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date: 03 August 2020


A Dictionary of Astronomy
Ian RidpathIan Ridpath

An astrometry satellite launched by the European Space Agency in 1989 August. The name is an acronym for High Precision Parallax Collecting Satellite. It measured the position, brightness, proper motion, and trigonometric parallax of 118 218 stars down to 12th magnitude to an accuracy of better than 0″.002. It did so by measuring angular separations between pairs of stars with a 0.29-m telescope as it scanned the sky continuously about a slowly varying rotation axis. Simultaneously, the satellite’s star mappers measured the magnitude and colour of over a million stars down to about 10th magnitude, plus their positions to about one-tenth the accuracy of the main survey; these observations were used to construct the ... ...

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