etymology - Where did the "art" in "Our Father who art in Heaven" go?

What happened to the art in "Our Father who art in Heaven"? And why is it art, and not is?

Answer

Art was the second person singular form of the verb "to be" in older versions of English. Over the centuries, it eventually became are (along with the disappearance of the -st suffix across all 2nd person singular verb forms). Is, on the other hand, would be the 3rd person singular form.

The reason are is used instead of is is because the prayer is speaking directly to "Our Father": i.e. "You, who are in Heaven".