Ephemeris

An Ephemeris is a table giving the positions of the heavenly bodies day by day, or even hour by hour, over years or centuries. For centuries they were published in books; today they are calculated with high-precision software such as Swiss Ephemeris, based on data from NASA's Jet Propulsion Laboratory. They are the indispensable tool for casting natal charts, calculating transits and dating retrogrades, lunations and eclipses. A good astrologer checks calculations against a reliable ephemeris rather than trusting approximations.