दिवि सूर्यसहस्रस्य भवेद्युगपदुत्थिता । यदि भाः सदृशी सा स्याद्भासस्तस्य महात्मनः ॥१२॥
divi sūrya-sahasrasya bhaved yugapad utthitā yadi bhāḥ sadṛśī sā syād bhāsas tasya mahātmanaḥ
divi — in the sky; sūrya — of suns; sahasrasya — of many thousands; bhavet — there were; yugapat — simultaneously; utthitā — present; yadi — if; bhāḥ — light; sadṛśī — like that; sā — that; syāt — might be; bhāsaḥ — effulgence; tasya — of Him; mahā-ātmanaḥ — the great Lord.
If a thousand suns were to blaze forth together in the sky, they would not match the splendor of that great form.
What Arjuna saw was indescribable, yet Sañjaya is trying to give a mental picture of that great revelation to Dhṛtarāṣṭra. Neither Sañjaya nor Dhṛtarāṣṭra was present, but Sañjaya, by the grace of Vyāsa, could see whatever happened. Thus he now compares the situation, as far as it can be understood, to an imaginable phenomenon (i.e., thousands of suns).