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astronomy × 26science × 10
ancient-history × 3
calendar × 3
mathematics × 3
time-keeping × 3
china × 2
persia × 2
age-of-discovery × 2
historical-accuracy × 2
navigation × 2
needham × 2
soviet-union × 1
religion × 1
india × 1
language × 1
cold-war × 1
ancient-egypt × 1
middle-east × 1
hitler × 1
ottoman-empire × 1
classical-antiquity × 1
16th-century × 1
renaissance × 1
prehistory × 1
