: The invisible grid used to track everything from a distant quasar to a nearby asteroid.
The Explanatory Supplement to the Astronomical Almanac is a companion volume to the Astronomical Almanac, which is an annual publication that provides astronomical data and tables for a specific year. The Supplement provides a detailed explanation of the data and tables in the Almanac, as well as background information on the astronomical phenomena and concepts that are covered. the explanatory supplement to the astronomical almanac pdf