On the use of natural references.

Criteria for the definition of references in space and time are reviewed, showing that, for the first time, two of the most important criteria will soon be satisfied simultaneously. Relevant questions concerning the relations among the reference system, the equator, and the equinox, concerning precession and nutation, concerning the reference point on the equator, and concerning ephemerides are posed and answered. The adaptation of traditional concepts and usages to this new situation is addressed.