Definition : Saptarishis in or indicating a nakshatram as per Vishnu Puranam
Reference : Vishnu Puranam Page : 334; 4th Amsa – 24th Adhyaya.
Time of this shlokam : Based on the shlokas that this verse is with, we can assume that it was written after the coronation of Nanda. See : Parikshit to Nanda – 1050 years – Vishnu Puranam.
It is relatively easy to understand the terms, when the sun or moon are in this nakshatram or the other. See : Nakshatras
It is much harder to understand how a set of 7 stars outside the zodiac belt can be ‘in’ or can ‘point to’ one of the fixed stars. I reproduce this diagram and explanation from Saptarishi Calendar. The first diagrams shows how different pairs of the saptarishis point to different nakshatrams. The second picture shows how different pairs of nakshatrams rise first during different seasons.
Which Nakshatram is presently indicated by the Sapta Rishis?
As you can see in the image below, different nakshatra-pairs of the Saptarishis indicate different nakshatras. In the picture below, it is not alpha and beta, but gamma and delta that point to Regulus or Makha Nakshatram, today. Eta indicates Arcturus (Swati) (yellow line). The pink line from Swati takes you to Spica (Chitra). The green line from epsilon takes you to Denebola (Uttara/Uttara Phalguni). Of course the line joining the alpha and beta line always points to polaris (Dhruva), north pole star.
Image source: http://stars5.netfirms.com/ourse.jpg
The Saptarishis rotate in the same direction as the 27 nakshatrams but much more slowly because they are closer to the axis through the pole star (Dhruva). So it appears as if they were “moving backwards” with respect to the fixed stars. Every hundred years it appears as if they were indicating the previous nakshatram. ie 25 years per nakshatra pada. (3.3333 deg), which is observable in a single life time. In fact Varahamihira quotes Vrddha Garga on this.
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