וַיַּ֣רְא אַהֲרֹ֔ן וַיִּ֥בֶן מִזְבֵּ֖חַ לְפָנָ֑יו וַיִּקְרָ֤א אַֽהֲרֹן֙ וַיֹּאמַ֔ר חַ֥ג לַיהוָ֖ה מָחָֽר׃
And when Aaron saw it he built an altar before it and Aaron made proclamation and said, Tomorrow is a feast to the LORD
Saw it? What did he see?
According to the ancient interpreters, what he saw was the murder of his nephew Hur. (I don't recall where this first appears, but believe its in the apocrypha. TPJ and TN both have it) Another possibly, also suggested by an ancient interpreter (again, I forget who) is that he saw the angry mob.
Both possibilities are cleverly captured in the Peshita, where it says not that Aaron saw, but that he was afraid (In Hebrew the words are the same.)
All of these explanations are needed; without them the verse tells us that "Aaron saw the golden calf and built an alter." This reading, though the plain intention of the verse, was untenable to the ancient interpreters who could not tolerate the idea that Aaron might have built the altar of his own volition.
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