Tropical zodiac

The tropical zodiac divides the ecliptic into twelve equal signs of thirty degrees, starting from the point where the Sun stands at the March equinox. That point defines zero degrees Aries, and everything else follows from it. The anchor is the cycle of the seasons; the word tropical comes from the Greek for turning.

The complication is that the equinox point moves. Earth's axis wobbles over a 26,000-year cycle, sliding the equinox slowly backward against the stars, about one degree every 72 years. The signs and the constellations they were named for parted ways roughly two thousand years ago and now disagree by about 24 degrees, most of a sign. Vedic astrology answers this by using a sidereal zodiac, anchored to the stars instead.

Neither zodiac is a mistake; they measure different things, seasonal geometry versus stellar backdrop. This is also how our 3D sky and our charts can both be honest while disagreeing: the planetarium shows the constellations as they really stand, the chart pages use tropical signs, and so the Sun's sign and the constellation behind the Sun usually differ.