Description
Setting circles consist of two graduated disks attached to the right ascension (RA) and declination (DEC) axis of an equatorial mount. The right ascension disk is graduated into hours, minutes, and seconds. The declination disk is graduated into degrees, minutes, and seconds. Since right ascension coordinates are fixed to the celestial sphere the RA disk is usually driven by a clock mechanism in sync with sidereal time. Locating an object on the celestial sphere with setting circles is similar to finding a location on a terrestrial map using latitude and longitude. Sometimes the right ascension setting circle has two scales on it: one for the Northern and one for the Southern hemisphere.
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