Signs and constellations, two zodiacs
The astrologers’ zodiac divides the Sun’s apparent path into twelve equal parts of 30 degrees, counted from the vernal point, where the Sun stands at the March equinox. Constellations, by contrast, are groups of stars of very unequal size.
A little over two thousand years ago, the two systems roughly matched. Since then the Earth’s axis has slowly wobbled, like a spinning top that is running down. This motion, the precession of the equinoxes, shifts the vernal point by about one degree every 72 years. The Greek astronomer Hipparchus was the first to describe it, in the 2nd century BC.
1930
The International Astronomical Union publishes the boundaries of the 88 constellations, drawn by the Belgian astronomer Eugène Delporte at the observatory in Uccle.
With these official boundaries, the Sun passes through thirteen constellations over the year, including Ophiuchus, the Serpent Bearer, from late November to mid-December. The signs have stayed at twelve. This site keeps both: the sign for tradition and pleasure, the constellation for what you can actually see.