hour circle
Definition
- Noun:
- A great circle on the celestial sphere that passes through both celestial poles: An "hour circle" is an imaginary line in the sky, similar to a line of longitude on Earth, used in astronomy for celestial navigation and coordinate measurement.
Usage Examples
- Noun:
- The star's position is defined by where its hour circle intersects the celestial equator.
- Astronomers use hour circles to map the right ascension of celestial objects.
Advanced Usage
- In Celestial Coordinate Systems: The term is used within systems like the equatorial coordinate system. An object's right ascension is measured eastward along the celestial equator from the vernal equinox to the point where the object's hour circle intersects the equator.
Variants and Related Words
- Meridian: A related concept; a great circle passing through the celestial poles and the zenith (the point directly overhead for an observer). While all hour circles pass through the poles, a meridian is a specific hour circle for a given location and time.
- Great Circle: Any circle on a sphere whose center is the center of the sphere. An hour circle is a specific type of great circle.
Synonyms
- Celestial meridian (in a specific, local context).
- Circle of declination.
Related Phrases
- Hour angle: The angular distance west of a local meridian along the celestial equator to an object's hour circle. This demonstrates how the "hour circle" is used in practical measurement.
Noun
-
a great circle on the celestial sphere that passes through both celestial poles
