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The mean solar days are all exactly the same length; they are equal to the length of the average apparent solar day; and they are each four minutes longer than a sidereal day.

The sidereal day and the mean solar day necessitate some means of registering time, such as clocks; therefore the original day in use must have been the apparent solar day.

This is the most natural and the most accurate measure of time for the navigator at sea and the unit of time adopted by the mariner is the apparent solar day.