mean solar day

The mean solar day is the average time — 24 hours — the Sun takes to go from its highest point in the sky on one day to its highest point the next.

• The Sun's highest point can be found simply by tracking a stick's shadow through the day — the shadow is shortest exactly when the Sun is at its highest.
• The "mean" in mean solar day matters because Earth's actual rotation and orbit make each individual solar day vary very slightly — averaging over many days gives the steady, dependable 24-hour figure we actually use.
• This cycle, caused by Earth rotating on its own axis, is the foundation of the day as a unit of time — the shortest natural cycle regularly used for timekeeping.
• Everything else in a calendar builds upward from this single unit: days combine into the Moon's month-long cycle, and months (or seasons) combine into the year.