Periods of seasons-enigmatic chapter

1
I wonder why Labban choses these degrees to start thge seasons:

Chapter Twelve: On the Periods according to the Views of the Ancients1
t
[1] To the ancients, who were the foremost people in this art concerning the judgment
of great events, there was a foundation and basis which they called periods.
The period consists of three hundred and sixty solar years. They regarded a year
as consisting of four seasons.
[2] The beginning of the first season is from the time when the Sun enters
twenty degrees and fourteen minutes of Pisces. In this place the anomaly of the
Sun is nine signs exactly according to the old Zijes. 2 Its mean <longitude> is
eleven signs and eighteen degrees,3 and here occurs the maximum equation which
is additive to the mean. The Sun begins rising from the mean <distance> of its
orb toward the place which the apogee follows.
[3] The beginning of the second season is from the time when the Sun enters
eighteen degrees of Gemini. In this place the Sun is at its apogee, th~ anomaly
is zero, and there is no equation belonging to it.
[4] The beginning of the third season is from the time when the Sun enters
fifteen degrees and forty-six minutes of Virgo. In this place the anomaly of the Sun
is exactly three signs according to the old Zijes, and its mean <longitude> is five
signs and eighteen degrees. The equation is at the maximum and subtractive from
the mean <longitude>. The Sun begins descending from the mean <distance>
of its orb toward the place which the perigee follows.
[5] The beginning of the fourth season is from the time when the Sun enters
eighteen degrees of Sagittarius, and in this place the anomaly of the Sun is exactly
six signs, and there is no equation belonging to it, and the Sun is in the perigee
of its orb.4
1The first part of this chapter (up to the end of [12]) was edited, translated, and discussed
in Yano [1984].
2'0ld Zfjes' here seems to mean some astronomical tables of Indian origin.
3This means that the solar apogee is located at 348? - 270? = 78?, which is the standard
value of the Sindhind derived from the Brahmapak~a of Indian astronomy.
4The relation of the four seasons and the solar longitude can be tabulated as:
season first second third fourth
anomaly 270? oo goo 180?
equation +2;14? oo -2;14? oo
mean longitude 348? 78? 168? 258?
true longitude 350;14? 78? 165;46? 258?
-[6] The first season consists of eighty-seven degrees and three quarters approximately,
5 and the days are in this number, and likewise is the second season.
The third season consists of ninety-two degrees and a quarter and the days are
in this number/ and likewise is the fourth season. Exactly in this manner they
established the periods and the parts therein, while a day of a season of a year
<corresponds> a year of a part of a period.
[7] Then they deemed the conjunction as having taken place two hundred
and seventy-six solar years before the Flood, and they regarded the beginning of
that year as the beginning of the period according to what I have said. They
made the ruler of the <first> period Saturn with the sign of Cancer, and <in> Saturn was then in Cancer. In this period there was the Flood after the
expiration of two hundred and seventy-six years. The tasyir reached the sign of
Cancer since the motion in each year since the beginning of the period was one
sign. 7 The motion of the periods is again one sign and one planet for each period.
The ruler of the second period was Jupiter8 with the sign of Leo, and the ruler of
the third period was Mars with the sign of Virgo, and so on in this arrangement.
[8]9 From the beginning of the period to the entry of the Sun into twenty
degrees and fourteen minutes of Pisces in the three hundred and eleventh Persian
year of Yazdagird, there expired four thousand and three hundred and twenty solar
years, <namely> there expired <exactly> twelve periods belonging to twelve
signs and twelve planets.10 The period arrived at Mercury with the sign of Cancer.
U
[9] They thought the first part of each period belonged to Mars together
with the ruler of the period, the second part to the Sun, the third part to Mercury,
and the fourth part to Saturn.12 The ascendant of the period is the ascendant at
the Sun's entry- <into> the place of Pisces which we mentioned13 -at each
beginning of a period. The ascendant of a year is the ascendant at its entry into
this place
5