I am constructing a Hertzsprung-Russel diagram for stars within a radius around Pleiades and have repeatedly come across stars that have negative parallaxes. For example, http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-info=XML&-out.add=.&-source=I/239/tyc_main&recno=161838
I found three reasons listed in the following article for where these values come from, but I do not fully understand why the values are quoted as negative.
When a distant star, whether bright or faint, is observed through a scattered cluster, or perhaps rather a layer, of relatively near and faint stars, a negative parallax must be expected.
The article goes on to say that this parallax is the positive parallax of the comparison stars with respect to the distant star.
Is the magnitude of this parallax the distance between the distant star and the comparison stars, or between the earth and the distant star?