28-05-2025, 11:52 PM
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.
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.