Rashi cites a Medrash that says that when Hashem told Avraham to go outside, He was not only referring to leaving the tent, but also to leaving his astrology behind. Avraham had read the zodiac signs and understood that he was not destined to have children. Hashem would change his name to Avraham and his wife's name to Sarah, and these newly created people would not be constrained by their previous zodiac signs. It appears that Hashem gave credibility to Avraham's concern about his astrological fate, writes the Mikdash Halevi. If so, what do the mazalot, the stars and the zodiac signs, convey?
Rabbi Frand cites Rav M. Shapiro that Hashem told Avraham to go out and count the stars. Without questioning the logic behind this, Avraham did so. To this, Hashem responded, "This will be your offspring." They too will refuse to acknowledge the impossibility of any command, and will try again and again to fulfill Hashem's command. This is how we change our future, continues Rabbi Frand, for we find new strengths and capabilities within ourselves that help us go beyond our perceived limitations. Doing a physical act, notes Zos Yaakov, further concretizes an abstract idea.
Rabbi S. R. Hirsch notes that Hashem wanted to teach Avraham that he and his descendants would not be bound by natural law. Just as the stars are more numerous than anything on earth and their existence is directly under Hashem's providence without intermediaries, so too would Avraham's offspring be directly under God's hand.
Nevertheless, the Gemara states that each Hebrew month has a different zodiac sign that contains a unique energy. For example, one shouldn't initiate a lawsuit with a gentile in the month of Av, but rather in the more propitious Adar. The Siftei Chaim, cites the Gemara that notes that before a child is born, Hashem contemplates the embryo and decides what strengths and challenges will be necessary for it to fulfill its mission on earth. Having children or earning an easy living, for example, are not based on merit but on one's mazel, like the word nozel, what flows down from above. Mazel is the potential within life's circumstances. It is up to each of us to use our gifts and challenges that flow from above, in a constructive way.
Rabbi A. Tatz in Living Inspired masterfully explains that the mazalot act as Hashem's emissaries and pipeline. They bring the energy of the upper world down into our world, but they act only under the direction of Hashem. Astrology is therefore not predicting the future, but reading the present energy and understanding how it can affect and influence the future. Mankind originally understood the difference between natural forces and the Source of it all, but reasoned that certainly the emissaries of the King also deserved honor and respect. Eventually however, they forgot about Hashem and worshipped only the messengers. Selfishness stood at the root of this mistake. Someone who values the King wishes to be of service to the King, while one who pays homage to the messengers wants something for himself. A God-fearing man understands that Hashem is the source of everything, and seeks to serve Him. An idolater's philosophy is that I am everything; let's see how the gods can serve me and what I can get. If Hashem counts and is the source of reality, then that is worship. But if I count and all that matters is what I desire, that is idolatry.
When Hashem took Avraham outside, He indicated that even though the stars rule the natural order, but you Avraham and your progeny are above that. You can define your own destiny by making My will your will. Through the mitzvot we can break through to the original Source without the need of the intervening mazalot. The Ohr Doniel explains that there is no "bad" mazal, for everything is part of God's plan for us. Nevertheless, we can change our perceived destiny, through prayer and good deeds. In fact, Avraham's stars predicted that he would remain childless, but through his work of bringing others closer to Hashem he merited to change his destiny.
Rabbi S. Pincus asks, if Hashem has already determined our mazal, what's the point of wishing people mazal tov on momentous occasions? He explains that it is not so much a wish but a prayer for good mazal. If Hashem has determined that the child or the new couples' lives will be difficult, may our prayers help break through to the Source and change that.
Torah study, prayer, and acts of loving kindness connect us directly to the original Source without going through the conduit of the mazal. They can thereby redirect the mazal itself. We must also accept that sometimes Hashem's answer will be no.
Chazal tell us, "The Nation of Israel has no mazal." Even though each of us has a personal mazal, our Nation and its history are not bound by nature, but by the hand of God only.