
Caesium Atomic Clock, 1955.
1955
1955
1956
1839 (original); circa 1988 (replica)
circa 1955
1955; 1830
1955
1954-1955
1955
1957
1955
1957
2000-100 BCE
1954
1958
1954
circa 1955
1957
1955
1958
1956
1955
1954
1955
1957
1954
1954
1957
1956
1957
1956
1957
1954
1957
1956
1958
1956
1956
1955
1955
1955
1955
1957
1957
1957
1955
1957
1957
1958
1954
1955