instance, to register the planet's place on a globe, to turn the globe one hour, or one twenty-fourth part of a revolution, from the position in which the principal star was under the meridian, and then we know that the planet which we have observed, will be somewhere under the meridian, in that new position of the globe. That is the result of the observations with the transit instrument.

The next thing is, by means of the observation of the Polar Star with the Mural Circle, and by determining how high any other object appears when it passes the meridian, to determine the angular distance of any object from the Pole. These two observations amount to this:—the first gives the angular distance of the Pole from the north horizon. It is, however, rather more convenient to refer the position of the Pole to the point which is exactly upwards usually called the Zenith. The change is very easily made; for as the angular distance from the Zenith to the horizon is ninety degrees, we have only to subtract the elevation of the Pole (or of any other object) from ninety degrees, in order to obtain its zenithal D2