How Big Is The Universe?

A primer on how we learned to measure it:

Mike Springer marvels:

The video was made for “Measuring the Universe: from the transit of Venus to the edge of the cosmos,” an exhibit that will be on display at the [Royal Observatory at Greenwich] through September 2. The exhibit is timed to coincide with this year’s rare transit of Venus, which will be visible from Earth on June 5 and 6 and won’t happen again until 2117. The transit of Venus played a key role in the history of astrometry. In 1663 the Scottish mathematician and astronomer James Gregory proposed a method of timing the movement of Venus across the Sun from two widely separated points on the Earth and using the differential to calculate the sun’s mean equatorial parallax and, by triangulation, the Sun’s distance from the Earth.