The problem of using Astronomy in dating the Vedas, Ramayana, Mahabharata
Often times, scholars use the astronomical signs mentioned in our ancient texts to date those texts. The problem with this is that they do not provide definitive dates in themselves, but provide dates with repetitious cycles unless additional information is taken into account.
There are two equinoxes during the calender year: the spring equinox and the vernal equinox and are days when the light of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of the same length.
An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year. The one in March 20 is considered the spring equinox and the September equinox is the vernal equinox.
Procession of the Equinox
The precession of the equinoxes refers to the precession of Earth's axis of rotation with respect to inertial space. The positions of the equinoxes move westward along the ecliptic compared to the fixed stars on the celestial sphere. Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, during which time the equinox regresses over a full 360°.
In terms of people from earth observing the stars, it looks like the constellations of the zodiac move counter-cyclical to the earth's motion around the sun.
This journey over 25,765 years covers the 12 constellations of the sky with the sun rising on the vernal equinox in the same zodiac for approximately 2,150 years in each constellation.
Since there is also a precession of the earth's axis, the equinox dates keep changing every year, and when seen from a stellar background the constellation in which the sun rises on the vernal equinox also keeps changing over a period of time.
Dating based on the precession of the Equinox
During the Vedic age, the vernal equinox is considered to have occurred in the month of Margashira (named after a star lambda orionis in the constellation of Orion). Today, the sun rises in the background of Meena Rashi (or Pisces) as compared to Margasira (or when Orion was in Gemini).
This means that the zodiac for when the run rose
In our times today, the sun rises in the background of Pisces (Meena Rashi) on the vernal equinox (we will transition to Aquarius from Pisces in the year 2,150 AD) . Whereas in the vedic period it used to rise in the background of Gemini (Margasira/Orion is in Gemini).
There are 4 constellations (Gemini, Taurus, Aries, and Pisces) that the sun has traveled through between the time of the Vedas and today. Multiplying 4 by 2,150 gives us 8,600 years and multiplying by 3 by 2,150 gives us 6,450 years. Thus the date range within which the Vedic civilization existed is between 8,600 and 6,450 years ago or 6600 BCE to 5540 BCE. However, this by itself is not definitive.
As mentioned earlier, a single complete processional cycle takes 25,765 years; so the dates could be multiples of 25,675 years older than these dates. Taking this into account, the vedic age was
(N x 25,675)+8600 to (N x 25,675)+6,450 years ago where N >= 0.
Dating our texts
Luckily, the Vedas, Ramayana, Mahabharata, etc contain many description of the skys with locations of the stars as well as descriptions of which Manavatar, yuga, etc. they were in. All of these have to be taken into account to get to a date. Even then, some of the dates seem to go back to a time which contradict our understanding of the timelines of human evolution and human expansion out-of-Africa.