*The meter is the length of the path traveled by light in vacuum during a time interval of* 1/*c of a second.*

To measure the 3-distance between two objects in an inertial frame we send a ray of light from the first object, to which is associated an atomic clock, to the second one, where it is reflected and then reabsorbed by the first object. The measure of the flight time and the 2-way velocity of light determine the 3-distance between the objects.

This convention is compatible with the Euclidean 3-space of inertial frames in Minkowski space-time. When the technology will allow one to measure the deviations from Euclidean 3-space implied by Post-Newtonian gravity we will need a modified convention taking into account a general relativistic notion of length.

In astronomy the unit of length, defined in the IAU (1976) System of Astronomical Constants, is the *astronomical unit AU*, approximately equal to the mean Earth-Sun distance (Resolution 10, 1976; IBWM, 2006). It is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with an angular frequency of 0.017 202 098 95 radians per day. Measurements of the relative positions of planets in the Solar System are done by radar (or by telemetry from space probes): one measures the time taken for light to be reflected from an object using the conventional value of the velocity of light *c*.

Both for objects inside the Solar System and for the nearest stars one measure the distance with the *trigonometric parallax* by using the propagation of light and its velocity *c* in inertial frames. One measures the difference (the inclination angle) in the apparent position of an object viewed along two different lines of sight at two different times and then uses Euclidean geometry to evaluate the distance. The used unit in astronomy is the parsec, which is 3.26 light-years or 3.26 1016 meters.

However this convention cannot be used for more distant either galactic or extra-galactic objects (UCLA, 2007; Carrol B.W. et al, 2007). New notions like standard candles, dynamical parallax, spectroscopic parallax, luminosity distance,..... are needed. These notions involve both aspects of light propagation in curved space-times and cosmological assumptions like the Hubble law (velocity-redshift linear relation). Therefore they belong to another type of metrology.
