Definition
Meridional parts express the distance of a parallel from the equator on a Mercator projection, measured in units of one minute of longitude at the equator. Tabulated in Norie's and other nautical tables, the difference of meridional parts between two latitudes allows exact Mercator sailing, where tangent of course equals difference of longitude in minutes divided by difference of meridional parts. This replaces plane sailing for long distances where meridian convergence matters.
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