Recently I’ve gotten interested in what has been called “astronomical astrology” or “true sidereal astrology” which is a form of sidereal or fixed-star-based astrology in which the planets are located in the actual constellations or star groups as they appear in the sky. For example, if you were born with the Moon in the middle of the group of stars we call Leo, your Moon would be considered a “Leo Moon” rather than being a Moon in the tropical sign Cancer. Constellational astrology is a kind of “what you see is what you get” form of the art, which is based strictly on direct observation of the position of planets in the sky against the backdrop of the fixed stars.
While both tropical and sidereal astrology divide the zodiac into equal 30-degree signs, “constellational astrology” partitions the zodiac circle (or path of the Sun) into thirteen constellations of unequal size. Some constellations are bigger than others and occupy more space along the zodiac. In addition, the foot of the physician snake-bearer constellation Ophiuchus crosses the zodiac circle, so that Ophiuchus is allotted a portion of the zodiac circle. The image below shows how Ophichus occupies about 18.6 degrees of the ecliptic (the blue line) between Scorpio and Sagittarius.
Ophiuchus is associated with the myth of Asclepius , the noted healer of Greek legend, but he may also be related to the Babylonian god Nirah, a serpent deity whose upper half was human and lower half consisted of serpents for legs (not unlike Sagittarius whose upper half is human and lower half is that of a horse).
As mentioned, each IAU designated constellation will occupy a distinct portion of the ecliptic circle. The following lists gives the number of degrees along the ecliptic that is allotted to each of the 13 zodiacal constellations (note that these are star-groups rather than the traditional “signs” of the zodiac which are always 30-degrees in length). The listing is compiled for the date J2000.0 and gives the number of ecliptic degrees in each constellation along the ecliptic, the duration in days and the date on which the Sun enters that constellation. In the IAU conventions for boundaries of zodiac constellations, the Sun spends only 6 days passing through the Scorpio portion of the ecliptic, which is rather bizarre and probably indicates how out of touch the IAU astronomers who developed this system were with the history of astronomy.
- Aries: 24.7303 degrees (lasts ~ 25 days, starting around April 18 Gregorian)
- Taurus: 36.7229 degrees (lasts ~ 37 days, starting around May 14 Gregorian)
- Gemini: 27.8479 degrees (lasts ~ 28 days, starting around June 21 Gregorian)
- Cancer: 20.0504 degrees (lasts ~ 20 days, starting around July 20 Gregorian)
- Leo: 35.8124 degrees (lasts ~ 36 days, starting around August 10 Gregorian)
- Virgo: 43.9593 degrees (lasts ~ 45 days, starting around September 16 Gregorian)
- Libra: 23.2372 degrees (lasts ~ 23 days, starting around October 31 Gregorian)
- Scorpio: 6.5905 degrees (lasts ~ 6 days, starting around November 23 Gregorian)
- Ophiuchus: 18.5999 degrees (lasts ~ 19 days, starting around November 29 Gregorian)
- Sagittarius: 33.4184 degrees (lasts ~ 34 days, starting around December 17 Gregorian)
- Capricorn: 27.8315 degrees (lasts ~ 29 days, starting around January 19 Gregorian)
- Aquarius: 24.1635 degrees (lasts ~ 24 days, starting around February 16 Gregorian)
- Pisces: 37.0368 degrees (lasts ~ 38 days, starting around March 12 Gregorian)
For dates other that J2000.0, the values in the above listing for the number of degrees per constellation along the ecliptic will vary slightly due to precession as the ecliptic will tilt slightly with respect to the Earth’s equator. For example, in the year 0, Ophiuchus occupies 18.5672 degrees along the ecliptic. By the year 1000, Ophiuchus was covering 18.5834 ecliptic degrees; in the year 2000, 18.5999 degrees; and in the year 3000, Ophiuchus, due to the tilting of the Earth’s axis in its precession, will span 18.6167 degrees.
Now that we have established the sizes of the IAU designated constellations, a standardized measure agreed to among astronomers, all that remains is to determine where to start the zodiac, that is, which point to choose as 0 degrees of the constellation Aries. As usual, there are differing opinions about which value to select as the ayanamsa (the difference in degrees between the starting point of tropical and the sidereal zodiac).
For example, one site by Bruce McClure, using the boundaries for constellations set by the International Astronomical Union in the 1930s. notes that on 19 April 2019 the Sun entered the constellation Aries at 29.09 ecliptic degrees of tropical sign Aries. This corresponds to a chart set for 11:33 AM BST in Greenwich, UK, on 19 April 2019 (as calculated in Solar Fire v9.0.26). In the sidereal zodiac with the popular Lahiri ayanamsa, this would place the Sun at 4 Aries 58 sidereal. Thus, if we were to add 4 degrees 58 minutes to the Lahiri ayanamsa for the year 2019, we would place the Sun at 0 Aries sidereal.
Another site, in a video by sidereal astrologer Athen Chimenti, uses a “custom” ayanamsa in which the fixed star Beta Aries (Sharatan) is set to occupy 2 Aries 15 in the “true” sidereal zodiac. This star near the beginning of the star-group Aries was at 3 Taurus 58 tropical in the year 2000. With the Lahiri ayanamsa this star would lie at about 10 Aries 07 sidereal. If we were to add about 7 degrees 52 minutes to the Lahiri ayanamsa for the year 2000, it would place Beta Aries at 2 Aries 15 sidereal.
