A ship sails around the world westward — does it gain a day or lose one?
In Lewis Carroll's 'A Tangled Tale', a ship's captain explains a puzzle of dates to his passengers: a vessel that circumnavigates the globe travelling west keeps setting its clocks back to match the sun, hour by hour, port by port. By the time it completes the loop and arrives home calling the day 'Wednesday', everyone on land insists it's already Thursday. How did the ship lose a whole day without anyone on board noticing?
Reveal the answer
Travelling west, the ship followed the sun, gaining roughly one extra hour of 'daylight bookkeeping' for each 15 degrees of longitude crossed — 24 hours in total over a full circuit, exactly one day, even though every individual day on board still felt like 24 hours. This is the same 'date line' puzzle Jules Verne used to spring the twist ending of Around the World in Eighty Days. Carroll posed it as 'The Dead Reckoning', the fourth of ten mathematical 'Knots' he published as a puzzle serial from 1880.
— Lewis Carroll, A Tangled Tale — The Monthly Packet, 1880-1885
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