mean time
Definition
- Noun:
- Mean Time: In astronomy, time measured by the motion of a hypothetical "mean sun" that moves at a constant average speed along the celestial equator, unlike the actual sun whose apparent motion varies. It is the basis for local mean time and time zones.
Usage
- Noun:
- The concept of mean time was developed to create a more uniform system for telling time than apparent solar time.
- Before time zones were standardized, each town kept its own local mean time based on its longitude.
Advanced Usage
- "Local Mean Time (LMT)": The mean solar time for a specific longitude, differing from the standard time of a time zone.
- Historically, cities just a few miles apart could have clocks set to different local mean time.
- "Greenwich Mean Time (GMT)": The mean solar time at the Royal Observatory in Greenwich, London, which historically served as the prime meridian (0° longitude) and global time standard.
- GMT, based on mean time, was the international standard for civil timekeeping for much of the 20th century.
Variants and Related Words
- Mean Solar Time: A more descriptive synonym for mean time.
- Apparent Solar Time: Time based on the actual observed position of the sun, which varies daily relative to mean time.
Synonyms
- Mean Solar Time: The standard term in astronomy and timekeeping.
- Civil Time: In historical contexts, this often referred to mean time as used for civil purposes.
Related Phrases
- Equation of Time: The difference between apparent solar time and mean solar time, which can be plus or minus several minutes.
- The equation of time shows the discrepancy between your sundial (apparent time) and your watch (mean time).
Noun
-
(astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)
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