Have you ever wondered how astronomers figure out the mind-boggling distances between the Earth and various astronomical objects? In this informative animated video from the Royal Observatory at Greenwich, we learn the fundamentals of the Cosmic Distance Ladder, the succession of methods used to determine those distances.
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 observatory 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.
Knowing the distance from the Earth to the Sun, we can then figure out the distances of some stars using the same method of trigonometric parallax. But as astronomer Olivia Johnson explains in the video, that technique can only be used to measure the closest stars. For distances greater than 500 light years, other methods are required. When the objects in question have a known luminosity--in other words, when they are "standard candles"--the inverse square law of light can be used to calculate distances. Those measurements, along with Hubble's Law and the Doppler Effect, enable even further calculations extending to the edge of the known cosmos.
"What's most incredible to me," says Johnson, "is how all these measurements build on each other. It's only by knowing the scale of our Solar System--the distance between the Earth and Sun--that we're able to measure distances to nearby stars using parallax. If we can learn how far it is to some nearby standard candles using parallax, we can then use comparisons between standard candles to measure the distances to farther stars and galaxies. Finally, by studying the motions of galaxies with standard candles, we learn we can use redshift to measure distances throughout our expanding Universe."