In plain words
If the brilliance of a thousand suns were to rise at once in the sky, it might resemble the splendour of that Mahātman.
Word for word
If in the sky the light of a thousand suns, risen at once, were to be, that would be like the radiance of that great-souled one.
दिवि सूर्यसहस्रस्य भवेद् युगपदुत्थितायदि भाः सदृशी सा स्याद् भासस्तस्य महात्मनः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
या पुनर्भगवतः विश्वरूपस्य भाः, तस्या उपमा उच्यते —
दिवि अन्तरिक्षे तृतीयस्यां वा दिवि सूर्याणां सहस्रं सूर्यसहस्रं तस्य युगपदुत्थितस्य सूर्यसहस्रस्य या युगपदुत्थिता भाः, सा यदि, सदृशी स्यात् तस्य महात्मनः विश्वरूपस्यैव भासः । यदि वा न स्यात् , ततः विश्वरूपस्यैव भाः अतिरिच्यते इत्यभिप्रायः ॥ १२ ॥
In plain words
But that radiance the Lord's universal form now blazes with — what can it be compared to? Here comes the answer —
Picture a thousand suns rising together in the sky, all at once, blazing as one. If that light could somehow equal the radiance of this great being's universal form — well, that is the comparison offered. And if it could not equal it, then the plain truth is this: the form's own splendor outshines even that. Nothing else measures up to it.
Close to the Sanskrit
Now, as for the radiance which belongs to the Lord's universal form, a simile for it is stated —
In the sky — in the atmosphere, or in the third, i.e. in heaven — of a thousand suns risen simultaneously, of that simultaneously-risen thousand of suns, the radiance which, arisen simultaneously, that radiance, if it would be similar to the radiance of that great soul, the universal form alone; but if it would not be, then the splendor of the universal form alone exceeds it — such is the intended sense.