Mean Solar Year. Average time taken by Sun to go round Ecliptic or, to be precise, for Earth to go round Sun. Due to slight irregularities in the movement of First Point of Aries the time varies slightly and irregularly. Mean value, when taken over about 100 years, gives 365 days. 5 hrs. 48 mins. 48 sees. (approx.).
Mean Sun. Fictitious body that travels along Equinoctial at a constant speed, and in the same time that true Sun takes to travel round Ecliptic at a variable speed.
Mean Tide Level. Average tidal height above chart datum. Found by evaluating half sum of Mean High Water and Mean Low Water at a given place. Name is sometimes applied to mean level of a tide.
Mean Time. Time regulated by Mean Sun.
Measured Distance. Accurately measured distance used for determining speeds of ships. Most of the so-called 'measured miles' are measured distances, and require adjustment when calculating speeds in knots.
Measured Mile. Measured distance of 6080 ft. See 'Measured Distance'.