Gemara

65b (Beraisa - R. Shimon): Me'onen is one who puts semen of seven men on the eye;

Chachamim say, it is a sleight of hand magician;

R. Akiva says, it is one who 'calculates' good or bad times, e.g. today is good to embark, tomorrow is good to buy, wheat is good in Erev Shemitah...

Menachesh is superstition, looking for or acting according to omens... He says 'do not collect the tax from me first, I do not want to pay my debt now, it is the start of the day or week or month.'

(Beraisa): "Lo Senacheshu v'Lo Se'onenu" refers to people who look for or act according to omens of weasels, birds and fish.

Nedarim 32a (Rebbi): Anyone who engages in Nichush (acts according to omens) will be affected by them - "Lo (to him) there is Nichush".

Objection: Lo is written with an Aleph, it means 'No'!

Correction: Rather, Rebbi's teaching follows from the fact that Hash-m treats people according to their actions.

(Ahavah, R. Zeira's son): Anyone who does not do Nichush is greater than the angels - "Yisrael do not do Nichush... (the angels will ask Yisrael about Hash-m's actions)".

Pesachim 113b (Rabah bar bar Chanah) Question: What is the source to forbid asking astrologers?

Answer: "Tamim Tihyeh Im Hash-m" (rely simply on Him).

Shabbos 156a - Avraham Avinu: I see in my Itztagninus (the Chachmah of reading the stars) that I cannot father a son!

Hash-m: Leave your astrology. There is no Mazal for Yisrael! I will move your Mazal to the east (to enable you to father children).

Kaldai told R. Akiva that the day his daughter gets married, she will be killed by a snake. He was very worried. The day of her Chupah she did a great Mitzvah. Later, she stuck a pin in the wall. It struck a snake and killed it.

Kaldai had told the mother of Rav Nachman bar Yitzchak that he will be a thief. She told him to always (properly) cover his head, so that fear of Shomayim will be upon him, and to request mercy (to be saved from the Yetzer ha'Ra). He did not understand why. One day he was learning under a date tree; his garment fell from his head. He looked up and saw the date tree (it was not his). His Yetzer ha'Ra overpowered him, he cut a cluster of dates with his teeth.

Horayos 12a (R. Ami): To know whether one will survive the coming year, he should hang a lamp during the 10 days of Teshuvah in a house without wind. If all the oil burns, this shows that he will live. If one wants to go on a trip and wants to know if he will return, he should stand in a dark house. If he sees a Bevo'ah d'Bevo'ah (a shadow of his shadow), he will return.

Rejection: He should not do this. Perhaps he will not see it, and he will worry, and this will spoil his Mazal (and cause that he not return).

Rishonim

Rif and Rosh (Sof 16b and 7:7): "Lo Senacheshu " refers to people who look for or act according to omens of weasels, birds and stars.

Hagahos Tur ha'Shalem (YD 179:6): Our text says weasels, birds and fish. Dikdukei Sofrim says that old texts said 'stars', and it seems that this was the text of the Rambam (Perush ha'Mishnayos Avodah Zarah 4:7, b'Sof, and of the Rashba brought below).

Rambam (Hilchos Avodah Zarah 11:8): Me'onen is giving times. They say based on astrology that this day is good, and another day is bad. This day is good for this task, this year or month is bad for this matter.

Rambam (9): Me'onen is forbidden even if he does not do an action, just he informed (others about) the lies that the fools believe is true Chachmah. If one acts based on astrology and timed his work or departure based on the times the astrologers said, he is lashed for Lo Se'onenu.

Rambam (15): All these matters are false. The early idolaters tricked people into following them. It is improper for Yisrael, who are Chachamim, to be drawn after them or believe that there is something to them. "Ha'Goyim... El Me'onenim Yishme'u v'Atah Lo Ken..." Anyone who thinks that it is true, but the Torah forbids it, is a fool.

Tosfos (Shabbos 156b DH Kaldai): Kaldai are astrologers. Rashi (Pesachim 113b) said that it is Ba'al Ov. This is wrong, for the Gemara forbids due to "Tamim Tihyeh Im Hash-m Elokecha". "Al Tifnu El ha'Ovos" explicitly forbids Ov! The Sifri forbids asking lotteries due to "Tamim Tihyeh". This is the same as astrology.

Beis Yosef (DH Kosav ha'Rambam): The Rambam rules like R. Akiva. He and Semag hold that the person decides which days are good based on astrology. The Aruch holds like Tosfos, that Kaldai are astrologers.

Rashba (Teshuvah 283 attributed to the Ramban, in Beis Yosef ibid.): The Gemara is like the Aruch. Astrology is not Nichush. The Halachah does not follow Rav Chanina, who says that Yesh Mazal l'Yisrael, but in any case it is not Nichush. Avraham was concerned for his astrology. Sanhedrin 66a discusses Nichush through the stars; this is not astrology. One may believe in and heed astrologers, for we find that Avraham and R. Akiva did so. However, sometimes Hash-m does a miracle for those who fear Him to overcome what was decreed in the stars. These are hidden miracles that the entire world depends on them. This is why we do not ask them. We go simply - "Tamim Tihyeh Im Hash-m." If one sees something against his will, he does Mitzvos and prays. If he sees in astrology that a certain day is bad for a certain task, he should beware and not rely on a miracle. I think that one may not oppose Mazalos and rely on a miracle. People do not begin things on Monday and Wednesday, because Mazal Levanah is not light, and things are not finished properly on these days. The Gemara discussed Mazal of days and hours; really, it depends on the position of the stars. The entire Chachmah is not precise; Chasidim should not engage in it. The Rambam says that all this is folly. The Sugya shows otherwise, even regarding Nichush of birds (Gitin 45a). These matters require investigation. One should not be too exacting with people about them.

Nimukei Yosef (16b DH Tanu): The custom is to marry only when the moon is increasing (until mid-month). One may not fix a Siman for himself based on the stars, but an astrologer who understands the constellations is not Nichush. It is a great Chachmah! Hash-m decreed to run His world through them. R. Akiva was concerned for what astrologers said. However, Chachamim said that one should not dote on these things. Rather, he relies on Hash-m; in His mercy, He will save His servants from mishaps. This is why the Gemara forbids asking astrologers. The Rambam holds that even the Chachmah of astrology is Nichush. This is wrong.

Poskim

Shulchan Aruch (YD 179:1): We do not ask astrologers or Goralos.

Shach (1): The Terumas ha'Deshen says that doing an act of witchcraft, etc. is forbidden mid'Oraisa, but asking is forbidden only due to "Tamim Tihyeh." Therefore, a sick person may ask these people. The Maharshal forbids unless he is dangerously sick or if the sickness came through these means.

Rema (OC 664:1): The Rishonim say that on the night of Hoshanah Rabah there is a Siman in the shade of the moon foretelling what will happen to one or his relatives during the year. Some say that one should not be particular about this, lest it ruin his Mazal, for many do not understand this properly. It is better to be Tamim than to delve into future events.

Source (Darchei Moshe 2 citing the Avudaraham): Horayos 12a says that one can test his Mazal similarly before embarking, but it is better not to.

Source (Gra 8): One should not seek Simanim (Horayos 12a). One may not ask astrologers (Pesachim 113b). Rather, he should be Tamim.