ZENITH

That point in the heavens which is vertical to the spectator, and from which a perpendicular line passing through him and extended would reach the center of the earth. From of old the documents of the Ancient and Accepted Scottish Rite are dated "under the Celestial Callopy of the Zenith which answers to" the latitude of the place whence the document is issued being then given to fill the blank space. The latitude alone is expressed because that indicates the position of the sun's meridian height. Longitude, however, is always omitted, because every place whence such a document is issued is called the Grand East, the specific spot where the sun rises. The theory implied is, that although the South of the Lodge may vary, its chief point must always be in the East, the point of sunrising, where longitude begins.
- Source: Mackey's Encyclopedia of Freemasonry