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Worse, using sextants and other optical instruments AT OR NEAR A COASTLINE is going to introduce further errors and inaccuracies caused mainly by the changing temperature of the air (which means a changing and unknown index of refraction) in those areas where the sea/land temperature is causing the air temperature to vary relatively widely.
The land may not be rocking, but the air at the coastline makes it just as "unstable" as a small boat rocking at sea.