What a zodiac does
A zodiac divides the ecliptic into twelve equal sectors of 30° each. A longitude is measured from a chosen starting point. Different starting points can assign different sign names to the same observed direction without changing where the planet actually is.
The tropical reference
The tropical zodiac places 0° Aries at the March equinox. Its reference is tied to the equinox rather than permanently fixed against the background stars. Astrology.mobi’s calculation tools use this convention unless a tool explicitly states otherwise.
The sidereal reference
Sidereal systems use a reference associated with the stellar background. An ayanamsha specifies the offset from the tropical zodiac, and different sidereal schools use different values. “Sidereal” alone is therefore not a complete calculation setting.
Why the labels diverge
The Earth’s axis slowly changes direction, producing precession of the equinoxes. Over long periods, an equinox-based reference moves relative to the stars. This explains the growing separation between tropical and sidereal labels; it does not establish one interpretive tradition as scientifically validated.
Signs are not constellations
Astronomical constellations have boundaries of unequal size. The twelve equal zodiac signs are a coordinate division, so they should not be presented as a map of those constellation boundaries. Discussions of Ophiuchus concern this distinction, not a new thirteenth sector in the standard twelve-sign zodiac.
Compare like with like
When checking a placement elsewhere, match the date, time, zodiac convention, ayanamsha if applicable, and apparent versus geometric position. A sign discrepancy near a boundary may be a settings difference rather than a calculation failure.
This guide describes symbolic conventions and traditions. Astrological interpretations are not scientifically established predictions about personality or events. Our sources and editorial approach.