# Happy Birthday John Harrison

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John Harrison  (1683-1776)  invented the first chronometer  accurate enough to determine longitude.  His best chronometer  would loose a second a month, which for its time was fantastically  accurate.

Latitude had been known and accurately measured  in ancient  China, ancient Egypt  and ancient Greece.   The tilt of the earth as a function of time  could be determined by measuring the length of the shadow produced by a vertical rod on the day of Spring or Autumn.  On the day of the equinox  the Sun is directly over the equator at noon.  Latitude could also be measured at night in the Northern Hemisphere by measuring the angle  to the North Star.   For daytime measurement  the angle to the Sun at noon is measured  and the measurement is adjusted by looking up the tilt of the Earth in a table, for that day of the year.  So latitude was fairly easy to measure and compute.  It is a matter of geometry and trigonometry.   Magellan might not have known exactly where he was at all times  but one he got  hias latitude eventually he could work has way back to Spain.  Magellan himself died mid voyage but his navigators  brought his ship home after circumnavigating the Earth (1519 - 1522 ce).  That was about 30  after Columbus.

The ability to measure one's  position on the vast oceans of the world  gave Britain a major advantage in domination of the seas and in exploration.

BTW the fact that latitude was well known around 1000 bce buries the the canard  that the spherical shape of the earth was not known in ancient times.  It was and around 250 bce Eratosthenes, the director of the Library at Alexandria,   determined the circumference of the Earth  to within 5 percent of its modern value.  All he needed was a stick, some paper and writing instrument  and his very fine brain.  The idea that the spherical shape of the earth was not known in the time of Columbus was a literary fiction  produced by Washington Irving as part of his highly inaccurate biography of Christopher Columbus.  Every educated person, in those days (the 15 th century) knew the Earth was an oblate spheroid to within a very small error.