How sailors found longitude before satellites: a race against the clock

£20,000 (30 nmi) £15,000 (40 nmi) £10,000 (60 nmi)

–clock error

–longitude error

–km off course

–

Clock drift per day 60 s Days at sea 42 Latitude 15°

Clock error in seconds ÷ 240 = degrees of longitude. One degree is 111 km at the equator and shrinks with the cosine of latitude. The prize zones are fixed distances, as in the 1714 Act. The H4 preset uses its Jamaica result of about 5 seconds over 81 days.