[pabianice]

But Menzies's tale, which looks at a well-documented voyage by a Ming Dynasty fleet in 1421, is more specific in its assertions than most theories. In his version, a fleet led by admiral Zheng He rounded the Cape of Good Hope and then split up. One group explored South America, Antarctica, and Australia, while other ships toured Central and North America, circled Greenland, learned to measure longitude, and established settlements. Menzies says all records of the voyages were later destroyed.

BZZZTTT! Sorry. Wrong answer! You don't "learn to measure longitiude." To measure and plot rough longitude you need a timepiece accurate to within several minutes after a journey of months (or, when I was a Navy nav in the 70s, to within ten seconds). No such watch or clock existed in 1421, and certainly not in the Chinese Navy. The Brits finally built such a clock in the Eighteenth Century.

[blam]

Ancient Navigation

[JudyB1938]

http://www.pbs.org/wgbh/nova/easter/civilization/navigation.html

What a coincidence you posted that link, Blam. I was thinking of that very thing as I was reading through this thread.

[Grand Old Partisan]

That's not true. As was known to the ancients, longitude could be determined by careful observations of the moon over several nights, which was impossible on a ship, hence the need for an accurate timepiece for shipboard calculation.

[Kenny Bunk]

FYI, there are a few methods of finding longitude. One pre-chronometer method was by taking sights on the moons of Jupiter (which are actually visible to the naked eye at sea on clear nights). Other methods involved complex astronomical observations. These difficult methods required a high level of skill and dense mathematics to be used.

Using a sextant, tables, and a chronometer is merely the easiest way to do it ... and to train people to do it who are not rocket surgeons einsteins.

[pabianice]

http://www.imdb.com/title/tt0192263/