How an ephemeris works

An ephemeris is a table of planetary positions over time. For any given date, it tells you where each body sat along the ecliptic. Before computers, astrologers and navigators relied on thick printed volumes, updated and reissued every year.

Those tables were themselves the product of centuries of careful observation and orbital mathematics. Astronomers modelled how each planet moves under gravity, refined the models against telescope measurements, and then computed positions far into the past and future.

Modern software like the Swiss Ephemeris compresses that high-precision astronomical data into a form a program can query in microseconds. Ask it where Mars was on a particular morning and it returns a position accurate to a fraction of an arc-second, which is far finer than any chart interpretation requires.

There is a subtlety worth knowing. Positions can be expressed as geocentric, measured as if from the centre of the Earth, which is the standard for astrology, or heliocentric, measured from the Sun. They can also be tropical, tied to the seasons, or sidereal, tied to the fixed stars. A good engine lets you choose, because Western and Vedic traditions make different choices.

AstroMasters computes every chart against this data rather than approximating. It is the difference between an estimate and a measurement, and it is the part of the craft that should never be left to guesswork.