מה שקראתי ומצאתי מעניין, זה באמת העניין שמקור המזלות הוא בקונסטלציות של הכוכבים בשמיים, אבל קיימת שאלה אם מחשבים את המזלות במיקום קבוע ומדוייק (30 מעלות למזל, תמיד באותו תאריך של שנה שמשית) או ממש לפי הקונסטלציות של הכוכבים. לפי מה שקראתי, האסטרולוגיה המערבית (שאנחנו לומדים) מבוססת על הלוח השמשי tropical zodiac בעוד שהאסטרולוגיה ההודית/וודית משתמשת בלוח של מיקומי הכוכבים ממש sidereal zodiac. בגלל תופעה אסטרונומית מסויימת של כדור הארץ, הלוחות "זזים" אחד לעומת השני, וחוזרים לנקודת ההתחלה כל 26000 שנים. והתופעה הזאת נותנת את השנה הגדולה, ושמחלקים ב-12, זה נותן את החישוב של העידנים.

לעיל זה סיכום של הטקסט המקורי (באנגלית) מתוך ספר אסטרולוגיה בשם "Mapping the Psyche"

הנה הטקסט המלא

Zodiac signs versus constellations

It is worth knowing at this stage that the twelve zodiac signs
are not the same thing as the twelve constellations (or star groups). As
we have seen above, the signs of the zodiac are twelve equal 30º sections of
the ecliptic, related to the seasonal year and anchored to the four cardinal
points of the Sun’s annual journey. Western astrology, which is based on the seasonal
year, uses the tropical or seasonal zodiac. Indian or Vedic astrology uses the
sidereal (or ‘star’) zodiac, based on the positions of the stars in the actual
constellations, which are star groups of very different sizes. Owing to a
phenomenon caused by the earth’s ‘wobble’ on its axis, the two zodiacs have
gradually, over the last two thousand years or so, started to drift out of
alignment with each other.

Audience: 
Can you say more about this? I am not sure that I understand.

Clare: 
Perhaps the best way to imagine this is as two concentric circles, the outer
circle representing the backdrop of the stars, containing the twelve
constellations, and the inner circle divided into the twelve signs of the
zodiac, each 30º in length. Approximately 2000 years ago these two circles
lined up, with the beginning of the constellation of Aries and the beginning of
the zodiac sign of Aries in the same place. Since that time, these two circles
have been moving in opposite directions at the rate of 1º every 72 years. They
will not line up again for approximately 26,000 years. This 26,000-year cycle
has itself been divided into twelve ‘astrological ages’ of approximately 2,000
years each.

 For the last two thousand years, at the spring equinox
the Sun has in fact been rising against the constellation of Pisces and will
gradually begin to rise against the constellation of Aquarius, heralding the
start of the new astrological age referred to in the 1960s musical Hair
as the dawn of the Age of Aquarius. In another 2,000 years, when the Sun begins
to rise against the constellation of Capricorn, we will enter a new ‘Age of
Capricorn’, which doesn’t have quite the same ring to it. The astrological ages
have their own meanings too, although these are on a quite different scale.
Western astrology uses the tropical zodiac as its primary frame of reference,
and eastern, Indian or Vedic astrology uses the sidereal zodiac as its primary
frame of reference. The difference between the two zodiacs is now approximately
26º.

החפרנים ממש יכולים לקרוא הרבה כאן  http://en.wikipedia.org/wiki/Astrological_age