The Menzel Gap — Harvard, 1953–1968

For generations, Harvard College Observatory photographed the night sky on glass plates. Each exposure preserved the brightness and position of stars at a particular time. Taken together, the plates became a record that later astronomers could search for changes nobody had known to look for when the photographs were made.

That record is not continuous. Harvard stopped taking astronomical plates in 1953 and did not resume until 1968. The fifteen-year interruption is now identified in material from the Digital Access to a Sky Century at Harvard project as the Menzel Gap.

The consequence is simple. If no plate was taken, no later instrument can recover what crossed that part of the sky on that night. The sky continued. Its photographic record did not.

Donald H. Menzel was acting director and then director of Harvard College Observatory across the relevant period. The stated institutional reason for ending plate production was financial pressure and the continuing cost of the program.

The gap is not the only documented loss. In her 2002 memoir, Harvard astronomer Dorrit Hoffleit wrote that Menzel instructed his secretary to destroy part of the plate collection without first inspecting it. Later committees approved the destruction of additional plate series. Those decisions permanently removed observations that could otherwise have been examined with modern methods.

Menzel also performed wartime and postwar military work, became a prominent public advocate of natural explanations for UFO reports, and promoted atmospheric inversion as an explanation for the 1952 Washington radar events. An atmospheric inversion occurs when layers of air at different temperatures bend radar signals in unusual ways.