Definition

Noun: A great circle on the celestial sphere that passes through both the observer's zenith (the point directly overhead) and the celestial poles (north and south). It is the projection of the observer's terrestrial longitude line onto the sky and divides the sky into an eastern and a western half from the observer's specific location.

Usage

The term is used in astronomy, geodesy, and navigation to define a local, observer-specific reference frame for measuring celestial coordinates. - To calculate the local sidereal time, you must first identify your observer's meridian. - The star transited, or crossed, the observer's meridian at exactly midnight.

Advanced Usage

• Local Meridian: Often used synonymously with "observer's meridian." It emphasizes the local, position-dependent nature of this celestial line.
• Meridian Passage or Transit: The moment when a celestial object crosses the observer's meridian, reaching its highest altitude in the sky for that day.
  • We timed the meridian passage of the Sun for our survey.

Variants and Related Words

• Celestial Meridian: A more general term for any great circle passing through the celestial poles. The observer's meridian is a specific celestial meridian.
• Prime Meridian (Greenwich Meridian): The terrestrial meridian at 0° longitude, used as the universal reference, in contrast to the local observer's meridian.

Synonyms

• Local Celestial Meridian
• Local Meridian

Related Concepts (Not Phrasal Verbs or Idioms)

• Zenith: The point on the celestial sphere directly above the observer, through which the observer's meridian passes.
• Hour Angle: The angular distance of a celestial object west of the observer's meridian, used in celestial navigation.

Noun

1. a meridian that passes through the observer's zenith