Pranavopanishat 7

7

sūryamaṇḍalamābhāti hyakāraścandramadhyagaḥ |
ukāraścandrasaṃkāśastasya madhye vyavasthitaḥ ||

The 'a'-sound shines like the solar disc, situated in the midst of the moon; / the 'u'-sound resembles the moon, positioned in the middle of it;