In reading the texts of Hellenistic and Persian astrologers, like Vettius Valens and Abu Ma’Shar, it becomes clear that in their case examples they are using the sidereal zodiac, which they inherited from the Babylonians. Even though Hellenistic astrologers around the time of Christ were aware of the findings of Hipparchus regarding the precession of the equinoxes, they persisted in using the sidereal zodiac.
For example, in the 1st century text De re rustica by Roman author Lucius Columella (4 – 70 CE), who wrote about agricultural topics, we find the following comment:
“Winter begins around 8 days before the first day of January, when the Sun is in the 8th degree of Capricorn. I am not persuaded by the argument of Hipparchus who teaches that the solstices and equinoxes do not occur in the 8th degree of the zodiacal signs but rather in the very 1st degree” (my translation into English).
Here we see a first century Roman agriculturalist, well acquainted with the seasons of the year, who rejects the tropical zodiac of Hipparchus and instead states his preference for a sidereal zodiac which locates the vernal equinox in the 8th degree of Aries (that is, between 7 and 8 degrees of Aries).
The ayanamsa of Usha-Shashi takes into account this difference postulated by Columella between the tropical and sidereal zodiacs. To illustrate this difference, I cast the vernal equinox for 50 CE in NYC in the tropical zodiac (on the left) and in the sidereal zodiac with the ayanamsa of Usha-Shashi (on the right) in the figure below.
As you can see, in the year 50 CE (at the time when Columella lived), in the tropical zodiac the Sun lies at 0 Aries 0 in the 1st degree of Aries, whereas in the sidereal zodiac the Sun lies at 7 Aries 03 at the beginning of the 8th degree of Aries.
Interestingly, in his book on annual revolutions, the Persian astrologer Abu Ma’Shar describes his own birth chart, which appears to be cast in the sidereal zodiac with what we would now call the ayanamsa of Raman. Here is what the vernal equinox of the year 50 CE would look like with the ayanamsa of Raman:
The above chart places the vernal equinox in the 5th degree of Aries of the sidereal zodiac in the year 50 CE rather than the 8th degree of Aries, which was in common use by Hellenistic astrologers of the first century CE.