Since the 1960s, the length of the Astronomical Unit (the mean distance between the Earth and the Sun) has been determined to a very great precision by laser-ranging. This technique is based on sending powerful laser signals to inner solar system objects with precisely known orbits and measuring the travel time of the signal to the object and its reflection back to Earth. As the travel distances are precisely known in astronomical units (this follows from Kepler’s Third Law), a precise determination of the travel times results in a precise determination of the Astronomical Unit.
Laser-ranging experiments made since the 1960s on the surfaces of Mercury and Venus and since the late 1970s on spacecraft on the surface of Mars, have resulted in the following value for the Astronomical Unit:
1 Astronomical Unit = 149597870.691 ± 0.003 km
which is equivalent with a horizontal solar parallax of 8.794144 seconds of arc.