Time determination at US Naval Observatory. US Naval Observatory Associate Astronomer John Edwin Willis (1901-1979) using the observatory's first photographic zenith tube (PZT). This device was acquired for variation of latitude work in 1914-15, having been designed by Frank Elmore Ross (1874-1960) and used by him at Gaithersberg, Maryland, USA, from 1911-14. This device was used regularly for time determination from February 1934, helping establish official time signals used in astronomical and navigational calculations. Photographed at the US Naval Observatory, Washington DC, USA.
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