mean time

Thân thiện

The astronomer calculates the mean time from the observatory's clock.

Definition

1. 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).

Học thuật

Noun

1. (astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)

Từ đồng nghĩa

• mean solar time