The most useful constellations for the beginner are those surrounding the north celestial pole, and we give a little circular chart showing their characteristic stars. The names of the months running round the circle indicate the times of the year when these constellations are to be seen on or near the meridian in the north. Turn the chart so that the particular month is at the bottom, and suppose yourself to be facing northward. The hour when the observation is supposed to be made is, in every case, about 9 o'clock in the evening, and the date is about the first of the month. The top of the chart represents the sky a little below the zenith in the north, and the bottom represents the horizon in the north. [Illustration: _Fig. 18. The North Circumpolar Stars._ ] The apparent yearly revolution of the heavens, resulting from the motion of the earth in its orbit, causes the constellations to move westward in a circle round the pole, at the rate of about 30° per month. But the daily rotation of the earth on its axis causes a similar westward motion of the heavens, at the rate of about 30° for every two hours. From this it results that on the same night, after an interval of two hours, you will see the constellations occupying the place that they will have, at the original hour of observation, one month later. Thus, if you observe their positions at 9 P.M. on the first of January, and then turn the chart so as to bring February at the bottom, you will see the constellations around the north pole of the heavens placed as they will be at 11 P.M. on the first of January. [Illustration: _Fig. 19. Key to North Circumpolar Stars._ ] Only the conspicuous stars have been represented in the chart, just enough being included to enable the learner to recognise the constellations by their characteristic star groups, from which they have received their names. The chart extends to a distance of 40° from the pole, so that, for observers situated in the mean latitude of the United States, none of the constellations represented ever descends below the horizon, those that are at the border of the chart just skimming the horizon when they are below the pole.