Three thousand years ago, the Babylonians observed that from Earth’s perspective, the Sun appears to move through 12 of the charted constellations over the course of a year. This led to the development of the signs of the zodiac, a foundational part of astrology today. But astronomers have pointed out in recent years that the Babylonians’ observations were not completely accurate, and that the 12 zodiac signs have become even less accurate over time.

The biggest inaccuracy is the number 12, since the Sun actually moves through 13 constellations in a year, including Ophiuchus. The zodiac calendar was also simplified, with each of the 12 constellations arbitrarily assigned a full month, although the Sun’s path is not that evenly divided. And third, because the Earth wobbles on its axis, the Sun’s apparent path through the stars today is different from the path it would have taken 3,000 years ago when the zodiac was devised. This difference actually results in a one-month shift in the stars' alignments.

Because astrology is not a science, these inaccuracies have not widely affected how people practice it. Although some astrologically-inclined people who used to identify as Sagittarius might now call themselves Ophiuchus, the field as a whole has not adapted to this new information.

Astrology has changed in response to other, less threatening, advances in astronomy. For example, the dwarf planet Pluto became incorporated into astrological models after it was discovered in 1930. This pick-and-choose reaction to new information is what differentiates pseudosciences like astrology from sciences like astronomy.