Modern Determinations
Precise measurements of the relative positions of the inner planets can be made by radar and by telemetry from space probes. As with all radar measurements, these rely on measuring the time taken for photons to be reflected from an object. These measured positions are then compared with those calculated by the laws of celestial mechanics: an assembly of calculated positions is often referred to as an ephemeris, in which distances are commonly calculated in astronomical units. One of several ephemeris computation services is provided by the Jet Propulsion Laboratory.
The comparison of the ephemeris with the measured positions leads to a value for the speed of light in astronomical units, which is 173.144 632 6847(69) AU/d (TDB). As the speed of light in meters per second (c0) is fixed in the International System of Units, this measurement of the speed of light in AU/d (cAU) also determines the value of the astronomical unit in meters (A):
The best current (2009) estimate of the International Astronomical Union (IAU) for the value of the astronomical unit in meters is A = 149 597 870 700(3) m, based on a comparison of JPL and IAA–RAS ephemerides.
Read more about this topic: Astronomical Unit
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