taru-adjective, adverb taking the 'to' particle
shining brightly; shining whitely
Literary, classical adjective/adverb describing something gleaming white or bright. Used in poetic or elevated prose; not part of everyday speech.
It was a night when the moon shone brightly.
His white hair gleamed whitely, just like snow.
The exact derivation is uncertain; the kanji spellings 皓然 and 皎然 are literary forms associated with brightness or whiteness, likely borrowed from classical Chinese.