World of Greyhawk

As is obvious, the Eastern portion of the continent of Oerik, the Flanaess, is the center of enlightened humanity. Oerth has four great continents and countless islands, and four great oceans and countless seas which surround these bodies of land. Little is known about the lands of the western portion of Oerik, less still about the savage inhabitants of the other continents, but such knowledge is, of course, of little use anyway and of no importance to humanity.
The heavens are far more important and interesting. We must study the stars, those which wander and those which are fixed, to properly understand Astrology. The sun travels once around Oerth in 364 days, visiting the 12 Lairs of the Zodiac in an appointed round which never varies. The Great Moon (Luna) waxes and wanes in fixed cycles of 28 days each, upon which our months are based; while the Handmaiden (Celene, the small aquamarine satellite) follows a path which shows Her in full beauty but four times each year, thus showing us the time for our Festivals. When both Mistress and Handmaiden are full, things of great portent are likely to occur, depending upon the positions of the five wandering stars in the Lairs, naturally. For complete details of Astrology, we recommend the reader to such texts as Selvor the Elder's “Secrets of Ye Skye Revealed” or Yestro Bilnigd's “Astrology, Divinity, and Mankind.”