The Riefler Clock at the US National Observatory in Washington, DC, determines the official time in the United States

PAN-EXT US Naval Observatory / MLS rotating dome atop observatory / VS astronomer looks thru telescope / MS Riefler Clock, the master clock of the United States, which determines official time / MS paper loaded into graph-making machine to create record of clock's beats / CU machine operating / CU relay device sends out exact time to entire nation, setting the official time / Note: exact year not known