Avatarika
आदित्यो ब्रह्मण पाद उक्त इति तस्मिन्सकलब्रह्मदृष्ट्य-र्थमिदमारभ्यते-
In plain words
The statement that the sun is a quarter of Brahman is now begun in order to cultivate the vision of the whole Brahman within it.
Close to the Sanskrit
The sun has been spoken of as a quarter of Brahman—for the purpose of the vision of the whole Brahman in that, this is commenced.