15. RISING AND SETTING OF THE STARS.--A study of the sky along the lines indicated in these questions will show that there is a considerable part of it surrounding the pole whose stars are visible on every clear night. The same star is sometimes high in the sky, sometimes low, sometimes to the east of the pole and at other times west of it, but is always above the horizon. Such stars are said to be circumpolar. A little farther from the pole each star, when at the lowest point of its circular path, dips for a time below the horizon and is lost to view, and the farther it is away from the pole the longer does it remain invisible, until, in the case of stars 90° away from the pole, we find them hidden below the horizon for twelve hours out of every twenty-four (see Fig. 9). The sun is such a star, and in its rising and setting acts precisely as does every other star at a similar distance from the pole--only, as we shall find later, each star keeps always at (nearly) the same distance from the pole, while the sun in the course of a year changes its distance from the pole very greatly, and thus changes the amount of time it spends above and below the horizon, producing in this way the long days of summer and the short ones of winter. [Illustration: FIG. 9.--Diurnal motion of the northern constellations.] How much time do stars which are more than 90° from the pole spend above the horizon? We say in common speech that the sun rises in the east, but this is strictly true only at the time when it is 90° distant from the pole--i. e., in March and September. At other seasons it rises north or south of east according as its distance from the pole is less or greater than 90°, and the same is true for the stars. 16. THE GEOGRAPHY OF THE SKY.--Find from a map the latitude and longitude of your schoolhouse. Find on the map the place whose latitude is 39° and longitude 77° west of the meridian of Greenwich. Is there any other place in the world which has the same latitude and longitude as your schoolhouse? The places of the stars in the sky are located in exactly the manner which is illustrated by these geographical questions, only different names are used. Instead of latitude the astronomer says _declination_, in place of longitude he says _right ascension_, in place of meridian he says _hour circle_, but he means by these new names the same ideas that the geographer expresses by the old ones.