*IAU(1991) RECOMMENDATION II*

Physical structures and quantities that will be used to establish the reference frames and time scales based upon the ideal definition of the system given by Recommendation I

The XXIst General Assembly of the International Astronomical Union,

considering,

a) the need to define a barycentric coordinate system with spatial origin at the centre of mass of the solar system and a geocentric coordinate system with spatial origin at the centre of mass of the Earth, and the desirability of defining analogous coordinate systems for other planets and for the Moon,

b) that the coordinate systems should be related to the best realization of reference systems in space and time, and,

c) that the same physical units should be used in all coordinate systems,

recommends that,

1. the space coordinate grids with origins at the solar system barycentre and at the centre of mass of the Earth show no global rotation with respect to a set of distant extragalactic objects,

2. the time coordinates be derived from a time scale realized by atomic clocks operating on the Earth,

3. the basic physical units of space time in all coordinate systems be the second of the International System of Units (SI) for proper time, and the SI meter for proper length, connected to the SI second by the value of the velocity of light c = 299792458 m/s.

Notes for Recommendation II

1. This recommendation gives the actual physical structures and quantities that will be used to establish the reference frames and time scales based upon the ideal definition of the system given by Recommendation I.

2. The kinematic constraint for the rate of rotation of both the geocentric and barycentric reference systems cannot be perfectly realized. It is assumed that the average rotation of a large number of extragalactic objects can be considered to represent the rotation of the universe which is assumed to be zero.

3. If the barycentric reference system as defined by this recommendation is used for studies of dynamics within the solar system, the kinematic effects of the galactic geodesic precession may have to be taken into account.

4. In addition, the kinematic constraint for the state of rotation of the geocentric reference system as defined by this recommendation implies that when the system is used for dynamics (e.g., motions of the Moon and Earth satellites), the time dependent geodesic precession of the geocentric frame relative to the barycentric frame must be taken into account by introducing corresponding inertial terms into the equations of motion.

5. Astronomical constants and quantities are expressed in SI units without conversion factors depending upon the coordinate systems in which they are measured.