In -The Qabalistic Tarot- Rober Wang states on p.48:
Why is the commencement of the zodiac at 0 degrees Aries arbitrary, as it starts after the death of winter.
I am not sure about the second part of your sentence; do you mean it should NOT be an arbitrary point as it comes after winter and because of that it must be there? Then we need to examine why 'after the death of winter' is important .
It is arbitrary as it is a point in a circle and the 'start' of any progression around a circle must be arbitrary.
A few points;
Aries was considered to relate to spring and the vigorous energy of new birth and growth. It was originally marked by stars and asterisms , back when the European (western astrology) was set 'in stone' - written down in tables and books - ephemeris (around 300 ad - excuse the slackness here I am going off the top of my head at the moment) the equinoctal point appeared to be in a position that marked the beginning of the 'constellation' of Aries so the zodiac was 'set' here at this point (the intersection of two 'great circle' ; one based on the plane of the Solar system and one based on the axial tilt of the earth) ; 0 degrees Aries and then they went about marking off a sign, 12 each at 30 deg.
Nice and ordered but perhaps not based on the 'naked-eye' observation of the heavens.
But that point moves across the backdrop of stars (about 1 deg every 72 years ???) because the Earths axial tilt wobbles - so by now we are nearly 1 whole sign out from the constellations.
So if you are (by ephemeris and western tropical astrology - the most common type of western astrology - although all other types are not) ' a Capricorn' and you look at the sky on your birthday the planets and Sun will not be in the stars and constellations you thought ... as western astrology ignores them and uses signs that have no reality based on stella position but in 30 degree segment progressions from the equinoctial point - regardless of where that is in the sky (IMO a ridiculous concept).
Magic has a long tradition of working with the stars and planets in their actual positions in the sky. Since this equinoctial point moves it is not a good marker so another type of marker that doesn't move is required ... a star.
The best ones to use are bright ones, easy to find and near the ecliptic. Regulus in Leo is a good candidate for this (as is Vega in Virgo - there might be one or two more but I think these are the main / obvious ones).
Regulus is NOT the beginning of Leo, it is into Leo a bit , being the 'heart of the lion'. In a way it is not the start of the GD zodiac but the marker of the GD zodiac and being a little way in - it is 'set' at regulus . The start is 0 degrees Leo but it is set at Regulus ( 5 degrees Leo, I think, in the GD Zodiac??? ).
I assume the GD used it as it was one of the 4 main stars that marked the 4 parts of the years and Being the Golden Dawn (Solar) and considering Leo's association with the Sun it seems the obvious selection. Also the idea of 'dawn' would go with the 'spring star' and not one that represents winter.
[Wang and Manly Hall have written a lot of speculative ( IMO rubbish) about it but 2 things
It is an arbitrary point from a geometric or astrological view (for calculations)
It is not arbitrary from THAT magical point of view (it may be arbitrary from A magical point of view e.g. ' The Order of Sapphic Star Children' may validly use Vega ). ]
