Thread by @edwardW2

On decans
1.The decans are one of the oldest parts of astrology—you will find them in ancient Egyptian texts. They are still used today, mostly in the context of essential dignity.

2.For starters, a decan is 1/3 of a zodiac sign. As each zodiac sign contains 30 degrees, it follows decans are 30/3=10-degree stretches of each sign. (hence the name)

3.So, for example, if the Moon is in 12 Cancer, she is said to be in the 2nd decan of Cancer (the 2nd decan of Cancer runs from 11-20 Cancer).

4. Some people have the idea that decans only apply to the Sun. This is unprincipled. In western astrology , the positions of *all* the planets in the decans are noted. In fact, without doing so, the system of essential dignity would basically collapse

books.google.com/books?id=cExRA…

5.To these decans, astrologers have assigned all manner of significations and associations. Let’s go through them

6.The first system, which is conventional in western astrology, is called the “Chaldean system”. It works rather simply by assigning the decans to the planets, in the following order:
Saturn—Jupiter—Mars—Sun—Venus—Mercury—Moon—Saturn--Jupiter (etc)

The system starts with the 1st decan of Aries being assigned to Mars, and the decans following in order

8.The first decan of Aries (i.e. 1-10 Ari) is assigned to Mars.
The 2nd decan of Aries (i.e. 11-20 ari) is assigned to the Sun
The 3rd decan of Aries (i.e. 21-30 Ari) is assigned to Venus
The 1st decan of Taurus (i.e. 1-10 Tau) = Mer
The 2nd decan of Taurus = Moon
Etc.

9.Oddly enough, by the time you end up with the final decan—the last 10 degrees of Pisces, it is also assigned to Mars. So mars ends up ruling 2 consecutive decans.

10.This is the method that ends up being used in places like William Lilly’s Christian astrology, and in most of western astrology. However, there are other systems.

11.In one system, the decans are not assigned to planets, but rather to signs.
This system is the one used in the Brihat Jataka, and has since been adopted in western astrology...archive.org/details/brihat…

12it is the system found in Mach and McEver’s only way to learn astrology, vol 2.

12.The method by which this system works is rather more complicated, but bear with me. This method is based on triplicities, or zodiac signs sharing the same element...

13/
The first decan of the sign is ruled by the sign itself.
The next decan of the sign, is ruled by next sign of that triplicty, going in order.
The last sign is the third sign of that triplcity, again going in order.

14.To give a concrete example:

The first decan of Aries is ruled by Aries
The second decan of Aries is ruled by Leo (The next sign of the triplicity)
The third decan of Aries is ruled by Sagittarius (the third sign of the triplicty,)

15.Another:
The first decan of Leo is ruled by Leo
The 2nd decan of Leo is ruled by Sgr (the next fire sign after leo),
and the 3rd decan is ruled by Aries.

16.This method found popularity in the west through the 20thc; you still find it listed as “traditional decan ruler” under astro,com, even though for western astrology the planet assignations are longer in use

(IIRC Bonatti hated this system lol)

17.This may be due to the fact that you can admit the outer planet rulers using this system, something you cannot do with the planet-based system. So, for example, the three decans of cancer:

Cnc 1: Cancer= Moon
Cnc 2: Scorpio = Pluto
Cnc 3: Pisces= Neptune.

<I should add that this works with all the signs; so the three Decans of Aries are ruled by Mars, the Sun, and Jupiter, respetively, as they are the rulers of Ari, Leo and Sgr.

https://twitter.com/edwardW2/status/1374704413424558082

This comes to the next question, of how are the decans used. Turns out, quite a lot. The most familliar use is in the astrological system of essential dignity.

For those unfamilliar with what that is, it is a system used to tabulate how strong a planet is

You may be familliar with the idea of planets ruling signs, so the Sun rules Leo; the moon rules Cancer etc.

In this context, a planet that is in the sign it rules is strengthened.

However, planets rule bits of signs, which includes the decans.

So, for exmaple, if Venus was in the 2nd Decan of Aries, she may have been in Mars' sign, but she is in her own Decan. It follows, rather than being entirely at the mercy of Mars, she does have some strength.

Thing is, the decans are reckoned to be the weakest possible level of essential dignity in western astrology. granted, it is good enough to prevent the planet from having absolutely *no* dignity at all, which is an even worse state

William Lilly rather evocatively described a planet having only essential dignity on the decan level as being like a man "about to be turned out of doors" books.google.com/books?id=cExRA…

In any case, the traditional method is to add up *all* the levels of dignity a planet happens to rule, no matter if they are the sign, triplicity, bound or decan, to see how strong a planet would be.

For more about essential dignity, see these articles by @DeborahHoulding web.archive.org/web/2020021817… <at time of writing, the original is for some reason, down>

But there are other uses of decans too. One method of predicting global events using astrology (a practise called mundane astrology) utilised eclipses. By looking at which decan a solar or lunar eclipse occurred, you could tell something of its effects.

the famous Early modern astrologer William Lilly provided a list of such predictions, which you can see here edwardwhite123.blogspot.com/2020/06/lillys…

So, for example, on 12th dec 2021, there will be a solar eclipse. The sun and moon would be in 12 Sagittarius 51; or in the 2nd decan of Sgr. William lilly would tell us this would indicate death amongst large domestic animals... oh dear

But possibly one of the more interesting traditions attached to the decans are that of the images that are associated with them. One of the more interesting examples comes form Johannes Angelus' "Astrolabium planum",books.google.com/books?id=aTMST…

It was translated into english as "astrological opticks" in the 17thc, and you can see it here. edwardwhite123.blogspot.com/2020/03/esoptr…. In addition to giving the images of the decans, it also gives characteristics of the decans (eg fortitude, nobility, etc)

Another extensive section of decans comes form the notorious compedium of magic, the Picatrix, an excerpt of the Attrell and Porreca translation is shown below

<Cw: Rape>

Some of the delineations are quite disturbing; for example, the third decan of libra is a face of "evil works, sodomites, adultery", and even more disturbingly the 3rd decan is of scorpio is one for "Joining oneself by force with unwilling women"

The Picatrix then provides another set of decans (p 120 of the Attrell and porreca translation) that it calls the "Adorguen", which corresponds to the sign-based system mentioned above. This system is used for the making of talismans, as you can see below.

The Brihat Jataka in turn provides another set of images of decans that you can see below. We are told that such images have natal import: archive.org/details/Brihat…

The decans mentioned here are divided by a fairly elaborate system of classification, so there are male and female decans; human and bestial decans, as well as "armed" decans. From such information, electional as well as horary (i think) advice can be gleaned: