Most people are familiar with the concept that there are twelve signs of the zodiac. This idea comes from the fact that throughout the year the sun appears to travel through twelve of the eighty-eight modern constellations. As we circle the sun throughout the year, we continue to get a different vantage point relative to the stars of space. Since we perceive earth as being stationary, it looks as if the stars that create the backdrop of night are moving. As such, the arrangement of stars that occupy the area of the eastern sky at the time of sunrise change from day to day. Therefore, it appears as if the sun is moving from one constellation to another. It takes a month for one constellation to fully move from this position to the next. The constellation that occupied this position was considered to be the constellation of the month. Since there are twelve constellations that cross this area, there are twelve signs of the zodiac. (see Rashi Rosh Hashana 11b)
While the above statement is certainly an accurate account of the system, it has one major issue with it (besides the fact that the attributed signs have shifted since this system was implemented, but that is something for another discussion). THERE ARE THIRTEEN CONSTELLATIONS THAT OCCUPY THIS AREA, NOT TWELVE!!! These thirteen are: T'leh/Aries, Shor/Taurus, Teomim/Gemini, Sartan/Cancer, Aryeh/Leo, Besulah/Virgo, Mozayim/Libra, Ophiuchus, Akrav/Scorpius, Keshes/Sagittarius, G'di/Capricorn(us), D'li/Aquarius and Dagim/Pisces. (In case you haven't guessed it, Ophiuchus is the one with no corresponding sign of the zodiac.) This was true even in the times of Chazal and the Ibn Ezra records all thirteen, yet does not mention the issue that there are only twelve signs.(see Ibn Ezra Reishis Chachma) And no, it does not make sense that one should be reserved for a leap year and correspond to Adar Sheini because the sun passes through all thirteen every year and at the time it is in Ophiuchus it is considered to be in the sign of Akrav/Scorpius.
After reading a comment by Rav Yonasan Eibshitz on this week's haftarah, I came up with a theory to resolve, at least partially, the mystery behind this discrepency. The haftarah starts with:
עם זו יצרתי לי תהלתי יספרו: ולא אתי קראת יעקב כי יגעת בי ישראל
"This nation I have created to tell of my praises. But Yaakov has not called to me; for Yisrael has become tired of me."
In his Ahavas Yehonasan, Rav Eibshitz explains that Hashem has thirteen attributes and it is through Klal Yisrael that the world can recognize that all these thirteen attributes are really just expressions of the one God. It is, therefore, through Klal Yisrael that Hashem's oneness is perceived. This is accomplished via the thirteen shevatim. Each shevet embodies a different one of Hashem's attributes. When the world recognizes that these thirteen tribes are really one nation, they also can comprehend that the thirteen atrributes of Hashem are not all separate entities. In this way Klal Yisrael was created to tell of Hashem's praises
Although we generally refer to the shevatim as being only twelve, if one counts Levi, who is a shevet, and Menashe and Ephraim, who divided Yosef into two shevatim, there are thirteen. They are: Reuven, Shimon, Levi, Yehuda, Yissachar, Zevulun, Dan, Naphtali, Gad, Asher, Ephraim, Menashe and Binyomin. Rav Eibshitz then cites from the pasuk in the haftarah that said עם זו, "this nation". The numerical value of זו "this" is thirteen. This displays that "this nation, meaning the thirteen tribes of this nation, were set aside to express Hashem's praises.
When I saw this I realized that the same issue that seems to be expressed in the constellations (there being thirteen and only recognized as twelve) exists within the shevatim, too. There are thirteen shevatim, but we generally refer to them as only twelve. When Levi is included then Menashe and Ephraim are counted as one, Yosef. When Menashe and Ephraim are included, then Levi is not. Since Chazal compare the twelve shevatim to the twelve signs of the zodiac (see Bamidbar Rabbah 2), this oddity, occurring in both the constellations and shevatim, must be more than coincidental.
The mystery unravels a little more when one delves deeper into the symbolism of Ophiuchus. Every constellation is depicted as some sort of object, creature or human. Ophiuchus is the Serpent Bearer and carries the snakes of the constellation Serpens. Ophiuchus is symbolic of man's familiarity with sin and his willingness to handle snakes, the symbol of sin (it was the snake that tempted the first humans to eat the forbidden fruit). Since the twelve signs of the zodiac correspond with the twelve tribes, then this element must be showing an element that still requries perfection.
The tribes only display Hashem's praises when they act in unison and follow God's commandments. Ophiuchus is displaying the element present in Klal Yisrael, and every individual, that has not yet decided to do so, rather, this element is still familiarizing itself with sin. Therefore, the tribes can only be expressed as twelve and not thirteen. The zodiac is, therefore, also expressed as being somewhat deficient. Perhaps, when Klal Yisrael perfects herself, we will find a more complete representation of her in the sky and the thirteenth constellation will be realized as a sign of the zodiac, as well.
Utilizing the same methodology of Rav Eibshitz, this can be seen in the pesukim cited above. The latter part stated:
ולא אתי קראת יעקב כי יגעת בי ישראל
"But Yaakov has not called to me; for Yisrael has become tired of me."
The wording "for Yisrael has become tired of me" has in it the word בי "of me". Technically, the Hebrew could also be read as "for Yisrael has reached בי". The word for tired has the same pronounciation and spelling as that of reached, יגעת. The numerical value of בי is twelve. The new meaning is then "For Yisrael has reached twelve." In this context, Hashem is saying that Yisrael is not telling His praises because they have reached twelve. Based on the understanding above, it is thirteen that they are striving for, but they are still falling short. We are still waiting for the realization of all thirteen shevatim.