Solar time

Solar time is a time system based on the Sun’s apparent position in the sky at a particular location. It can be measured from the Sun’s highest point or from its apparent daily motion.

Apparent solar time: Time measured directly from the Sun’s apparent position, with apparent noon at its daily highest point. This is the sundial-based form of solar time, shaped by Earth’s orbit and axial tilt.
Solar day: The interval between successive passages of the Sun across a local meridian. This is the natural daily cycle underlying solar time.
Sundial: An instrument that uses a shadow cast by sunlight to indicate time. Sundials made local apparent solar time measurable before mechanical clocks.
Solar noon: The moment the Sun crosses the local meridian and reaches its highest daily altitude. Solar installations use this event to identify the midpoint of local daylight.
Mean solar time: Time measured using a fictitious mean Sun that moves uniformly along the celestial equator. It smooths the irregular apparent motion that makes sundial days vary in length.
Sidereal time: A time system based on Earth’s rotation relative to distant stars. A sidereal day is shorter than a solar day because Earth advances along its orbit.
Mechanical clock: A clock that measures time using a mechanical oscillator and regulated motion. Mechanical clocks encouraged uniform hours that did not follow the Sun’s variable apparent motion.
Solar azimuth: The Sun’s horizontal direction, measured as an angle around the horizon. Solar time helps estimate the Sun’s direction for orientation and tracking.
Equation of time: The difference between apparent solar time and mean solar time, varying through the year. It quantifies the seasonal offset between a sundial and a clock tracking mean time.
Earth's rotation: Earth’s spin around its axis, which produces the apparent daily motion of the sky. The Sun’s apparent passage across the sky supplies solar time’s basic daily rhythm.

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