|MadSci Network: Astronomy|
Thank you, Amanda, for your question. There are a number of ways which we can now measure the distance to Venus thanks to technology, including radar. Electromagnetic energy (includes light and radio waves) travel at a fixed, maximum speed of about 186,000 miles per second in the vacuum of space. By sending a radio pulse in the direction of venus and precisely timing the return pulse, one can measure the distance to Venus [distance = 186,000 x time /2 (divide by 2 since the radio pulse goes to Venus then returns, hence it travels twice the distance from earth to Venus)].
A far more interesting question though is how was it measured prior to the 20th century? This distance has been known for centuries. The way is basic geometry. Galileo observed the phases of Venus. By observing when Venus was half-full phase and determining the angle between the sun and Venus, he was able to work out the distance relative to the earth-sun distance. Now all one needs is the a way to measure that. The transit of Venus (Venus passing across the face of the sun, between earth and the sun) provided that data. Interestingly enough, the transit of Venus will occur again in a few weeks. [2004 June 8]
For more information, and a more detailed expanation of the geometric approach to measuring the distance, please see
Try the links in the MadSci Library for more information on Astronomy.