hour circle

9.9

A circle on the sky that passes through both celestial poles and a star to show its right ascension

noun
/ˈaʊər ˈsɜrkl/

Specialized

: círculo horario

The hour circle is essential for locating stars in the sky using right ascension.

Examples

Each hour circle intersects the celestial equator at a unique right ascension.
Astronomers use the hour circle to measure the right ascension of celestial objects in the night sky.
Astronomers use the hour circle along with declination to pinpoint celestial objects.
The hour circle helps astronomers locate stars.
An hour circle intersects the celestial equator.
Each hour circle represents a line of constant right ascension on the celestial sphere.
To determine the position of a star, astronomers refer to its hour circle and declination.

Surface Forms

hour circle singular

hour circles plural

Morphology

hour + circle

Although 'hour' and 'circle' are individually transparent, the specific sense of 'hour' here is a technical measure used in celestial coordinates (right ascension) rather than the everyday period of sixty minutes, and the full definition (a great circle through the celestial poles and a given object) cannot be inferred from the parts. This is a specialist astronomical term whose meaning is not predictable for a B1 learner who only knows the common senses of the constituents.

Etymology

Hour circle gives a simple picture: imagine the sky as a globe with lines that run north and south on Earth, where each circle passes through both poles and marks an hour like a clock on the sky. That's why an hour circle is used 'to measure right ascension' and to help locate stars.