John Pazmino
 NYSkies Astronomy Inc
 2008 August 31 initial
 2009 October 4 current
    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 
    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 
    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 
    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 
    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
    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