Overview
Solar time is a system of timekeeping derived from the apparent motion of the Sun across the sky as seen from a particular location. It measures intervals between successive crossings of a chosen meridian by the Sun; the most familiar unit is the solar day, the period between two successive local solar noons when the Sun reaches its highest point. Solar time underlies the operation of sundials and provided the basis for local timekeeping before the adoption of standardized clock time.
Apparent and mean solar time
Apparent solar time is based on the actual observed position of the Sun and can be read directly from a sundial. Because the apparent speed of the Sun along the ecliptic varies, apparent solar time is not uniform. To provide a uniform civil day, astronomers define mean solar time using a fictitious "mean Sun" that moves at constant speed along the celestial equator. Mean solar time produces the conventional 24‑hour day used in clocks and underlies historical standards such as Greenwich Mean Time.
Causes of variation and the equation of time
The interval of an apparent solar day varies through the year mainly because Earth’s orbit is elliptical (changing orbital speed) and because Earth’s axis is tilted relative to its orbital plane (changing the Sun’s declination). The difference between apparent solar time and mean solar time is called the equation of time. Its value changes predictably through the year and can amount to up to roughly a quarter of an hour; plotted against calendar date it produces the familiar figure‑eight curve known as the analemma.
Solar day versus sidereal day
A sidereal day measures Earth's rotation relative to distant stars, not the Sun. Because Earth advances along its orbit while rotating, the Sun appears to shift slightly each day relative to the stars; a sidereal day is therefore shorter than a mean solar day by about four minutes (about 23 hours 56 minutes in length). This distinction is important in astronomy for pointing telescopes and for converting between solar and stellar coordinate systems.
Sundials, corrections and local time
Sundials show apparent solar time. To compare a sundial reading with clock time one must correct for the equation of time and for the difference in longitude between the sundial’s location and the reference meridian of the clock (standard zones). Longitude differences change solar time by roughly four minutes per degree of longitude. Historically, local solar time governed daily life until the 19th century when railways and telecommunication prompted adoption of standard time zones.
Civil time, Universal Time and modern practice
Modern civil time is tied to mean solar time only historically: Universal Time (UT) is based on Earth's rotation and approximates mean solar time at Greenwich. Coordinated Universal Time (UTC), the present international standard, is maintained by atomic clocks and is kept within a second or so of UT by occasional leap seconds to allow for irregularities in Earth’s rotation. Time zones assign a common civil time to wide regions rather than local solar time, so clock noon need not coincide with local solar noon.
Uses and historical significance
- Sundials and solar observations were the earliest practical clocks and remain educational and decorative instruments.
- Navigation and astronomy relied on conversions between solar and sidereal time to determine longitude and to locate celestial objects.
- Understanding solar time clarifies why sunrise, solar noon and sunset shift with the seasons and why clock time differs from apparent Sun time.
Solar time continues to be a useful conceptual and practical tool in astronomy, historical studies, sundial design, and for explaining the relationship between Earth's rotation, its orbit, and the familiar cycle of day and night.