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It has been suggested that because the original list of 27 nakshatras contains only 24 distinct names, these represent the 24 half months of the year. Later, as astronomy developed further, the naks.atra list was expanded to describe the motions of the moon.<ref name=":9" />
It has been suggested that because the original list of 27 nakshatras contains only 24 distinct names, these represent the 24 half months of the year. Later, as astronomy developed further, the naks.atra list was expanded to describe the motions of the moon.<ref name=":9" />
== Astronomical Evidences Related to Nakshatras<ref name=":9" /> ==
The earth's axis of rotation is tipped at an angle of 23 1/2 degree with respect to the direction of its orbital motion around the sun. This is what causes the changing seasons because the length of the day keeps on varying. The longest and the shortest days, also called summer and winter solstices, occur roughly near the 21st of June and and 21st December, respectively. The date of a solstice can be marked by noting that around this date the sun appears to linger at the same extreme at dawn. The days when the days and nights are equal are called equinoxes. The two equinoxes, vernal in spring and autumnal in fall, mark the halfway points between summer and autumn. The equinoxes occur at the two intersections of the celestial equator and the ecliptic. The motion of the moon is more complex since its orbit is inclined approximately 5 degree to the earth's orbit around the sun, and the earth's gravitation perturbs the moon in its orbit. The resultant precession completes a cycle in 18.61 years.
Due to the precession of the earth's polar axis the direction of the north pole with respect to the �xed background stars keeps on changing. The period of this precession is roughly 26,000. Polaris (� Ursae Minoris) is the Pole star now but around 3000 B.C.E. it was � Draconis which was followed later by � Ursae Minoris; in C.E. 14000 it will be Vega. The equinoxes and the solstices also shift with respect to the background stars. The equinoxes move along the ecliptic in a direction opposite to the yearly course of the sun (Taurus to Aries to Pisces rather than Pisces to Aries to Taurus and so on).
The vernal equinox marked an important day in the year. The sun's position among the constellations at the vernal equinox was an indication of the state of the precessional cycle. This constellation was noted by its heliacal rising. The equinoctial sun occupies each zodiacal constellation for about 2200 years. Around 5000 B.C.E. it was in Gemini; it has moved since into Taurus, Aries, and is now in Pisces. The sun spends about 13 1/3 days in each nakshatra, and the precession of the equinoxes takes them across each nakshatra in about a 1000 years.
Thirteen and a half nakshatras ending with Vishakha were situated in the northern hemispheres; these were called devanakshatras. The remaining nakshatras ending with Bharani that were in the southern hemisphere were called yamanakshatras (yama: twin, dual). This classi�fication in Taittiriya Brahmana (1.5.2.7) corresponds to 2300 B.C.E.
As mentioned above, the list beginning with Krttika indicates that it was drawn up in the third millennium B.C.E. The legend of the cutting off of Prajapati's head suggests a time when the year began with Mrgashirsha in the �fifth millennium B.C.E. Scholars have also argued that a subsequent list began with Rohini. This view is strengthened by the fact that there are two Rohinis, separated by fourteen nakshatras, indicating that the two marked the beginning of the two half-years.
Shatapatha Brahmana speaks (21) of a marriage between the Seven Sages, the stars of the Ursa Major, and the Krttikas; this is elaborated in the Puranas where it is stated that the rshis remain for a hundred years in each nakshatra. In other words, during the earliest times in India there existed a centennial calendar with a cycle of 2,700 years. Called the Saptarshi calendar, it is still in use in several parts of India. Its current beginning is taken to be 3076 B.C.E. On the other hand, notices by the Greek historians Pliny and Arrian suggest that, during the Mauryan times, the calendar used in India began in 6676 B.C.E. It is very likely that this calendar was the Saptarshi calendar with a beginning at 6676 B.C.E.22
Around 500 C.E., a major review of the Indian calendar was attempted by astronomers. Aryabhata, Varahamihira and others used the nakshatra references that the Saptarshi were in Magha at the time of the Mahabharata war to determine its epoch. Aryabhata declared the war to have occurred in 3137 B.C. (the Kaliyuga era begins 35 years after the war), and Varahamihira assigned it 2449 B.C.E. It has been suggested that this discrepancy arose because the change in the number of nakshatras from the earlier counts of 27 to the later 28 was differently computed by the two astronomers. It is quite likely that the fame of the Kaliyuga era with its beginning assigned to 3102 B.C.E. prompted a change in the beginning of the Saptarshi era to about the same time, viz. to 3076 B.C.E.
The shifting of seasons through the year and the shifting of the northern axis allow us to date several other statements in the books.(23) Thus the Shatapatha Brahmana (2.1.2.3) has a statement that points to an earlier epoch where it is stated that Krttika never swerve from the east. This correspond to 2950 B.C.E.
Maitrayaniya Brahmana Upanishad (6.14) refers to the winter solstice being at the mid-point of the Shravishtha segment and the summer solstice at the beginning of Magha. This indicates 1660 B.C.E.
Vedanga Jyotisha (Yajur 6-8) mentions that winter solstice was at the beginning of Shravishtha and the summer solstice at the mid-point of Ashlesha. This corresponds to about 1370 B.C.E.
It should be noted that these dates can only be considered to be very approximate. Furthermore, these dates do not imply that the texts come from the corresponding period; the text may recall an old tradition. A chronology of the Vedic period by means of astronomical references was attempted by the historian of science P.C. Sengupta.(24) Amongst other evidence, Sengupta uses the description of the solar eclipse in RV 5.40.5-9 to fi�x a date for it. Unfortunately, this work has not received the attention it deserves.
The changes in the beginning of the Nakshatra lists bring us down to the Common Era; at the time of Varahamihira (550 C.E.) the vernal equinox was in Ashvini.<ref name=":9" />
== References ==
== References ==
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