Firstly it is not an association with a modern astrological concept of decans ( cardinal, fixed and mutable )
The decans originally started out as a way of keeping track of time. In the earliest civilizations the only observable constant was the rotation of the earth (or the passing overhead of the heavens) which was measured by the rising and setting of certain stars or groups of stars. Also these stars or asterisms were associated with deities.
Their importance lingered in astrology for centuries. . In later times the concept of star-deity or star-beings survived in talismanic magic (e.g., the Picatrix), but in astrology they became only minor dignities and played special roles only in astrological medicine and physiognomy.
Originally, the Egyptians associated 36 constellations with the calendar . They would note what stars were rising with the sun. After ten days, they’d consider it the start of the next decan (but that doesn’t quiet work as there are 365 days in the year not 360 … so the calendar would get out of sync).
When the Greeks synthesized prior traditions and innovations of their own, the zodiac became more common, and the decanal system was incorporated onto it. The decans became part of astrology, with three decans of 10 degrees each being assigned to each zodiacal sign.
The decans also seem to have been associated with star deities during this period. It was in Hellenistic astrology (from the first few centuries B.C. until approximately the 1st century A.D.) that the decans played a prominent role in astrology. The Roman astrologer Firmicus Maternus (3rd - 4th century A.D.) says the decans have “infinite power and freedom in indicating the fates of men.”
Some of the decans were considered lucky or unlucky. Also, the decan that was rising on the horizon at birth was taken to indicate whether a native would have a good or bad life. This was the basis for the later Ascendant and the concept of the astrological houses.
There were two versions of rulership given to the decans in the ancient world: the ‘Chaldean System’ rulership, and rulership by Triplicity. The Chaldean rulership orders the planets by planetary speed, and associates them in order with the decans, starting with Mars as the ruler of the first decan of Aries, and then working around the zodiac:
Chaldean Decans
Aries Decan 1 Mars
Aries Decan 2 Sun
Aries Decan 3 Venus
Taurus Decan 1 Mercury
Taurus Decan 2 Moon
Taurus Decan 3 Saturn
Etc.
[ The scheme of rulership by Triplicity assigns the sign ruler to the first decan of any given sign, and then the domicile/sign rulers of the other two signs of that element to the remaining ones, in order.]
As time went on, the decans declined in their significance for astrologers.
Moving closer to understanding the associations with Tarot we have a period where people used ‘The Images of the Decans’ or the ‘Faces’ of the decans, the Greeks, Hindus, and Babylonians attributed a human or divine figure to each one. Later, the images were used for magical purposes (i.e., the Picatrix, and Agrippa).
I use three versions of the images associated with the decans:
• Ibn Ezra’s description of “the Hindus’” images. These are more motley images, often featuring animals and humans with animal parts.
• The Picatrix’s images. These often include the animals or unusual features of “the Hindus’” images, but come with more explicit meanings similar to Agrippa’s.
• Agrippa’s versions of the images, along with what they signify. These lose their animal symbolism, are more streamlined, and come with specific meanings relating to human emotions and pursuits.
Some have compared the descriptions to the constellations that rise with the decans, as well as the two systems of decan rulership (by Chaldean order or speed, and by the rulers of the Triplicity), but have so far been unable to find consistent reasons why the images and meanings are what they are, both between different decans and in terms of how the three sources differ. The answer may lie in the divinities associated with the decans.
[There is also the issue of ‘memory theatre’ ie. an image (the more unusual the easier to remember) stays in the memory and is easily more accessible (for many ), these images were associated with astronomy / astrology as a memory aid as to the perceived energy of stars or asterisms in a decan.
Memory Theatre:
https://www.google.com.au/search?q=..._ioATMv4GQDw&ved=0CAcQ_AUoAQ&biw=1220&bih=497
http://en.wikipedia.org/wiki/Giulio_Camillo ]
First Decan of Aries
• Ibn Ezra: A head in the form of a dog with a candle in its left hand and a key in its right hand. Banbakha says it is a Moor; his eyes are black, and his eyebrows are straight, and he is of the giants’ race. He is self-laudatory and is wrapped in a large white cloak with a rope girdle on it; he is irascible and stands on his feet.
• Picatrix: The form of a black man, restless and great in body having red eyes and holding a cutting axe in his hand, girded about with white garment; and there is great worth in this face. And this is a face of strength, high rank, and wealth without difference.
• Agrippa: A black man, standing and clothed in a white garment, girdled about, of a great body, with reddish eyes, and great strength, and like one that is angry; and this image signifieth and causeth boldness, fortitude, loftiness, and shamelessness.
In some instances one can see a similarity between the decanic image and the meaning of a card that is associated with it.
Sometimes things just match anyway; that is, a modern astrological view (what Mars in Aries might mean to a modern western astrologer) may match up with a cards meaning. Sometimes it doesn’t, then what I do is ; I look at the individual stars that are in the decan (yes individual stars have predictive and associative meanings ) and construct the energy of the decan. Sometimes I cannot find a correlation but mostly I have. [ So it seems the image on the card and its meaning can relate to a descriptive meaning, but often the star association meaning can relate to the title . ]
So …. How did they do it? Its relatively simple; Aces are unity and match with the Princesses as a type of Alpha and Omega ‘outside’ the system, that leaves 2-10 of each suit and 36 cards. There are 36 decans so …. 9 cards of each element (4x9=36) – three of each go to one of the three signs OF that element (the first 3 wands –fire – go to the first sign ruled by fire, the next three wands go to the next sign ruled by fire and so on). So that ‘staggers’ the numerical and suit progression of the cards as they are attributed around the zodiac/decan wheel, as after Aries (fire) is Taurus (earth), then Gemini (air) Cancer (water) and so on (the suits progress in reverse of normal order ( fire water air earth.
So, yes, in a way they just did lay them out using association and common sense), but by laying them out they associated them with the planets that annote each decan and that is the order of speed (how fast they appeared to move across the sky).
Two systems of Decans
Decans Chaldean System Triplicity System
Aries 1 Mars Mars
Aries 2 Sun Sun
Aries 3 Venus Jupiter
Taurus 1 Mercury Venus
Taurus 2 Moon Mercury
Taurus 3 Saturn Saturn
Gemini 1 Jupiter Mercury
Gemini 2 Mars Venus
Gemini 3 Sun Saturn
Cancer 1 Venus Moon
Cancer 2 Mercury Mars
Cancer 3 Moon Jupiter
Leo 1 Saturn Sun
Leo 2 Jupiter Jupiter
Leo 3 Mars Mars
Virgo 1 Sun Mercury
Virgo 2 Venus Saturn
Virgo 3 Mercury Venus
Libra 1 Moon Venus
Libra 2 Saturn Saturn
Libra 3 Jupiter Mercury
Scorpio 1 Mars Mars
Scorpio 2 Sun Jupiter
Scorpio 3 Venus Moon
Sagittarius 1 Mercury Jupiter
Sagittarius 2 Moon Mars
Sagittarius 3 Saturn Sun
Capricorn 1 Jupiter Saturn
Capricorn 2 Mars Venus
Capricorn 3 Sun Mercury
Aquarius 1 Venus Saturn
Aquarius 2 Mercury Mercury
Aquarius 3 Moon Venus
Pisces 1 Saturn Jupiter
Pisces 2 Jupiter Moon
Pisces 3 Mars Mars