As long as 40 years ago, scientists were proposing that Ophiuchus was the 13th sign of the zodiac. One author (who was actually an art historian, not an astronomer), Owen Rachleff proposed this in 1973 in his book Sky Diamonds. It was not accepted by astrologers at that time and the notion faded away. Now it's back, and younger astrologers will have to decide its fate.
First of all we have to make two terms clear, "sign" as in "sign of the zodiac" and "constellation." They are often treated as if they were the same thing but they are not. A sign in astrology is now and always has been exactly 1/12th of the circle, or 30 degrees of 360 degrees. A constellation consists of a visual figure, such as a ram, bull, crab, archer, etc. superimposed upon the stars by the human imaginations of many different cultures. Our system of constellations had its origins in the culture of Mesopotamia (modern day Iraq) with finishing touches put on it by the Greeks. Constellations vary tremendously in size and none of them are exactly 30 degrees in extent.
Signs had their origin in about the sixth century B.C. when the astronomer/astrologers of the time (they were both), developed signs as a measuring system to be used in tracking planetary movements. As it developed they also began to attribute qualities of various kinds to these divisions. These divisions did not have then, nor have they ever had, anything to do with the stars located in them. Because the signs were also used to help keep track of the seasons for agricultural purposes, they have always started with some point in the area of the zodiac where the Sun is located at the beginning of spring. At the time in question this was within the constellation we know as Aries. The other signs, then, were also named after the main constellations located in each. In the case of Scorpio, more of that constellation appears in the sign also given that name than the stars of the constellation Ophiuchus. Hence that 30 degree segment (the eighth counting Aries as the first) was given the name of the scorpion.
However, as it turned out, two major zodiacal systems of signs came into being. One was a "sidereal" zodiac based on the constellations of fixed stars, that is, the same constellations remained in the signs of the same name more or less permanently. The other has come to be known as "tropical" because of its relationship to the latitudes on earth called the tropics. At the time the system of signs was first conceived, the two systems coincided. The ancients at first did not know that this would not continue to be true. The first astronomer that we know of for certain who realized that the two systems were changing with respect to each other was Hipparchus (c.190 BC – c.120 BC). This created a problem. Where should the zodiac begin and how should its beginning be defined? A number of Greek astronomers (though not all by any means) decided that seasonal issue was more important than the constellational one. These astronomers placed the beginning of the signs of the zodiac exactly at the point in the zodiac where the Sun seems to be at the beginning of spring. The most important of these (but not the first) was Claudius Ptolemy (c. 90 AD – c. 168 AD) of Alexandria. Over the course of the next few centuries this definition became the standard. (In fact this system of measuring stellar and planetary positions is still in use in all of the major national astronomical tables of positions, or ephemerides, with the exception of India.) However, astronomers and astrologers (still mostly the same people) were aware that the stars were moving with respect to this kind of zodiac and dealt with it in various ways. In India the astronomers decided to fix the signs with respect to the constellations and adopted a sidereal zodiac. This is why the Indian national astronomical ephemeris is the only one (that we know of) that gives the positions of the planets in a sidereal zodiac. However, even in India a sign consists of an arc of exactly 30 degrees.
There is a reason why there are 12 signs and not 13. It has to do with the lunation cycle. There are approximately twelve cycles of the Sun and Moon (or lunations) in the year. A sign stands for a month even though it is a bit more than a lunar month. The exact division is one-third of a season, and there are four seasons defined by the equinoxes which define the beginnings of spring and autumn; and the solstices which are the beginnings of summer and winter. A constellation is a pattern of stars, whose pattern is not in any way related to the equinoxes or the solstices. A constellation of stars as a pattern seen from earth will eventually disappear due to the movement of its component stars. There will always be seasons which can be divided into three parts, or signs. And there will always be a part of the zodiac, eighth in order from whatever is defined as the first sign. It is called Scorpio but it need not be.
Now about Ophiuchus... Ophiuchus is not a "sign." It is a constellation of stars that forms a pattern in the part of the sky which crosses the ecliptic in the tropical sign of Sagittarius and toward the end of the sidereal sign of Scorpio. Other than the fact that their light appears to shine in the same part of the sky as seen from earth, the stars in Ophiuchus have no necessary astronomical association with each other. They move individually on their own courses in the galaxy and eventually the constellational image will disappear (after some millions of years). For this reason Ophiuchus cannot ever be a "sign." The signs Sagittarius and Scorpio (not the constellations of the same names) are not things composed of actual stars. They are concepts and measuring tools which we visualize as a counterclockwise, 12 fold symbolic system. If they are regarded as defined by the first point of spring, they make up a tropical zodiac. If they are regarded as fixed with respect to the fixed stars, they make up a sidereal one. The motion of these two systems with respect to each other are what astrologers call "the Precession of the Equinoxes."
But a final question has to be answered. Do signs of either zodiac, tropical or sidereal, actually have the kinds of properties attributed to them by astrologers and, if so, which zodiac, the tropical or the sidereal?
Astronomers would answer "No" to this question, and astrologers would answer "Yes." Most astrologers in the West use the tropical system as defined above. Most astrologers in India and parts of the world that have adopted Indian methods use a form of sidereal zodiac. Many astrologers would also accept that the stars in Ophiuchus, as individual fixed stars, have some influence. However. these influences have nothing to do with the constellational image of Ophiuchus as a whole. these issues have to do with the validity of astrology as a whole and not with the matter of whether or not astrologers have correctly defined signs of either zodiac. If astronomers who charge that astrologers have no idea what they are doing in regard to the zodiac would take the trouble to do a little research, either in astrological literature or in the literature of this history of science, they would discover that astrologers are well aware of the issue and have been since the times of Hipparchus and Ptolemy.
What we have said concerning the signs may strike some persons who otherwise take astrology seriously as a bit abstract and mathematical. So we want to conclude with a few words which relate mostly to the tropical form of astrology.
As the earth moves about the Sun the seasons change. If you could speed up this motion and look at the earth from space, you would see on the continents a movement of green (from vegetation) moving up and down. At the beginning of the northern hemisphere's spring (beginning of Sun in Aries) you would see it moving up to the north of the equator, reaching its greatest development in the north at the beginning of the northern hemisphere's summer (Sun in Cancer). Then it would begin to move south until at the beginning of the northern autumn, the southern hemisphere's spring (Sun in Libra) it reached the most southerly part at the beginning of the southern summer (Sun in Capricorn). After that the belt of green begins to move north again. This is the "pulse of life" that the late Dane Rudhyar referred to in his book of the same name. We believe that the origin of the qualities that astrologers associate with the signs comes from this movement of life's energy on earth. In the last analysis, the zodiac may not be so much due to the astronomy by itself as it is to the earth's biosphere. The signs are simply a measure of where we are in the year and the flow of life on earth as it relates to the heavens.