mean solar day

As a result, our global standard of time, known as Coordinated Universal Time, or UTC, occasionally becomes misaligned with UT1—the marker used to measure the actual length of one mean solar day.

A mean solar day — based on the time it takes the Earth to rotate once — is calculated to be 86,400 seconds.

The length of the sidereal day is to the mean solar day as .99727 to 1, and various attempts have been made by trains of wheels to obtain this relation—but all are somewhat complicated.

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 Second is the time of one swing of a pendulum making 86,164.09 swings in a sidereal day, or 1/86,400 part of a mean solar day.