Page:Astronomy for Everybody.djvu/329

The present chapter is intended for those who wish to be able to recognise the principal constellations, and to know where to look for the several planets. The problem of pointing out the constellations is complicated by the effect of the twofold motion of the earth; on its axis and around the sun. In consequence of the former the constellations change their apparent position in the course of the night, and the result of the latter is that different constellations are seen at different seasons.

We explained in a former chapter how, in consequence of the motion of the earth in its orbit round the sun, the latter seems to us to perform an annual circuit among the constellations. Hence, if a star is east of the sun, we shall see it approach nearer to the sun every day. If we look out night after night at the same hour we shall find it farther and farther advanced toward the west. In consequence of this change it must rise and set earlier every day than it did the day before. More exactly, the time between two risings and settings of the same star is twenty-three hours fifty-six minutes four and a half seconds. While in the course of a year the sun rises three hundred and sixty-five times, a star rises three hundred and sixty-six times. The latter will therefore during the year have risen at every hour of the day and night.