At the dedication of Van Vleck Observatory on June 16, 1916, the observatory’s first director, Frederick Slocum, outlined an agenda for the new institution: determining the distances of stars. This stellar parallax program constituted the observatory’s main research effort for the majority of the twentieth century.
The parallax program involved researchers as well as many human “computers” who were aided by tools, such as the Millionare mechanical calculator, to perform the necessary calculations. Advances in technology allowed for more accurate measurements and faster computations, changing the kinds of work people did. The Mann comparator, which the Astronomy Department purchased in the late 1950s, was one of the observatory’s most important acquisitions in this regard.
The Mann was an important aid to the people tasked with measuring the distances between the different stars recorded on glass plate images of the sky. The stellar parallax program’s results were contingent on both how the plates were recorded and how they were interpreted, so accurate measurements were crucial for exact results. Previously, plate measurers had worked solely relying on their eyes to gauge the minute distances that would be used in their calculations. With the Mann, the measurers could insert each plate and look into a viewfinder, using a mechanical crank to move the plate very slowly by degrees in order to pinpoint the exact locations of the stars and make measurements between them. It was still tedious work that required a practiced eye, but the comparator made accuracy easier to achieve.
When you take a closer look at our Mann comparator, you can see that there are two small name plates attached to it. One, close to the crank, indicates the manufacturer and serial number of the device. The other, larger plate is affixed to the base and reads, “PROPERTY OF U.S.A.F – 866477 – DO NOT REMOVE THIS TAG.” What do these two tags tell us?