http://www.csicop.org/si/show/the_voynich_manuscript_the_book_nobody_can_read/

It is hardly more illuminating to take a look at the astronomical and the cosmological sections, which contain pictures that can be identified as the Zodiac signs still familiar today (Aries, Taurus, Libra, and so forth). Scarcely another illustration in the Voynich manuscript is as unambiguous. Unfortunately, this observation does not result in further insight into the book's origin. The celestial bodies illustrated in the astronomical section cannot be identified and probably are only figments of imagination. Some Voynich researchers believe they recognize in these pictures the Andromeda fog or the Pleiades, but this again is just speculation.

Did you account for the relative change in position of the stars?  Constellations seen from earth wouldn't necessarily appear the same from another celestial position especially more distant from earth.