A blink comparator enables astronomers to look at two different photographic plates taken of the same region of the sky on different nights, using the same telescope and plate exposure. If something "blinks" as the view rapidly switches from one illuminated plate to the other, the object has either changed brightness or moved.
This apparatus and technique has been used to detect asteroids, comets, and variable stars. Plates taken a few years apart have been used to detect nearby fast-moving stars or to distinguish between binary stars that orbit a common center of mass, and two stars that happen to be close to the same line of sight, or an optical double.
The German physicist Carl Pulfrich (1858-1927) developed the device while working for the Carl Zeiss Optical Workshop. Blink comparators were soon being used by observatories around the world and led to the discovery of hundreds of variable stars. The most important discovery made with a blink comparator was the existence of Pluto, by Clyde W. Tombaugh in March 1930. At the time, "planet X" was believed to be the gravitational perturber of Uranus and Neptune, orbiting near the outer edge of the solar system. More recently, Pluto has been found to be a low-mass object and has been demoted from planetary status.
Blink comparators are still used today, although the photographic plates have been replaced by digital images that can be stored on a computer. A new application has been proposed to allow radiologists to more accurately compare new and old X-rays, and CAT and MRI scans.
