For all practical purposes, one probably need not go further back than Ptolemy who lived in the 2nd century CE. He reused the principles he knew from Hipparchus, but Hipparchus' work has not survived.
By this time, the twelve zodiac signs had been established, and were the basis for his star catalogue.

While nowadays celestial coordinates are based on the celestial equator, in those days they were based on the ecliptic. Precession was well understood and its size had been measured quite accurately. Every star had two coordinates and precession only caused a linear change in one of the two, namely the longitude. Longitude was measured as 'degree in zodiac sign'. 
While the stars are fixed, the areas covered by the zodiac signs gradually moved across the sky.
The intersection of the ecliptic and the equator marked the vernal equinox, the point where the sun was on 21 March, and where it took one year to return.

The gradual drift of this date over the centuries was the result of the fact that one leap day every four years was a good approximation to model the length of the year, but not exact.

While Ptolemy marked a stable point in the definition of al the above, this was also the basis of astronomical understanding all the way to the times of the Voynich MS.

In modern terms this is all very simplified, but it reflects the knowledge from the 2nd to the 16th (ish) centuries. Even Copernicus' work moving the centre of the solar system from the Earth to the Sun did not change any of this. This just eventually helped to understand why the planets move in the way they do.

The Ggregorian reform put the calendar back on the right track, but it had no significant impact on the choice which month best fits with which zodiac sign.
I have a vague recollection that the Voynich MS isn't quite unique in this, and similar exceptions exist but are rare. It could well be useful to find such examples. My 'vague memory' is known to be wrong from time to time.

This seems to me to be a good historical description except for one important nuance. The planets moved within closed, crystalline spheres. Even Copernicus believed this. These closed spheres are what may explain the tubes and all that plumbing we see in the Voynich.

If we are talking about the  marginalia on the vms zodiac illustrations, then i do not see the problem.
Zodiac signs are spread over 2 months, the Start month and the End month.
If you are only going to assign 1 month to each sign you need to choose either Start or End month.
Vms astrology marginalia person chose End month.

   

(29-05-2025, 03:10 PM)RobGea Wrote: You are not allowed to view links. Register or Login to view.If we are talking about the  marginalia on the vms zodiac illustrations, then i do not see the problem.
Zodiac signs are spread over 2 months, the Start month and the End month.
If you are only going to assign 1 month to each sign you need to choose either Start or End month.
Vms astrology marginalia person chose End month.

Yes, and that is indeed the month that contains the largest part of the sign's period as it is defined today.  But whoever wrote those month names could have used that rule only after the Medieval convention of "one sign = one month"  was abandoned in favor of the 'astronomically correct' convention astrologers use today, namely "one sign = one 30 degree sector counting from the equinox".

Wouldn't the two systems have existed simultaneously, depending on the type of text?

Seems to me this whole medieval astrology thing was hella complex.

Found this Masters thesis from 2013 by Irene Meekes - van Toer "Practical Advice in a Late-Fifteenth-Century Astrological Calendar Manuscript. Amsterdam UB MS XXIII A 8."
about a manuscipt in the Amsterdam University Special Collections Library.
The codex Amsterdam UB MS XXIII A 8 basically consists of a twelve-month calendar dating from about the last quarter of the fifteenth century.(1475-1500 AD)

The thesis itself is choc-a-bloc with medieval goodness and imho worth a read just for that.

I cannot find how the Zodiac signs were specifically dated but she does say this.

Quote:Tradition dictates that the transition from one sign to another take place in the middle of each month.
This is reflected in our calendar,
where we find the Latin formulation "Sol in [aquario etc.]"
written in the margin of each calendar month.
The rules for each sign are thus valid for a month at a time.

Thesis is in English. Link to its description (Msc link on that page)   -> You are not allowed to view links. Register or Login to view.

As I recall - from previous research of other investigators - there are examples of circular, astronomical / astrological charts where Pisces was labelled with March and so on. And most of the dating was later than the VMs C-14. That's how it would have been if there were a significant span of time between the VMs creation and the addition of "non-liturgical" month labels.

Welcome Jorge! 

The manuscript is full of labels, roots, "ladies", stars etc, yet something that is expected to have a label, is left bare. 
This in itself is very suspect to the dubious side of me. Obviously if we had these labelled in "voynichese" the text would presumably unravel.

JKP wrote about the Zodiac and labels here - You are not allowed to view links. Register or Login to view., it is from 2013 so maybe some of his thoughts have changed by now but either way it may be of interest to you, and others reading this thread. He notes the labels are French(ish) and 15th-17th century, it would be interesting to ask him if that could be narrowed any further now in his opinion, hopefully he comes back to the forum sometime soon.

Her is one of the sources that claim that Medieval astrologers equated each sign with one whole calendar month: You are not allowed to view links. Register or Login to view.  
I don't know whether those sources are to be trusted.  If they are, I don't know when the switch to the 'astronomically correct' periods took place...

The  Stammheim Missal { Ms. 64 (97.MG.21) } at the Getty museum seems to have the Zodiac - Months alignment we are looking for.

the Stammheim Missal was produced in Lower Saxony around 1170 at Saint Michael's Abbey at Hildesheim

Link is for  March as Pisces  -->You are not allowed to view links. Register or Login to view.

Link for full details ->You are not allowed to view links. Register or Login to view.