Precession position of the vernal equinox through the constellation over a 6000 year span. Image credit: Wikipedia.
The Zodiacal Ages
There are twelve zodiacal or astrological ages named after each sign of the zodiac. A complete cycle of ages assigned to each sign of the zodiac takes approximately 25,800 years (actually 25,780 years though some references state 25,920 years).

An age of a particular sign is determined by the time spent by the precessional position of the vernal equinox within the sector allocated to that sign (also see our page on Precession). Alternatively, in can be thought of as the time spent within the sector allocated to a constellation of the zodiac by the rising Sun on the day of the vernal equinox. Because the light of the Sun dispersed in the earth's atmosphere will hide the stars at sunrise, the zodiacal constellation occupied by the Sun on the vernal equinox is best observed just before the light of dawn hides the stars (see our Images Page for what this might look like).

Zodiacal Ages
 Constellation   Year entering   Year exiting 
 Leo   10750 BCE   8600 BCE 
 Cancer   8600 BCE   6450 BCE 
 Gemini   6450 BCE   4300 BCE 
 Taurus   4300 BCE   2150 BCE 
 Aries   2150 BCE   1 BCE 
 Pisces   1 CE   2150 CE 
 Aquarius   2150 CE   4300 CE 
Given that the Zoroastrian zodiac relegates the constellations of the zodiac to sectors of 30° each (regardless of the size of the constellation), the age of each sector allocated to a constellation of the zodiac is estimated at approximately 2,150 years. It appears that some Zoroastrian texts round this figure to an even 2,000 years, sometimes calling the two millennia, the bimillennium, a millennium.

Nowadays, astrologers use a variety of different methods to calculate the ages - there is no consensus. The year of change between the Ages of Aries and Pisces is variously stated as 234 BCE, 0 BCE/CE, i.e.1 BCE/CE (favoured by Christians), 63 CE, 221 CE, 234 CE and 285 CE. Using the estimated centre-point of a constellation as a measuring point, we prefer the 0 BCE/CE option. As there is no 0 year in the Gregorian calendar, we will use 1 BCE/1 CE as the boundary years between Aries and Pisces based on which we have the following Gregorian calendar years for the zodiacal ages in the chart to the above right.
Zodiac Constellation and a time map. Note anticlockwise progression of the constellations
but a clockwise progression of the years.
For the purposes of outlining a rough cosmology of the world, ancient Zoroastrian astrologers rounded measurement of the ages to thousands of years, millennia, and spoke of events taking place in groups of three millennia.

However, in either the English/Western translations or in the writing of the medieval Zoroastrian texts we can see an obvious error, for each zodiacal age is called a millennium, i.e. a period of a thousand years rather than two thousand years.

In the Greater Bundahishn (GB) 5B 15 we have, "Again, there is this that till the advent of the Adversary, six thousand years of time had elapsed: three thousand years in spirituality, and three thousand years of materiality in purity; and those six thousand years were from the Ram/Lamb (Aries) up to the Ear of Grain (Virgo), and each constellation ruled a millennium (one translation/thousand years (another translation)." The closing phrase "each constellation ruled a millennium/thousand years" is either an error or an indication that we should read the 'millennium' / 'a thousand years' to mean 'two thousand years'. In the following quote we see confirmation of this assertion:

GB 5B 16: As the rule of the millennium came to the Balance (Libra), which is the (house of?) fall, the fall of the Sun, the Adversary entered from underneath.

We therefore propose here that the so-called 'millennium' in the translations of the Bundahishn means an astrological or cosmological age of 2,000 years.

[Regarding the 'advent of the Adversary' see our page on World Horoscope. The aggression of the Adversary, i.e. the attack of the hordes of Ahriman on Aryan kingdom of its first king Gaya Maretan (later Gayomart) is the singular primal event that defines the start of Aryan history.]

In the concluding chapter of both the Middle Persian Bundahishns [the Lesser (LB) and Greater (GB)], we see that the authors do indirectly assign a period of two thousand years to a millennium: