8.3.100
नक्षत्राद्वा
Operational rule vidhi
ष् is optionally substituted for the स् when the preceding word is the name of a Lunar mansion.
Word by word (padaccheda)
Left context नक्षत्रात् pañcamī · ekavacanaIndeclinable वा avyaya
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Full reading
नक्षत्रात् वा
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ष् is optionally substituted for the स् when the preceding word is the name of a Lunar mansion.
Thus रोहिणिषेणः or रोहिणिसेनः, भरणिषेणः or भरणिसेनः ॥ But not here शतभिषक् सेनः were the preceded letter is ग् ॥ These two sutras 8.3.99 and 8.3.100 are really Gana-sutras, being read in the सुषामादि Gana. The author of Kasika has raised them to the rank of full sutras.
All cerebral ष changes, when not referable to any specific rule, should be classified under this Sushamadi gana_.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.