The inclusion of a 13th Zodiac in Astrology is just -NOT- true.
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Your zodiac symbol has not changed
Take note of this:
“In the Western or tropical zodiac, the Sun enters the tropical sign Aries the day of the vernal equinox each March. That’s the day that the Sun’s rays meet the equator directly overhead — the first day of spring in the Northern Hemisphere. (In the prior draft and in the audio I said ‘at a right angle’. Same idea.) The Sun enters the tropical sign Cancer when the Sun’s rays square the Tropic of Cancer — the first day of summer in the Northern Hemisphere, or summer solstice. The Sun enters Libra when the Sun’s rays square the equator again in September. The Sun enters Capricorn when the Sun’s rays square the Tropic of Capricorn each December, which is the first day of winter in the Northern Hemisphere (the seasons are reversed in the Southern Hemisphere).”
“You then take those four cardinal points and divide them equally and you have the 12 signs of the tropical horoscope. There are no ‘extra signs’ added — the tropical zodiac is a division of the 360-degree wheel of the year into 12 equal slices of 30 degrees. This is not rocket science — but it is science.”

To sum up:
Astronomers say, “3000 years ago, astrologers divided the zodiac equally into 12 sections which correlated with the 12 constellations. Today, due to shifts in the Earth’s axis, those constellations no longer correlate with the original dates. But tropical astrologers still use the original model, regardless of these physical changes.”
And the tropical astrologer replies, “we still use the original model, regardless of these physical changes. You stupid astronomers!”
The astrologer isn’t really disagreeing with the astronomer. He’s just denying that the facts make any difference to his beliefs.
The underlying thing your missing is that the ‘divided the zodiac equally’ part, while yet the constellations position changed do to our wobble the math to determine the segments of the zodiac has indeed, not changed.
While yes, he agree’s there is no denying the facts of the astronomer, the way the determination of the zodiacs in astrology have not changed.
The thirteenth Zodiac is Steve.