Because of the enormous distances to the stars their parallaxes are very small and this is a common problem when measuring them. Even for the nearest star that angle is smaller than 1”. The parallax of a stars was finally measured 300 years after Copernicus.

In 1838, almost at the same time, the Russian astronomer Vasilij Struve, the German Fridrih Vessel and the Scottish Henderson succeeded in measuring the distances to Vega - 225x10 12 km, Alfa Centauri - 41x10 12 km and to 61 Cygni - 103x10 12 km.

From this time on the picture of the star sky has gained much more depth. New units have been accepted – the light year and the parsec. One light year (l.y.) is the distance, which the light covers for one year. That is about 9.5x10 12 km. With this unit the distance to the stars shown above is: to Alfa Cen - 4.3 l.y., 61 Cyg - 11 l.y. and Vega - 27 l.y..