That we may better appreciate the earliest observations on astronomy, it may be well to mention briefly what are the varying phenomena which may most easily be noticed. If we except the phases of the moon, which almost without observation would force their recognition on people who had no other than lunar light by night, and which must therefore, from the earliest periods of human history have divided time into lunar months; there are three different sets of phenomena which depend on the arrangement of our planetary system, and which were early observed. The first of these depends upon the earth's rotation on its axis, the result of which is that the stars appear to revolve with a uniform motion from east to west; the velocity increasing with the distance from the pole star, which remains nearly fixed. This circumstance is almost as easy of observation as the phases of the moon, and was used from the earliest ages to mark the passage of time during the night. The next arises from the motion of the earth in her orbit about the sun, by which it happens that the earth is in a different position with respect to the sun every night, and, therefore, a different set of stars are seen in his neighbourhood; these are setting with him, and therefore also a different set are just rising at sunset every evening. These changes, which would go through the cycle in a year, are, of course, less obvious, but of great importance as marking the approach of the various seasons during ages in which the hour of the sun's rising could not be noted by a clock. The last depends on the proper motions of the moon and planets about the earth and sun respectively, by reason of which those heavenly bodies occupy varying positions among the stars. Only a careful and continuous scrutiny of the heavens would detect these changes, except, perhaps, in the case of the moon, and but little of importance really depends on them; nevertheless, they were very early the subject of observation, as imagination lent them a false value, and in some cases because their connection with eclipses was perceived. The practical cultivation of astronomy amongst the earliest people had always reference to one or other of these three sets of appearances, and the various terms and signs that were invented were intended for the clearer exposition of the results of their observations on these points. In looking therefore into extreme antiquity we shall find in many instances our only guide to what their knowledge was is the way in which they expressed these results.