By now you may have heard that, despite the recent freak out, the zodiac probably isn't changing and the possible 13th sign, Ophiuchus, may not be part of the zodiac after all. But even though this is undoubtedly a relief to us all, why isn't Ophiuchus in the zodiac? After all the uproar, it does seem a little weird.
The short answer to why the constellation Ophiuchus isn't an astrological sign is that people are probably not going to stop using the traditional astrological calendar with which we're all familiar. Despite some rumors to the contrary, NASA isn't changing the star signs because, well, NASA has no power to or interest in doing so. NASA can explain to you why ancient peoples probably used the constellations they did, but astrology isn't astronomy, and NASA is in the astronomy business. That means that no one is changing the signs.
So why do we use the calendar we use? And why was Ophiucus left off of it in the first place?
The current astrological schema used in the Western world was first developed by the Babylonians, and was adapted later by the Greeks. In this system (which isn't the only one in use, but is the one you're probably familiar with), the sky is divided into 12 sections, one for each of the 12 months or moon cycles in the year. They then chose 12 constellations, one in each of those sections. And as the year progresses and the Earth rotates around the sun, the sun appears to pass through each of the 12 sections and 12 constellations.
Of course, there are way more than 12 constellations, and the Babylonians actually recognized 13 important ones — the 12 we're familiar with, plus Ophiucus. But they decided to leave Ophichus out, as it was never a formal astrological sign.
Nowadays, due to an astronomical phenomenon that the Babylonians and Greeks didn't know about called the precession of the equinoxes, the constellations no longer line up the way they used to. So whatever sign you were supposedly born under based on the traditional system, you almost certainly weren't actually born under it at all, based on the actual movement of the heavens. This has caused some people to suggest updating the astrological charts, potentially even including a 13th constellation, but no one has made that change yet — and since we're so used to the old system, odds are no one ever really will.
Thus, Ophiucus still isn't a zodiac sign. Poor guy.