We need to first set the stage, The Indian astronomical model. The model was one of nakshatra-s and raasi. Our ancient Indians they divided the sky into several segments of 30 1/3° segments. They started from the eastern horizon said ‘let me divide the first 30 1/3° into the first nakshatra, the second 30 1/3° as the second nakshatra, the third and the fourth all the way to the western horizon’. But they didn’t stop there. If this was night sky they knew there was a day sky. They divided that portion, also, of the sky into 30 1/3° degree segments and they formed 27 of them. These are the 27 nakshatras. If you multiply 27 by 30 1/3 you will get 360°. Full 360°.
Now it is not enough to divide it into nakshatras. You must also be able to recognize them tomorrow. So what they did was the first 30 1/3, they said ‘what is the brightest star in that segment of the sky?’. That brightest star for example might have been Aldebaran. Aldebaran is the Arabic name. The Indian name is Rohini. So they said because Rohini is there, I am going to call it Rohini nakshatra. The next segment maybe the brightest star was Spika. Spika is the star which in India we call Chitra – Chitra nakshatra. Next we have perhaps Zeta Piscean. Zeta Piscean is Revati nakshatra. So this way every nakshatra was identified by the principle brightest stars present in that segment of the sky and identified accordingly.
So those are the 27 nakshatras that you see in this outer circle over here. They straddle the month, what you see over here is the nakshatra Chitra and below there is the Chaitra masa. So when the full moon appears over the Chitra nakshatra that month is the Chaitra month. That is how ancient Indians observed months and so on. Similarly for Falguni the Falguna masa, Magha and other ones. They also divided the sky in 30° segments called the raasi and these are all the familiar raasi-s that we know.
So, here is the listing of all the nakshatra-s in two of our ancient books the Vedanga Jyotishya and the Surya Siddhanta. Vedanga Jyotishya is conservatively dated to 1400 BCE because of the phenomena contained there. So that has got a listing of all the nakshatra-s. And these are those nakshatra-s. Surya Siddhanta is conservatively dated to 700 BCE or 400 current era. Take your pick, there is a lot of controversy over there. And they too contain a list of nakshatra-s which seem to match each other. And here are the principle stars. Kritika is Eta Tauri, Rohini is Alpha Tauri, Mrigashira is Landa Orionis and so on, all the way to Revati Zita Piscean and so on.