Positional astronomy plays an important role in keeping track of time

Apparent solar time is based on the apparent motion of the Sun across the celestial sphere, which varies over the course of the year
Mean solar time is based on the motion of an imaginary mean sun along the celestial equator, which produces a uniform mean solar day of 24 hours
Ordinary watches and clocks measure mean solar time
Sidereal time is based on the apparent motion of the celestial sphere

Apparent solar time is based on the apparent motion of the Sun across the celestial sphere, which varies over the course of the year