I remembered seeing this answer somewhere else. So I did a quick copy n' paste.
I agree with Eshelman. The sequence beginning with Regulus presented in Liber T is just a way to catalogue the cards.
...Ummm yeah, gota start somewhere ... again where is the beginning point of a circle? Starting at Aries is also 'just a way to catalogue the cards'.
Jim Eshelman said:
A couple of reasons. One, Mathers was an early advocate of a form of the Sidereal zodiac, though he was about 5° off on the correct starting point - he thought Regulus marked 0° Leo, whereas it's really at 5° Leo. In any case, he thought this marked the beginning of the actual zodiac.
So why is -5° Regulus the 'start' of Leo? To include the entire constellation of Leo? That won’t work either in a sidereal system i.e. start marking off 30° segments (from Jim's point) and see where that gets you, especially as you go through Libra and Scorpio. Jim offers no explanation as to why Mathers chose Regulus or why -5° Regulus is the start of Leo. Jim just seems to be making statements without explanations (or maybe he does explain elsewhere outside THIS quote)? Mathers didn’t just draw the name out of a hat. Look at the influence attributed to Regulus.
Jim Eshelman said:
But also, this allowed the discussion of the decanates to begin with a Saturn-ruled decanate. That's really no more than an organizational device for the monograph, of course.
But an 'organisational device with a reason; there is a system where it is believed that the 'cosmic' or 'constellational' or 'stellar' energies are then picked up by Saturn (traditionally the planet 'furthest out' or 'closest to the stars) and their energy and was passed on through the Solar system eventually to the Moon then the earth. So it could be for that reason as well (as the Leo Sun relationship) and Regulus being one of the four main stars (and what Regulus represents in that context).
Jim Eshelman said:
"The sequence has no relevance for the Tarot itself - it's just a catalogue device.
It may well have no significance to the Tarot just as attributing angels based on letters in an order in a sentence in a Jewish Scripture that even some Rabbis now say was written up as fiction i.e. not some Holy Word of God ... [as I heard a Rabbi say (sorry, can’t remember his name but it is in a doco called 'Kingdom of David')] ... 'Well, it may be just a story - but it’s a great story, it doesn’t matter if it is 'true' or not, it’s the message that counts and that message has helped the Jewish people to .... (Etc.)"
But if one is going to get into that stuff and its Tarot attribution (whether it is relevant or not is surely up to the individual) and why it is that way it might help to understand why that device was set up that way and not another." It's just a catalogue device" seems a bit of an oversimplification to me.
I think it is a catalogue device (just as listing the beginning with Aries is, but that starts the listing with Mars, and taking the above idea re Saturn into account ... why start with Mars) but maybe it is a device that was constructed for specific reasons (like some of the above)?