How time works
Solar time vs clock time
Why the Sun is not always highest at 12:00 and why sunrise changes with both longitude and the calendar.
Apparent solar time follows the real Sun
Local apparent solar noon occurs when the Sun crosses the local meridian and reaches its highest point for the day. A sundial follows this apparent motion directly.
Mean solar time smooths the variation
Earth’s tilted axis and non-circular orbit make apparent solar days vary. Mean solar time imagines a smoothly moving average Sun, giving equal clock days and producing the seasonal difference known as the equation of time.
Civil time adds zones and seasonal policy
A time zone places many longitudes on one shared clock, while daylight saving policy may shift that clock again. As a result, solar noon can occur well before or after 12:00 civil time, especially near a zone boundary.