mean sun
Noun: A theoretical or imaginary sun used in astronomy and timekeeping. It is defined as moving eastward along the celestial equator at a constant, average speed, completing one revolution in the same total time (one tropical year) as the actual Sun takes on its apparent path along the ecliptic, which it traverses at a variable speed.
The "mean sun" is a conceptual tool used to define and measure uniform time, specifically Mean Solar Time. It serves as a reference point because the apparent motion of the real Sun across the sky is uneven due to the Earth's elliptical orbit and axial tilt.
- The difference between the position of the mean sun and the apparent Sun causes the equation of time.
- Our clocks are set according to the motion of the mean sun, not the actual Sun in the sky.
- Local Mean Time is based on the hour angle of the mean sun.
- Mean Solar Day: The interval between two successive upper transits of the mean sun across the same meridian. This is the basis for the standard 24-hour day.
- Equation of Time: The key application. It is the difference between apparent solar time (based on the real Sun) and mean solar time (based on the mean sun), and can vary by roughly +/- 15 minutes throughout the year.
- Apparent Sun (n): The real Sun as observed from Earth, whose apparent motion is variable.
- Mean Solar Time (n): Time based on the motion of the mean sun.
- Mean Time (n): Often used synonymously with Mean Solar Time.
- Fictitious sun
- Theoretical sun
This term is purely technical and used in the specific contexts of astronomy, celestial navigation, and the science of timekeeping. It has no common, everyday meaning outside these fields.
- a theoretical sun that moves along the celestial equator at a constant speed and completes its annual course in the same amount of time the real sun takes at variable speeds