Athen Chimenti also uses values that differ some from the IAU constellation boundaries. Because the constellations along the ecliptic may overlap or have small gaps between them, Chimenti uses the midpoint between constellation boundaries as the demarcations points between them. Because the boundaries are not sharply defined, he considers the region within 3 degrees of such boundaries to reflect a blend of both constellations. The following list gives the approximate dates for which the Sun traverses each constellation in Chimenti’s system, which appears to be more consistent with observation of the heavens and the traditional constellation boundaries than the somewhat arbitrary IAU divisions.
Looking at the sky with the program Stellarium, we can see the issue more clearly. According to the sky maps, the Sun in the year 2020 should enter the constellation Aries around April 20, 2020, as can be seen in the following images, which show the transiting Sun moving along the ecliptic at a position around the end of the constellation Pisces and the beginning of the constellation Aries.
These images are set for the period when the Sun is leaving the constellation Pisces and entering the constellation Aries. On April 20, 2020, the star Alrescha at the end of Pisces has a tropical ecliptic longitude of 29d 40m 04s. At the same time the earliest star in constellation Pisces on this map, Mesathrim, has an ecliptic longitude of 33d 28m 23.8s. Thus, the boundary between the constellations Pisces and Aries should lie somewhere between these values. If we were use the midpoint between these two stars, we would place the boundary between Pisces and Aries at 31d 34m 14s of ecliptic longitude on 20 April 2020. In the sidereal zodiac with the Lahiri ayanamsa this boundary would like at 7 Aries 26 of the Lahiri sidereal zodiac. Thus, if we were to add 7 degrees 26 minutes to the Lahiri ayanamsa for the year 2020, we would place the Sun at 0 Aries sidereal in this star map. This value which we derived from Stellarium is very close to that used by sidereal astrologer Athen Chimenti, namely a “custom” ayanamsa in which the fixed star Beta Aries (Sharatan) is set to occupy 2 Aries 15 in the “true” sidereal zodiac as a standard of reference.
Below is a section of a chart cast in the “true” sidereal zodiac with Beta Aries set at 2 Aries 15 as a reference point.
Here is the same chart as the one above but cast in the tropical zodiac with Placidus houses.
Author Vasilis Kanatas, who wrote about about incorporating the constellation Ophiuchus into the zodiac circle, gives a somewhat different set of values than those discussed above. According to Kanatas, the number of days the Sun spends in each constellation as it travels around the ecliptic are as follows:
Number of Days in Each Sign
Aries: 25.5 days
Taurus: 38.2 days
Gemini: 29.3 days
Cancer: 21.1 days
Leo: 36.9 days
Virgo: 44.5 days
Libra: 21.1 days
Scorpio: 8.4 days
Ophiuchus: 18.4 days
Sagittarius: 33.6 days
Capricorn: 27.4 days
Aquarius: 23.9 days
Pisces: 37.7 days
The following table gives the values suggested by author Vasilis Kanatas for the year 2000.
For further comparison of the “true sidereal” chart with the tropical and sidereal zodiacs, I compared a chart cast in true sidereal with the Kanatas ayanamsa in the program Prometheus with its tropical and Lahiri counterparts. The chart is cast for 30 April 2017 at Noon EDT in NYC. Note the unequal sizes of the 13 constellations along the ecliptic.
In the above “true sidereal” chart (Kanatas ayanamsa), the Sun lies at 11 Aries 33. The tropical chart calculated for the same moment and location in Solar Fire has the Sun at 10 Taurus 29, a difference of 28 degrees 56 minutes between the “true sidereal” and the tropical positions. The same chart cast in the sidereal zodiac with the Lahiri ayanamsa places the Sun at 16 Aries 23, a difference of 4 degrees 50 minutes from the Kanatas value. For comparison, below is the same chart (30 Apr 2017) cast in the tropical zodiac with Placidus hosues.
It appears that different authors are in the same ballpark but are not in strict agreement about when to start the “true sidereal” zodiac (0 Aries sidereal) or about the size of the portion of each of the 13 constellations that lies across the zodiac circle.
Addendum (5 May 2020): The two systems in common use appear to be those of Athen Chimenti and Vasilis Kanatas, so I put together a table comparing them. The table lists the starting point of each “true sidereal” constellation as measured in ecliptic longitude from the Vernal Equinox of 19 March 2020 and also gives the span along the ecliptic of each sidereal constellation in each system. Chimenti’ takes into account the overlap and gaps between constellations on the ecliptic and uses the midpoints of their boundaries as points of demarcation. He claims that his measurements are more in sync with the actual view of the sky. Kanatas appears to be using the IAU astronomical conventions for the sizes of the constellations. I also included the mean value of the Chimenti and Kanatas systems.
Because the boundaries between the actual constellations in the sky are somewhat a matter of opinion, Chimenti advises considering planets within about 3 degrees of the boundaries as having the properties of both constellations. As you can see in the table below, the greatest area of disagreement appears to be for the constellations Scorpio and Ophiuchus. In my opinion Chimenti’s approach is more consistent with traditional constellation boundaries that the IAU-based approach of Kanatas.
Addendum (12 May 2020):
In addition, I came across an excellent video by Gemini Brett which critiques the constellation boundaries drawn by IAU and used by NASA, especially the area of the zodiac where Scorpio and Ophiuchus overlap. It is a video well worth watching. The discussion of constellation boundaries begins a little over an hour into the video.
Finally, I also found a listing of constellation boundaries and sizes in the book Scientific Hindu Astrology by P.S. Sastri, Vol. I, p.187 (2001), which I have copied and pasted below:
All original material in this blog is copyright Anthony Louis 2020.