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Estimating distances from parallaxes
Parallax is defined as the reciprocal of distance. But when we measure a parallax, this measurement is always noisy, and its simple reciprocal is not necessarily a good estimate of the distance. In fact, it has been known for some time that once the fractional parallax error is more than about 20%, the reciprocal parallax is a rather poor and biased estimate of the distance. This is an issue for Gaia, because about 80% of all stars which Gaia observes will have fractional parallax errors larger than 20%.
Credits: C. Bailer-Jones
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