More about radiant
Radiant, “bright with joy and hope,” is based on Latin radiāns “shining,” the present participle of the verb radiāre “to radiate light, shine,” which is based on the noun radius “beam, ray.” Radius is also the source of radian, radio, radium, and ray. While English uses -ing to mark its present participles (seeing, going), as we learned from the recent Word of the Day gallantly, Latin uses -āns, -ēns, or -iēns—depending on the type of verb—for the same purpose. For phonetic reasons, the stems of these three Latin endings swap the s for t, which is how Latin radiāns becomes English radiant, pungēns “piercing” becomes pungent, and conveniēns “coming together” becomes convenient. Radiant was first recorded in English in the late 15th century.