The Book of Zoroaster, in which some of the earliest notices of astronomy are recorded, states that the length of a summer day is twice as long as that of winter. This fixes the latitude in which that book must have been composed, and makes it 49 deg.. Whence it follows, that to such a place must we look for the origin of these spheres, and not to Egypt or Chaldea. [Illustration: PLATE III.--CHALDEAN ASTRONOMERS.] Diodorus Siculus speaks of a nation in that part of the world, whom he calls Hyperboreans, who had a tradition that their country is the nearest to the moon, on which they discovered mountains like those on the earth, and that Apollo comes there once every nineteen years. This period being that of the metonic cycle of the moon, shows that if this could have really been discovered by them, they must have had a long acquaintance with astronomy. The Babylonian tablets lead us to the belief that astronomy, and with it the sphere, and the zodiac were introduced by a nation coming from the East, from the mountains of Elam, called the Accadians, before 3000 B.C., and these may have been the nation to whom the whole is due. On the other hand, the arguments for the Egyptians, or Chaldeans being the originators depend solely on the tradition handed down by many, that one or other are the oldest people in the world, with the oldest civilization, and they have long cultivated astronomy. More precise information, however, seems to render these traditions, to say the least, doubtful, and certainly incapable of overthrowing the arguments adduced above. CHAPTER IV. THE ZODIAC. The zodiac, as already stated, is the course in the heavens apparently pursued by the sun in his annual journey through the stars. Let us consider for a moment, however, the series of observations and reflections that must have been necessary to trace this zone as representing such a course.