In order to explain why the zodiac is divided into 12 signs and not 13, we must consider the basis or foundation upon which the zodiac has been devised.  The zodiac is a scientific or mathematic standard of measurement of 12 equal divisions. Including Ophiuchus into the equation as a 13th zodiac sign, is like arbitrarily adding another hour to the 12-hour clock or adding another month to our 12- month calendar. The statement that there are 13 zodiac signs is based on a misunderstanding and confusion of constellations with star signs. When one understands the principles of the zodiac, you get to see that the 13th zodiac sign statement makes no sense.

What are the foundations of the zodiac then? And which principles are deployed? The zodiac is not ‘real’, it’s not visible in the sky. It is a mathematical division of the ecliptic. The ecliptic is the ‘apparent path of the sun’ as viewed from our perspective. The sun appears to revolve around the earth when we track it. In the morning we look to the east and say, “here comes the sun”! Then it follows a path in the sky during the day, until it goes down in the west again. All the planets follow this apparent path through the sky from our geocentric perspective. Technically speaking the ecliptic is the astronomical circle that tracks the annual interaction between the earth and the sun. All the planets including the earth remain in a similar path (the ecliptic) in an anti-clockwise position around the Sun. Ancient astronomers observed the Sun’s annual cycle against certain star groups and along the ecliptic and from this the calendar was derived.

Where does the zodiac fit into all of this? The ecliptic line, cuts through the equator at an angle of 23 degrees. The angle between the ecliptic and the equator is the reason why we experience seasons. The 12-sign zodiac is projected along this ecliptic. It’s a band in space that spans 9 degrees on either side of the ecliptic. The beginning of the zodiac is a mathematical point related to the Northern hemisphere’s spring or vernal equinox, which we call the sign of Aries. Two thousand years ago the spring equinox occurred against the backdrop of fixed stars. This backdrop shifts …..of the equinox.

Now that we have the background, we can get to why the zodiac is divided into 12 equal divisions. It’s for the same reason why we have 12 calendar months and 12 hours on a clock. A year has 365 days (one cycle of the earth around the Sun). But 365 is not a easy number to divide. As mentioned, the zodiac is a circle, called the ecliptic, that maps out the Sun’s position throughout the year. The Babelonians attributed 360 degrees to a circle to facilitate the correspondence between the solar cycle with the calendar. The calendar is devised from an interplay between the Sun, Moon and Earth. Within the annual solar cycle are 12 lunar cycles.

The zodiac therefore follows the same template that is used in calendars. It consists of 360 degrees, divided into 4 quarters, which is separated by 90 degrees. This angle relates to the intersection of the ecliptic and the equator, which relate to our four seasons. Each quarter is 30 degrees (sign) and it takes an approximate 30 days (month) for the Sun to move through the 30 degrees. Although the degrees are not precise, the zodiac is a reference system, like a clock or calendar and monitors planetary movements.

The early astronomers were also astrologers. The zodiac was devised to aid the mathematical tracing of the planets. Philosophers such as Pythagorus was exploring the mystical nature of shapes and geometrical relationships and generated theories. Thus, there is great philosophical depth embedded in the astrological archetypes. The zodiac does not relate to the star constellations. The zodiac took imagery form some of the constellations.

To sum up the debate. The 13th zodiac sign assumes that constellations are directly related to the zodiac (signs), which it is not. The zodiac took on some of the names and archetypes of the constellations but there are many more constellations than there are zodiac signs (88 in the Northern and Southern hemisphere) and the zodiac signs and constellations are not related. The rumour that keeps making its way into the media assumes that the ecliptic runs through the constellation Ophiuchus and therefore the 13th sign. However, this statement is based on incorrect information about the building blocks of the zodiac. The 12-sign zodiac is based on the same scientific standardised measurements of that of a 12-month calendar or 12-hour clock. An equal division of 360 degrees (the ecliptic) in 4 quarters (each being 90 degrees) which again relates to a division of 30 degrees each. 12 zodiac signs of 30 degrees each straddling the 12 months of the year.

Reference: Deborah Houlding http://www.skyscript.co.uk/