#TIL Pluto was discovered using a machine called a blink comparator. This machine dates back to 1904 and — like many things — has been superseded by modern technology. In 1930, scientists knew there was something pulling the orbits of the planets but they didn’t know where it was.

The blink comparator helps you find something that moves by showing you two photographic plates in quick succession. Anything in the same spot on both plates appears stationary. Anything moving in the frame will stand out as it appears to jump from one spot to another.

https://hackaday.com/2019/12/06/chandrayaan-2-found-by-citizen-scientist-reminds-us-of-pluto-discovery/