Definition
Navigators treat the sky as an infinite sphere concentric with the Earth on which bodies are located by declination and hour angle. The origin for star coordinates is the First Point of Aries, the ascending node where the sun crosses the celestial equator going north at the March equinox. Precession moves this point slowly westward, roughly 50 arcseconds a year, which is why star SHA and declination values must be taken from a current almanac.
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