The decans, originally developed in the 3rd millennium BCE Egypt, were incorporated into Hellenistic astrology more or less when the Age of Pisces was coming 'round (roughly speaking). Each decan spans 10˚ of the sky, and there are three per zodiac Sign. We know for sure that Sirius marked the beginning of the new year at the summer solstice. Below is an image depicting the three decans of Cancer.
However, other than possibly Orion and the Big Dipper, we don't know for sure which other stars or asterisms mark the other decans. There has been at least one list compiled to attempt to identify potential candidates. However, for fun, but also for potential use as a sky astrologer, I compiled my own initial, cursory list, using Stellarium.
First, I had to choose a time and place. Since the earliest decans appear on coffin lids during the 10th dynasty around 2100 BCE, I set my date for then, at the exact coordinates of that part of Egypt. However, it was immediately obvious that Sirius was not visible at that time, since it would have been hidden by the bright Sun's rays. The way the Egyptians used Sirius was this: when you could see Sirius rising in the east just before dawn, after several weeks of it being hidden by the day (called the heliacal rise), then that means the annual flooding of the Nile was imminent. It has been said that this coincided with the Summer Solstice, so I adjusted my date until these criteria were met. I had to guess exactly which date and time, since I am not sure when Sirius becomes invisible again before dawn. I chose July 16th, 2700 BCE. I also had to take into account that the other stars that I chose would not be as bright, so the Sun had to be pretty far below the horizon. I settled on the Sun at 10˚ below the horizon and Sirius just over the horizon. Then I fast forwarded 10 days, adjusted the time by up to a few minutes in order to keep the Sun at -10˚ altitude, and chose a bright star near the horizon and usually near the ecliptic, since, I imagine, that's what I would look for if I were an ancient priest responsible for the fate of a kingdom.
It is important to note here that while there are 360 degrees in a circle, or the Sun's path around the sky during a year, there are ~365.25 days in a year. Therefore, the Sun does not move forward exactly one degree per day. However, dawn only comes once per day. Therefore, by choosing 10 days per decan, we are left with five extra days at the end of the year, which throws off the decans. There is much to explore here, but not in this post. I will only say that this whole star list experiment could be redone to square these two separate circles, on the equinoxes and solstices, which would correct the drift every quarter.
Sometimes there was more than one star, and I had to choose somewhat arbitrarily. I often chose one that is most familiar, such as Algol, even though the bright star in Aries was a contender. The relationship of these stars to the decans is extremely rough, and only approximates the decan. Also, the star list would change from one location to another. As well, there has been some proper motion of the stars over the last 5,000 years.
Here is a short movie showing all 37 stars. If you expand the movie with the expand button, you can pause it at any point to see each star (or cluster, in the case of the Pleiades):
The idea is, that we could know which part of the zodiac the Sun (or other planets if nearby) is in, by identifying one of these bright stars' heliacal rise. It's potentially a practical tool, that helps to ground us into the sky, rather than always rely on our computers for astrological information. However, like I said above, it's preliminary at this stage, and would have to be tested. I am sure that many many changes would be made, if I ever live in a place with a low eastern horizon to try it out!
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