Among the stars, the moon is most prominent at night, and thus the moon represents Krsna. BG 1972 purports

Expressions researched:
"Among the stars, the moon is most prominent at night, and thus the moon represents Kṛṣṇa"

Bhagavad-gita As it is

BG Chapters 7 - 12

Marīci is the controlling deity of the heavenly spaces. Among the stars, the moon is most prominent at night, and thus the moon represents Kṛṣṇa.

BG 10.21 (1972), Translation and Purport

Of the Ādityas I am Viṣṇu, of lights I am the radiant sun, I am Marīci of the Maruts, and among the stars I am the moon.

There are twelve Ādityas, of which Kṛṣṇa is the principal. And among all the luminaries twinkling in the sky, the sun is the chief, and in the Brahma-saṁhitā the sun is accepted as the glowing effulgence of the Supreme Lord and is considered to be one of His eyes. Marīci is the controlling deity of the heavenly spaces. Among the stars, the moon is most prominent at night, and thus the moon represents Kṛṣṇa.