By Daniel Hudon
Journal of the Royal Astronomical Society of Canada (February 2004)
Introduction: How is it possible to measure distances in the solar system? Today, astronomers simply bounce a radar signal off the planet Venus and calculate the distance between Earth and the Sun using Kepler’s laws. But in the 18th century, astronomers had to be much more clever in their measurements. Like surveyors out to measure their surroundings, astronomers set sail around the world with a surveyor’s method to measure the solar system.
The surveyor’s method is called triangulation or parallax. By sighting an object from two widely separated positions and measuring how its image appears to shift against the background, the unknown distance to the object can be determined through basic geometry. It’s the same as holding your thumb at arm’s length: by measuring how its angular position changes first with one eye, then the other, and knowing the distance between your eyes, you can calculate the length of your arm.