A couple of thoughts on Noah's rainbow:
Is the rainbow "bad"?
Ever since I was a child, I have been taught in yeshiva that the rainbow is a "bad" sign, rather than "good", because even though it represents God's promise not to destroy the earth, it also indicates that God is angry at us for our sins.
Frankly, I find this notion that the rainbow is a "bad" sign to be perplexing. On Yom Kippur, we believe that God, after having reviewed all our many sins, nevertheless forgives us, and spares us punishment. So in that case (using the flawed logic above), Yom Kippur must be a "bad" thing, since it centers around our sins, for which we deserve punishment. Haza"l, of course, did not see things that way: In fact, R. Shimon b. Gamliel said (Mishna Taanit 4:8) that Yom Kippur was one of the two most joyous days of the year -- in celebration of the very fact that we gained God's forgiveness on that day.
The rainbow, then, is like Yom Kippur. It, too, indicates that we have sinned -- and that God, in his mercy, chooses nevertheless to forgive us. Therefore, like Yom Kippur, the rainbow, too, ought to be seen as a happy, joyous thing -- and not a "bad" thing.
Were there rainbows before Noah?
Rashi (as opposed to the Ramban) posits that the first rainbow that ever existed was the one witnessed by Noah. And presumably, Rashi feels compelled to say this because he finds it untenable to believe that rainbows already existed previously -- since the rainbow is described by God, after the Flood, as a sign that He will not destroy the world again. Therefore, Rashi seems to be saying, the rainbow must have been something "new under the sun", first created at the time of Noah, rather than having been a pre-existing part of the original Creation.
However, it should be pointed out that there exists a completely different school of though in Haza"l -- one that says that the laws of nature are permanent and inviolate, and that these laws have never changed since Creation -- not even in the context of things that are commonly regarded as "miracles". This idea is the basis for the mishna in Pirkei Avot (5:9) which tells us that the following ten things were created during the six days of Creation: The pit that swallowed Korah, Miriam's well, the mouth of Bilam's talking donkey, the manna that fell in the desert, Moshe's staff, the shamir (with which the Temple was built), the alphabet, the inscription of the Ten Commandments, the two tablets of the Ten Commandments -- and the rainbow.
This mishna is saying that nature doesn't change, ever: Even things we call "miracles" do not actually constitute changes in the order of nature, but rather, were built into the fabric of the universe from the very beginning. And by including the rainbow in the list, this mishna is rejecting Rashi's idea that the rainbow was something "new under the sun", and supporting the Ramban's contention that it was, in fact, always part of nature.
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