DECANATES
-------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2008 August 31 initial
2009 October 4 current
Introduction
----------
From time to time I get an inquiry about the 'decans' or
'decantes' of the zodiac. I didn't until recently take a closer look
at this curious feature, but it seems that there are two major and
distinct meanings for the term. The older sense comes from ancient
Egypt as a celestial clock. A newer definition comes from Greek
astronomy and astrology.
Egyptian decanates
----------------
There is a dense litterature about the use of decans by Egypt but
one crucial piece is always, as if deliberately, omitted. I give here
a summary of the intent of the decans, since there is enough
discussion in material about Egyptian timekeeping.
By day Egyptian kept time by the Sun using sundials or other
shadows. At night, with no shadows, they used the stars. They already
employed the zodiac as we know it from the babylonians and we have
reasonable correct depictions of it in temples. The star groups off of
the zodiac were peculiar to Egypt. Because their star charts were
schematic it is tricky away from the zodiac to precisely correlate
their groupings with the real stars. even the zodiac was schematic,
but at least there we have a rich history for the location of the
figures among the stars. We assume the Egyptians kept close to that.
The idea at night was to spot certain groups of stars as they rose
and look up on a chart to know the associated hour. Such charts were
plentiful, like on the lids of coffins and in temple murals. They
index every ten days thru the year to account for the seasonal
migration of the stars.
At nightfall, a certain set of stars rose, so that marked the
first hour of full night. Every 40 minutes (of modern time) the next
set rose, so there were many chances at night to get a time fix. As
the seasons advanced, the groups rose earlier in the night, until the
first ones were already up at nightfall and new ones were rising near
dawn.
We know the names of these groups and have pictures of the deities
they represent. There were 36 of these groups, spaced 40 minutes apart
around the sky. Probably by custom the 'first' in sequence or rank, of
the groups was the star Sirius.
Note well that these groups, the decanates of Egypt, were NOT in a
belt around the sky like a zodiac. They were supposedly easily
recognized patterns, asterisms, that could be well separated in
latitude or declination but rose somewhere along the eastern horizon.
making things more uncertain is that the rising in twilight was
impeded by the bright sky. A brighter pattern would be more easily
spotted than a dim one. Hence it is virtually certain the decans were
irregularly spaced over the sky and included asterisms of various
prominence.
Now comes the killer. I have never and never found any positive
delineation of these decans! The very best I found were pictures of
the creatures they stood for and their names. Except for Sirius and
maybe the Belt of Orion and Pleiades, I can find no other definite
chart or list of these asterisms.
Playing with a planetarium doesn't help. I set it to Cairo (a mid
point in ancient Egypt) and to the Middle Kingdom (about 1600BC),
Stepping thru the night at 40 minute intervals reveals a hodge-podge
of plausible asterisms. None are obvious and many are rising far from
a 40-minute mark. A few may correlate to the schematic skymaps but not
for sure.
This is a bit frustrating because at starparties and other
allnight observing, a system of modern decantes could be a useful
celestial clock! It would be easier to understand than the attitude of
the Big Dipper or the location of stars in the zenith. It would be nice
to note a pattern rising and check a chart. 'It's 03:30 and I didn't
find that galaxy cluster yet.'
So, you'll read LOTS about what a decan is supposed to be and do
but, unless you're a better hunter than I, you won't so easily
discover WHAT stars constituted each decan. Once this is sussed out,
it is trivial to precess the sky to today and shift the latitude and
develop a new set of decans.
From what I found in my playing around with a planetarium is that
there seems to be no obvious set of asterisms that rise at uniform
intervals, whether 40 minutes, an hour, or any other comfortable span
of time. I can get a few, then nothing for many hours. This is
allowing for faint groups, like those likely visible from a darksky
site and ignoring the twilight periods.
Egyptian astronomy
----------------
In just about all cultures, astronomy like other learned pursuits,
was engaged in by the educated class of the population. Even today,
astronomy is a personal endeavor is very much a niche market in our
culture. Recall the typical fanatical and sometime futile effort to
attract new astronomy club members or attendance at public sessions.
When we speak of 'Egyptian astronomy' we mean the profession
practiced by a special corps of people, trained and exercised to look
after the heavens. They were employed by the government and were duly
paid and cared for.
