nakshatras were quite important for determining times of the year, as can be seen in combination with Vedic mythology, and can also be used to determine how far back in history Vedic astronomy extended.
The myth of the god Janus shows both of these factors, the determination of the age of Vedic astronomy and different periods of the year. Janus had four heads, each of which represented a phase of the moon in Sagittarius (one of the nakshatras) which marked the four seasons. One head was the full moon (in Sagittarius) which gave the time of the spring equinox, another was the new moon, during which time the autumn equinox fell, still another was the half waning moon, marking the winter solstice, and finally came the head representing the half waxing moon, during which time came the summer solstice. From current knowledge of the movement of the sphere of stars surrounding the earth, it can be calculated that the observations leading to the myth of Janus were made around 4000 BC. Additionally, within the Rg Veda is a verse observing the winter solstice in Aries, which would have placed it at around 6500 BC.
It is possible to date the Rg Veda like this because a complete cycle in the procession of the equinoxes takes place either every 25,870 to 24,500 years according to modern astronomers (the exact time period is still disputed by modern day astronomers), meaning that the moon is only full in Sagittarius during the spring equinox every 25,000 years or so. Modern astronomers, however, were not the first to make the difficult calculations to discover the length of this cycle. The Vedics were able to do this and came up with the value of 25,870 years. How these ancient people were able to make these calculations,