Transit Of Venus Across The Sun

• 1874 and 1882
Transit observations in 1874 and 1882 allowed this value to be refined further. Several expeditions were sent to the Kerguelen Archipelago for the 1874 observations. The American astronomer Simon Newcomb 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 telemetry 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 As a result, the need for parallax calculations has been superseded.
• 2004 and 2012
A number of scientific organizations headed by the European Southern Observatory (ESO) organized a network of amateur astronomers and students to measure Earth's distance from the Sun during the transit. The participants made a measurement of the astronomical unit (AU) of 149 608 708 km ± 11 835 km which had only a 0.007% difference to the accepted value.