The man-in-the-street had nothing to do with the stars, except for
an ignorant contemplation of them. He relied on the government
officials to tell him what's coming down the track as foretelled in
the stars and other natural occurrences. The layman was concerned with
workaday chores. Maybe he sat by the Nile in a summer breeze to admire
the Moon or a bright star. It was the king's agents who did the real
study of the heavens.
Yet, most of what I read about the Egyptian decans seem to
describe them as a means for ordinary folk to mark the hours of the
night. I read, for example, that 'an Egyptian traveller could check
the horizon for the next decan and keep track of his progress'.
If this was the intent of the decans, distinct from some official
use of them in the king;s court, we have one hell of a sensational
aspect of astronomy history! It could be the one flash of popular
appreciation of astronomy in human history!!
To recognize ANY specific pattern of stars is a challenge for
modern people, Consider the hassle to tutor starparty visitors in
star-id. Factor in the diurnal rotation and seasonal drift, and, well
you get the picture. The layman simply accepts what you tell him and
goes home.
Are we to believe that 4,000 years ago an Egyptian peasant or
toiler actually recognized 36 star patterns and tracked them thru the
nights and seasons? Did he really hang on his icebox a chart of these
decans?
This is all the more astounding since many of the asterisms could
be, to our mind, made of only dim stars. Could you hand out a
starchart and sincerely believe the starparty visitor will know how to
apply it on his own?
An other question is: why look for a rising decan? Isn't is easier
to ride a decan in the west until it sets? You can far more reliably
find a star group already in the sky and watch it drift to the horizon
than to anticipate a group in the east that is only partially in sight
until it fully clears the horizon on rising. The answer may be partly
in the geography of the Nile River.
Egyptians were fanatics about the east, where the Sun rose and
began the new day. The west was the side for the end of life, where in
fact the necropolites were built. The east side of the Nile was
favored to live on, not the west.
Also, looking east in the northern part of Egypt obtains a low,
about ideal, horizon over flat terrain. Only low hills intervene
between the Nile and the Red Sea. To the west the Nile has cliffs that
block the ideal horizon. Only be scaling them can you see the ideal
horizon over the western Egypt desert. I can hardly imagine a person
climbing the cliffs just to know what time it is.
Greek zodiac
----------
In Greek astronomy, things firmed up a lot better. Thanks to the
handing down to us of so much Greek litterature, we have pretty full
understanding of what those folk were doing with the stars. First, for
calculation sake, they systematized the zodiac into 'signs' of 30
degrees each, starting from Aries at the vernal equinox. This removed
the problem with calculating within the varied lengths of each
constellation along the zodiac.
The one major gotcha is that some astronomers numbered the degrees
of a sign from 0 thru 29 while other numbered them 1-30. The former is
mathematicly correct because when a planet just crosses the frontier
from one sign to the next, it is the zeroth degree, like the zeroth
hour of a day right after midnight. You do have to mind this '0 or 1'
custom when interpreting observations cited in the sign-degree system.
An other glitch is that some astronomer when rounding their
positions to the degree either truncate or bump to the nearest degree.
A truncated longitude keeps the planet in its proper degree. The
minutes and seconds are stripped off. For a rounded longitude the
planet is moved to the closest whole degree, where its location is
better preserved. It's likely not all that critical for casual
skywatching, the error being at most one degree.
The alignment of signs with constellations slipped over the
centuries due to precession. As long as you treat the signs as merely
names of th zodiac zones, there's no problem. It's like the names of
our months of the year. December is no longer the tenth month; it's
the twelfth. No one campaigns to rename the months!
Greek decantes
------------
The Greeks adopted a system of divination from the stars practiced
in Babylonia, where a planet sends down to Earth certain influences
according to where it stands in the zodiac. This developed into
astrology, but in that time it was commingled with astronomy and
shared many common features with that profession.
Somewhere along the road, the signs were divided into third parts,
each 10 degrees long, and imparting additional properties to a planet
standing in them. As the farther planets were discovered, Uranus thru
Pluto, they were fitted into the scheme, too. I don't know about the
other farther bodies like Sedna or Eris.
In addition, the Greeks formalized the whole sky into 48
constellations, the core set we still use today. with 12 in the
zodiac, the other 36 were elsewhere in the heavens. It became
convenient to associate three of the other constellations with each
zodiac constellation, and moreover with each of the third parts of the
signs. Thus each of the 36 third parts, the decanates, 'rules' one
other nonzodiac constellation.
