The question
Why can the same birthday be Aries in one system and Pisces in another?
The short answer
Western astrology starts the zodiac at the March equinox; Vedic astrology starts it at fixed stars. Precession has pulled the two apart by about 24°, so most planets shift back one sign.
What science knows
Established ScienceThe tropical zodiac sets 0° Aries at the March equinox, so the signs follow the seasons. The sidereal zodiac anchors 0° Aries to the stars.
The difference is the ayanamsa. By the widely used Lahiri method it is about 24.2° in the 2020s, and it was close to zero around the 3rd century CE.
The pattern
Established MathematicsSidereal position = tropical position − ayanamsa. A sun at 15° Aries tropical sits near 21° Pisces sidereal today.
What is still uncertain
Mixed EvidenceSidereal astrologers disagree on the exact ayanamsa: Lahiri, Raman, Krishnamurti and Fagan-Bradley differ by up to about 2°. There is no astronomical fact that settles which star should mark 0°.
Why humans find it interesting
Cultural TraditionWestern astrology mostly uses the tropical zodiac; Jyotish in India uses the sidereal. A Western sidereal school also developed in the 20th century.
CUE perspective
CUE PerspectiveCUE uses the Western tropical signs, by calendar date. That is a choice of tradition, not a claim that it is the “real” zodiac.
Knowing both helps explain why astrology apps disagree, and why none of them contradicts the sky.