नक्षत्रान्येवर्क्चन्द्रमाः साम तदेतदेतस्यामृच्यध्यूढं साम तस्मादृच्यध्यूढं साम गीयते नक्षत्राण्येव सा चन्द्रमा अमस्तत्साम ॥ १.६.४ ॥
In plain words
The constellations are the Ṛc and the moon is the Sāman. This Sāman rests upon that Ṛc; therefore the Sāman is sung as resting upon the Ṛc. The constellations are sā and the moon is ama, and that makes sāman.
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
नक्षत्राणामधिपतिश्चन्द्रमा अत स साम ॥
In plain words
The moon is the lord of the stars, and therefore he is the sāman.
Close to the Sanskrit
The moon is the lord of the stars, hence he is the sāman.