We have already remarked that the rotation of the earth on its axis causes all the other heavenly bodies to _appear_ to revolve round it once every twenty-four hours, and we must now add that the earth's revolution round the sun causes the same bodies to _appear_ to make another, slower revolution round it once every year. This introduces a complication of apparent motions which it is the business of astronomy to deal with, and which we shall endeavour to explain.