Geodetic datum defines the size and shape of the ellipsoid earth and
the origin (or position) and orientation (or direction) with respect to
the Earth
The direction of the minor axis of the ellipsoid. This is classically
defined as being parallel to the mean spin axis of the earth
The position of its center, either implied by adopting a geodetic
latitude and longitude (Φ, λ) and geoid / ellipsoid separation (N) at one,
or more points (datum stations), or in absolute terms with reference to
the Earth centre of mass
The zero of longitude (conventionally the Greenwich Meridian).
True geodetic datums were employed only after the late 1700s when
measurements showed that the earth was ellipsoidal in shape. The
science of geodesy.