ब्रह्मणः सोम्य ते पादं ब्रवाणीति ब्रवीतु मे भगवानिति तस्मै होवाचाग्निः कला सूर्यः कला चन्द्रः कला विद्युत्कलैष वै सोम्य चतुष्कलः पादो ब्रह्मणो ज्योतिष्मान्नाम ॥ ४.७.३ ॥
brahmaṇaḥ somya te pādaṃ bravāṇīti bravītu me bhagavāniti tasmai hovācāgniḥ kalā sūryaḥ kalā candraḥ kalā vidyutkalaiṣa vai somya catuṣkalaḥ pādo brahmaṇo jyotiṣmānnāma || 4.7.3 ||
3. [The swan said,] ‘O Somya, let me tell you about one foot of Brahman.’ [Satyakāma replied,] ‘Yes, lord, please tell me.’ [The swan] said to him: ‘Fire is one part, the sun is another part, the moon is a third part, and lightning is a fourth part. O Somya, these are the four parts that make up a foot of Brahman. This foot is named Jyotiṣmān, the Luminous’.
Somya, O Somya; brahmaṇaḥ te pādam bravāṇi iti, I will now tell you about a foot [quarter] of Brahman; bravītu me bhagavān iti, yes, lord, please tell me; tasmai ha uvāca, [the swan] said to him; agniḥ kalā, fire is one part; sūryaḥ kalā, the sun is one part; candraḥ kalā, the moon is one part; vidyut kalā, ūghtning is one part; eṣaḥ vai somya catuṣkalaḥ pādaḥ brahmaṇaḥ, O Somya, these are the four parts that make up a foot of Brahman; jyotiṣmān nāma, named Jyotiṣmān, the Luminous.
The swan is speaking here of luminous things, such as fire, the sun, etc. Its partiality for luminous things gives rise to the suspicion that it is itself something luminous. It is, in fact, Āditya, the sun.