(Not a reply to anyone in particular....)
It's important to distinguish between the zodiac signs, and the corresponding constellations.
A zodiac sign is one of twelve equal parts of the great circle called ecliptic.
This is the path that the Sun follows against the background of stars. The moon and the planets also travel more or less near this circle.
At some point in time, these twelve parts roughly coincided with the twelve constellations they are still associated with, but of course these constellations are not all the same size, and in fact the ecliptic crosses a thirteenth one: Ophiuchus, the snake holder.
The ecliptic has a reasonably stable orientation in space, but its zero point, which is one of the two points where the celestial equator crosses it, moves. Thus, the locations of the twelve equal parts which are the zodiac signs also move with respect to the stellar background.
This effect (precession) was already reasonably accurately measured at the time of Ptolemy. There has been some controversy among historians of science, since Ptolemy claimed to have measured the star positions in his catalogue himself, while it can be argued quite convincingly that he took them from Hipparcus, and just moved them taking into account the precession for the interval of time since Hipparcus.
By now, the zodiac signs no longer coincide with the constellations they were named after, but this effect was accuractely predictable already 2000 years ago (or more).
It's important to distinguish between the zodiac signs, and the corresponding constellations.
A zodiac sign is one of twelve equal parts of the great circle called ecliptic.
This is the path that the Sun follows against the background of stars. The moon and the planets also travel more or less near this circle.
At some point in time, these twelve parts roughly coincided with the twelve constellations they are still associated with, but of course these constellations are not all the same size, and in fact the ecliptic crosses a thirteenth one: Ophiuchus, the snake holder.
The ecliptic has a reasonably stable orientation in space, but its zero point, which is one of the two points where the celestial equator crosses it, moves. Thus, the locations of the twelve equal parts which are the zodiac signs also move with respect to the stellar background.
This effect (precession) was already reasonably accurately measured at the time of Ptolemy. There has been some controversy among historians of science, since Ptolemy claimed to have measured the star positions in his catalogue himself, while it can be argued quite convincingly that he took them from Hipparcus, and just moved them taking into account the precession for the interval of time since Hipparcus.
By now, the zodiac signs no longer coincide with the constellations they were named after, but this effect was accuractely predictable already 2000 years ago (or more).