As far as i can discover, the nonzodiac constellations were never
schematized into some kind of 'sign' such as bands of latitude away
from the ecliptic.
Precession displaced the classical signs from their original
subsidiary constellations, but most astrologers (and astronomers) keep
the classical associations for tradition sake. Most astrologers do not
factor in the nonzodiac stars and there seems to be no credible lore
for them in horoscopy. About the only remanent of nonzodiac influence
is the charming names given to several nebulae and clusters visible by
eye and known to the ancient world. The Andromeda galaxy is named
'Vortex', for example.
The decantes are also about 10 days long for the orbital travel of
the Sun thru the ecliptic. Hence, each has a date range.
I note the classical constellations and the modern set that
accounts for precession. There being no formal modern decanates I
indexed the classical ones by one zodiac sign to better align them
with the present constellation figures.
-------------------------------------------------
solar dates : planet : classical : current
--------------- ------- ----------- ----------
Aries ---------------------------------------------
Mar 21 - Mar 31 : Mars : Triangulum : Cepheus
Apr 1 - Apr 10 : Sun : Eridanus : Andromeda
Apr 11 - Apr 19 : Jupiter : Perseus : Cassiopeia
Taurus
Apr 20 - Apr 30 : Venus : Lepus : Triangulum
May 1 - May 10 : Mercury : Orion : Eridanus
May 11 - May 20 : Saturn : Auriga : Perseus
Gemini ----------------------------------------
May 21 - May 31 : Mercury : Ursa Minor : Lepus
Jun 1 - Jun 10 : Venus : Canis Major : Orion
Jun 11 - Jun 20 : Uranus : Ursa Major : Auriga
Cancer ----------------------------------------------
Jun 21 - Jun 30 : Moon : Canis Minor : Ursa Minor
Jul 1 - Jul 11 : Pluto : Hydra : Canis Major
Jul 12 - Jul 22 : Neptune : Argo Navis : Ursa Major
Leo ------------------------------------------------
Jul 23 - Aug 1 : Sun : Crater : Canis Minor
Aug 2 - Aug 12 : Jupiter : Centaurus : Hydra
Aug 13 - Aug 22 : Mars : Corvus : Car, Pup, Vel PYx
Virgo ------------------------------------------
Aug 23 - Sep 1 : Mercury : Bootes : Crater
Sep 2 - Sep 12 : Saturn : Hercules : Centaurus
Sep 13 - Sep 22 : Venus : Corona Bor. : Corvus
Libra ------------------------------------------
Sep 23 - Oct 2 : Venus : Serpens : Bootes
Oct 2 - Oct 13 : Uranus : Draco : Hercules
Oct 14 - Oct 22 : Mercury : Lupus : Corona Bor.
Scorpio -----------------------------------------
Oct 23 - Nov 1 : Pluto : Ophiuchus : Serpens
Nov 2 - Nov 11 : Neptune : Ara : Draco
Nov 12 - Nov 21 : Moon : Corona Aus. : Lupus
Sagittarius ------------------------------------
Nov 22 - Dec 1 : Jupiter : Lyra : Ophiuchus
Dec 2 - Dec 11 : Mars : Aquila : Ara
Dec 12 - Dec 21 : Sun : Sagitta : Corona Aus.
Capricorn -------------------------------------------
Dec 22 - Dec 31 : Saturn : Cygnus : Lyra
Jan 1 - Jan 10 : Venus : Delphinus : Aquila
Jan 11 - Jan 19 : Mercury : Piscis Aus. : Sagitta
Aquarius ------------------------------------------
Jan 20 - Jan 29 : Uranus : Equuleus : Cygnus
Jan 30 - Feb 8 : Mercury : Pegasus : Delphinus
Feb 9 - Feb 18 : Venus : Cetus : Piscis Aus.
Pisces ------------------------------------------
Feb 19 - F 28-29: Neptune : Cepheus : Equuleus
Mar 1 - Mar 10 : Moon : Andromeda : Pegasus
Mar 11 - Mar 20 : Pluto : Cassiopeia : Cetus
-------------------------------------------------
Conclusion
--------
Apart from the historical curiosity the Greek decantes have no
utility today. They may have enjoyed little utility in classical
astrology as well. The Egyptian decantes, on the other hand, could be
handy way to know the hour at night, given a clear view to the eastern
horizon.