The North Star
The precession of the earth
and the changing pole star over 26,000 years.
Image credit:
A star immediately above either pole will appear to be stationary while other stars will appear to rotate around it. It is called a pole star. Since the ancient Zoroastrians lived in Central Asia which lies in the Northern Hemisphere, the 'fixed' star that they observed was above the earth's north pole. The present north star is commonly called Polaris. However, the specific star that is the north star changes in time because the earth slowly wobbles, a wobble called the precession of the earth.

Precession is the wobble of a rotating body around its axis is the manner a wobbling top. The earth wobbles very slowly around its centre. The axis is the imaginary line that runs through the centre of the earth and connects its two poles. We can extend the imaginary axial line into space beyond the poles. The axis of the earth not only tilts in relation to the earth's orbit around the Sun, but it wobbles as well.

This tilt and wobble and the consequent change in the angle the Sun's rays hit the earth, creates a change in climate in the short term of one orbit around the Sun, one year, and in the long term wobble, about 26,000 years.

Axial Precession of the Earth
The angle of the earth's tilt from the axis is approximately 23 degrees 27 arc minutes or say 23.5 degrees.

Presently, the precession takes place at a rate 50.27 arc seconds per year. At this rate the imaginary line extending from the pole will shift 1° (= 60'x60" = 3,600") in 3,600/50.27 = 71.6 years (say 72 years) and map out a complete circle in up in the sky in 360°x60'x60"/50,27 = 25,780 years (say 26,000 years).

Zodiacal Age
If the precession of the earth means that in 25,780 years the Sun will rise on the spring equinox at the same place in the zodiac, the passage of the Sun through each of the twelve sectors of the ecliptic in which a zodiacal constellation resides can be taken as a measure of time - a zodiacal age of 2,148 years, say approximately 2,150 years.

We believe that early Zoroastrian astronomers used the zodiac not so much as a means of prophecy, but as a means of measuring time and maintaining a calendar. The so-called Zoroastrian astrology of the world, is more an attempt to record history and learn from the lessons of history that is both cyclical in theme and linear in progression - an optimist progression.

Changing North Stars

Because of earth tilts on its axis and slowly wobbles (as it continues its rotation and orbit around the Sun) around its axis, a line drawn through both poles will trace out a cone in the skies above the poles. The specific star that becomes the pole star will change over time.

The brilliant Vega in the constellation Lyra was the North Star around 12000 BCE and will again become the North Star around the year 14000 CE. However, it is not a 'stationary' North Star as it never comes closer than 5° to the axial line extending into space, into the celestial sphere, from the earth's North Pole - the North Celestial Pole.

Gamma Cephei (also known as Alrai) situated 45 light-years away, will come closer to the North Pelestial pole than the present North Star Polaris around 3000 CE. Iota Cephei will become the pole star some time around 5200 CE.

Precession of the Equinoxes
The two points at which the ecliptic intersects the celestial equator (in the imaginary celestial sphere) are called nodes or equinoxes. Because of the precession of the earth, the nodes or equinoxes will gradually move along the ecliptic and this movement is called the precession of the equinoxes or the precession of the equator.

Retrograde Precession of the Equinoxes
Because the earth's precession occurs in the opposite direction to the earth's orbit, the ecliptic and the celestial equator move in opposite directions. In other words during the year, as the Sun 'moves' westwards through the sectors, the precession 'moves' in the opposite direction i.e. eastwards. This opposite precession of the equinoxes relative to the 'movement' of the Sun is called a retrograde precession.

Precession of the Vernal Equinox Through the Zodiac
In Western astrological terminology, the spring or vernal equinox presently 'resides' in the sign of Pisces (this terminology differs in Zoroastrian Astrology where a sign of the zodiac occupies a sector that changes in time - see World Horoscope). However, because of the retrograde or backwards precession of the equinoxes, the equinoxes have been moving backwards through the zodiac.
Precession of the vernal equinox. Image Credit: Wikipedia
The variable size sector boundary system.
Image credit: Covertrees
In the images above, The red line is the path traced by the Sun through the Earth's year. The red/green line is the Earth's equator. The crossing point of these two lines is the spring or vernal equinox.

In 1500 BCE the equinox was near the end of the constellation of Aries. In 500 BCE it was near the beginning of the constellation of Aries. In 150 CE (the time of Ptolemy) it was in the constellation of Pisces.

The are differences in the allocation of the vernal equinox to a sector of the zodiac (Western terminology) depending on how the size of the sector is allocated. One method is to assign each sector 30°, the Zoroastrian method, and to then calculate when the sun crosses the boundary of the sector. Using this method, according to one such calculation, the Age of Aries began in c. 2150 BCE and ended in c. 1 BCE. However, while the number of degrees assigned to each sector is a constant 30°, there is no consensus as to the Greogrian calendar equivalents of these ages.

The other method is to draw a sector boundary depending on the size of the constellation. This results in the sectors being of different sizes. Using this method, one interpretation is that the Age Aries began c. 1875 BCE and ended c. 100 CE.

Precession position of the vernal equinox through the constellation over a 6000 year span. Image credit: Wikipedia.