Hour Circle

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Hour circle of a star

The hour circle is a virtual great circle on the celestial sphere (the projection of our globe into outer space) which passes through the poles of the celestial sphere and intersects the celestial equator at right angles.

So called because the angle between the planes of the hour circles (or the difference in longitudes) of two objects is sometimes measured in hours (or hours, minutes, and seconds), one revolution (360°) being equivalent to 24 hours, or 1 hour being equivalent to 15°.
In other words, it is analogous to meridians or longitudes on a globe.

The declination of an object as observed on the celestial sphere is the angle measured along the hour circle of that object to the celestial equator.

Sometimes reference is made to the hour circle of a planet or fixed star. This is an imaginary line or circle lying perpendicular to the celestial sphere which runs through the planet or fixed star in question.

See also