 Transit observations in 1874 and 1882 allowed this value to be refined further . Three expeditions — from Germany , the United Kingdom and the United States — were sent to the Kerguelen Archipelago for the 1874 observations . The American astronomer Simon <unk> combined the data from the last four transits , and he arrived at a value of about 149 @.@ 59 million kilometers ( ± 0 @.@ 31 million kilometers ) . Modern techniques , such as the use of radio <unk> from space probes , and of radar measurements of the distances to planets and asteroids in the Solar System , have allowed a reasonably accurate value for the astronomical unit ( AU ) to be calculated to a precision of about ± 30 meters . As a result , the need for parallax calculations has been superseded . 
