I had heard that the word Mazal comes from the word Mazalot, meaning constellations in the Torah. If this is so, then wishing someone Mazal-Tov as either good luck or congratulations would be considered a form of astrology. I am curious why this is considered permissable when other forms of astrology are considered Asur.
There are two sources discussing Mazalot: A. Everything depends on Mazal, even the Sefer Torah in the Mikdash. B. Israel (the Israeli people) has no Mazal. The answer is that there are constellations in the world, such as the solar system and the stars, and the world is run according to them, on Hashem's command - but Jews have the power to can change that with their prayers. In addition, one may study astrology, like Shmuel said in the Gemara: I now the paths of he skies like I know the paths of Nehardea (where he lived). It all depends on the purpose one studies for: do I attribute powers to the Mazalot from themselves, and not from Hashem - or do I understand that there is nothing other than Hashem, everything is from him, and I'm learning just to be more knowledgeable like any other subject. Rabbi Ro'i Margalit