अमावस्यद्-अन्यतरस्याम्

The word अमा means 'along' or 'together'. अमा in composition with the root वस् 'to dwell', takes the affix ण्यत् in the sense of location of time. Optionally the Vriddhi is not substituted in such a case. The time or the day on which the sun and the moon dwell together in the same constellation is called अमावास्या or अमावस्या.

The final त in अमावस्यत् is indicatory and is for the sake of showing where should the proper accent fall.

The two forms अमावस्या or अमावास्या are the same word, and a fortiori a rule made with regard to one will be made applicable to the other. And to this effect there is a paribhasha which declares एकदेशविकृतस्यानन्यत्वम् ; 'that which has undergone a change in regard to one of its parts, is by no means in consequence of this change, something else than what it was before the change had taken place'. Therefore sutra4.3.30, though it, in terms, says अमावास्याया वा is made applicable to the form अमावस्या also.,