Most people meet the Minor Arcana as a memory problem. Fifty-six cards, four suits, ten numbers and four court ranks, all needing meanings. The Golden Dawn's answer was not to memorise them but to derive them — and the derivation runs through the sky.
Thirty-six and thirty-six
The zodiac divides into twelve signs of thirty degrees. Each sign divides again into three decans of ten degrees, giving thirty-six divisions of the year, each about ten days long. The decans are old — considerably older than tarot, going back to Egyptian star clocks — and by the Hellenistic period each had acquired a planetary ruler.
Now count the numbered pips. Two through Ten in each of four suits is thirty-six cards. The Aces are excluded, because in this system an Ace is the root of its element rather than a station in the year.
Thirty-six decans, thirty-six pips. The Golden Dawn mapped them one to one, in order, starting at the first decan of Aries and running round the wheel.
How the mapping runs
The pips follow their sign's element. Aries is fire, so the three Aries decans take the Two, Three and Four of Wands. Taurus is earth, so it takes the Five, Six and Seven of Pentacles — continuing the numbering rather than restarting it. Gemini takes the Eight, Nine and Ten of Swords. Cancer picks up the Two, Three and Four of Cups, and the cycle continues until Pisces closes it with the Ten of Cups.
Separately, each decan has a planetary ruler drawn from the seven classical planets in the Chaldean order — Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter — cycling continuously from the first decan of Aries. Because thirty-six is not divisible by seven, the cycle never repeats against the signs, which is why every sign gets a different combination of rulers.
Where the pip titles come from
Combine the sign and the ruler and you have the card's character. The Golden Dawn gave each pip a title accordingly. The Two of Wands is the first decan of Aries, ruled by Mars — Mars in its own sign, the most concentrated fire in the zodiac — and its title is Lord of Dominion. The Ten of Swords is the third decan of Gemini under the Sun: Lord of Ruin. The Five of Cups is the first decan of Scorpio under Mars: Lord of Loss in Pleasure.
Those titles are not decoration. They are the reason a Five is difficult in every suit and the reason the Ten of Cups and the Ten of Swords — the same number, the same position in the arc — end up at opposite emotional poles.
What it is actually useful for
Two things. First, timing: because each decan covers about ten days of the calendar, a pip can be placed against a date rather than only a theme. The decan pages list every one of them with its dates, ruler and card.
Second, coherence. Once the mapping is in place the Minor Arcana stops being fifty-six arbitrary meanings and becomes a structure you can reason about. If you know that a card is the middle decan of Leo under Jupiter, you can say something sensible about it before you have remembered anything at all.
One caveat worth stating: the outer planets rule none of the decans. The system predates their discovery and has never been retrofitted, so Uranus, Neptune and Pluto touch the trumps but never the pips. If you are doing timing work with the decans, you are working with the classical seven and nothing else.