The tropical zodiac
Twelve equal sections of a circle, measured from the March equinox.
The physical explanation
A tropical sign is a 30-degree section of ecliptic longitude, measured from Earth. Zero degrees Aries begins at the March equinox direction; Taurus begins at 30 degrees, Gemini at 60, and so on. A position of 140 degrees is therefore 20 degrees Leo. The signs share names with constellations, but constellations occupy unequal regions of the sky. Precession changes their relationship to the equinox over time. The tropical zodiac stays anchored to the equinox rather than to fixed stars.
Explore real dates in the Time Machine ↗One circle. Twelve equal sections.
An illustrative angle, separate from your selected sky date. Choose a sign or move the angle to cross its boundary.
140° longitude = 20° Leo
Leo starts at 120° and ends just before 150°. At 360°, the circle wraps to 0° Aries. These equal divisions are not the uneven boundaries of astronomical constellations.
The traditional association
Western tropical astrology attaches symbolic qualities to these twelve sections. That interpretive tradition is separate from the coordinate system used to locate a body. Choosing tropical signs is a convention; other astrological traditions use different reference systems.
Reference: Astrodienst · tropical signs, constellations and precession ↗
Editorially reviewed 2026-10-24 · prepared 2026-10-24.