'97 Hipparcos and Tycho catalogues: negative parallax distribution

NASA description of catalogues: HIPPARCOS - Hipparcos Main Catalog

Scatter plots from catalogues.
Size of dots is proportional to negative milliarcseconds of parallax.

d = df[df.parallax_mas < 0]
plt.scatter(d.ra_degrees, d.dec_degrees, -d.parallax_mas/3)
plt.title('Hipparcos: negative')

For Tycho catalogue half of object have big negative parallaxes so for plot only those with < 50 milliarcsec are included and magnitude is divided by 2000.

d = df[df.parallax_mas < -50]
plt.scatter(d.ra_degrees, d.dec_degrees, -d.parallax_mas/2000)
plt.title('Tycho: negative')

I don’t know how to interpret it.


Edit:

Tycho for positive parallaxes looks ~same

d = df[df.parallax_mas > 50]
plt.scatter(d.ra_degrees, d.dec_degrees, d.parallax_mas/2000)
plt.title('Tycho: positive')

Whole mission Hipparcos satellite was stuck on extremely elliptic geostationary transfer orbit (GTO) with period of ~10.5h.


Edit2:

To make it worse I’ll add an ecliptic line

Ok.

So they measured parallaxes using ICRS reference system, which „Its origin is at barycenter the of the Solar System”.

The ~small issue is - barycenter isn’t static. Here is position of Sun (or position of barycenter in relation to Sun) between AD2000-2050, in AU units.
s0

It’s mean speed in this time range is ~13m/s.

Why with the heliocentric model they are using dynamic barycenter coordinates?

Because the sun is always moving?

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