The graphic is intended to lighten up distance-determination of fixed stars as it was done at the beginning of the 19th century. Exclusively to precise telescopes, it was possible to calculate from astronomic measurements the distance "only" for fixed stars which are not too far from the solar system (around 10 light years).
Such fixed stars in a position as shown above can be recognized by the fact that they move move on an ellipse over the year in the observation sky ("Celestial sphere") next to very distant unmovable stars (10,000 light years, here "a" and "b".
The principle of the "parallactic effect" can already be seen when looking at the position of the thumb on an outstretched hand in front of a distant background with only the left eye, then only with the right eye.
Astronomically, one uses two positions of the earth way arround sun that are half a year apart. The diameter of the earth's orbit is the basis of an extremely acute triangle with the fixed star at the top. The graphic is therefore not true to scale and only shows the principle.
The actual angle of the acute triangle is about 1/12,000 degrees.
Determining this small angle was a challenge for astronomers like Bessel (61CygA/B; Königsberg / East Prussia), Struve (Wega; Dorpat / Estonia) and Henderson (aCent; Cape Town / South Africa) around 1800. Friedrich Wilhelm Bessel published first.