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Apparently, Hipparcos did not very well with douple stars (spectroscopic and close binaries).
This is became pretty obvious for me when I was playing with Celestia.. I wanted to see what Pleiades look like when viewed from side.. so I went to the same distance from Sol and looked at them and the cluster was grossly elongated along the line of sight (towards Sol), something I did not expect but I immediately concluded something went very wrong with measurement accuracy..
Hope Gaia will do better...
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