hour-circle

The hour-circle is drawn on the celestial globe.

Definition

1. Noun:
   • Astronomical coordinate: An "hour-circle" is a great circle on the celestial sphere that passes through the celestial poles and is perpendicular to the celestial equator. It is the astronomical equivalent of a meridian of longitude on Earth, used to measure the hour angle of a celestial object.

Usage Examples

• Noun:
  • The astronomer used the hour-circle to determine the exact position of the star. (The great circle helped locate the star's hour angle.)
  • Each star's hour-circle intersects the celestial equator at a specific point. (The circle passes through the poles and equator.)

Advanced Usage

• "Hour-circle of a celestial body": The specific great circle that passes through a given star or planet and the celestial poles.
  • The hour-circle of Sirius is used to calculate its rising time. (The circle through Sirius and the poles.)

Variants and Related Words

• Hour angle (n): the angular distance measured westward along the celestial equator from the local meridian to the hour-circle of a celestial object.

  • The hour angle of Mars is 45 degrees. (The angle from the local meridian to Mars's hour-circle.)

• Celestial meridian (n): the hour-circle that passes directly overhead at a given location.

  • The celestial meridian is the observer's local hour-circle. (The overhead great circle.)

Synonyms

• Meridian: in astronomy, a meridian is a specific hour-circle, typically the one passing through the zenith.
• Great circle: any circle on the celestial sphere whose plane passes through the center of the sphere; an hour-circle is a type of great circle.

Related Idioms