soltid

Soltid, or solar time, is a timekeeping concept based on the position of the Sun in the sky. It is the time determined by the Sun’s apparent motion, typically the local apparent solar time, which corresponds to solar noon when the Sun is highest in the sky. Sundials measure soltid, aligning their gnomon’s shadow with the Sun’s position.

Earth’s orbit is elliptical and its axis is tilted, causing the Sun’s apparent speed to vary through the year. This leads to a difference between apparent solar time and mean solar time. The difference is captured by the equation of time. Mean solar time assumes a fictitious Sun moving at a constant pace along the celestial equator, and it forms the basis of civil time and time zones. The equation of time can swing the difference between apparent and mean solar time by about −14 to +16 minutes over the course of a year.

Longitude and time zones influence how soltid translates to standard time. Local solar time advances by about one hour for every 15 degrees of longitude eastward. In practice, civil time is set to local mean solar time within a time zone, and adjustments such as daylight saving time can further shift the relationship between civil time and true solar time.

In summary, soltid describes time in relation to the Sun’s actual position, while civil time relies on mean solar time and standardized time zones. Soltid remains a fundamental concept in astronomy, navigation, and sundial-based timekeeping, even as modern life relies on standardized clock time.