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Local noon is found by observing the meridian passage of the sun. From this, latitude can easily be determined.
Published in 1599, this Mercator map of the world is attributed to Edward Wright an English mathematician.
A modern Mercator overlay reveals that the old chart was quite accurate in latitude but away from the well known areas of Europe and the Mediterranean Sea, longitude was at best a guestimate.

The problem of measuring longitude was resolved when, after a long struggle with the British Board of Longitude, instruments like John Harrison's watches finally came into common service.

H1 (1737)



H2 (1741)		 John Harrison (c. 1765)

and his wonderful clocks.


H-3 (1759)		 H-5(1770)



H-4 (1760)