Astrology Birth Chart

Western tropical astrology anchors Aries to the spring equinox, not to the constellation Aries. Earth's axis precesses on a roughly 26,000-year cycle — one degree every 72 years — so the two zodiacs have been sliding apart ever since the scheme was fixed. Astronomers and astrologers use the same word for the gap, the ayanamsa, and it now stands at about 24°, four-fifths of a full 30° sign. That is why the Sun is usually not in the constellation your birth date assigns it, and why the offset will grow by another degree within a lifetime.

Your sun sign comes from your birth date alone, but the moon and rising signs also need your exact time and place. The rising sign — the zodiac sign climbing over the eastern horizon at the moment of birth — shifts to a new sign roughly every two hours as Earth rotates. So two babies born the same day in the same city can share a sun sign yet have completely different rising signs, which is why an inaccurate birth time breaks the rising sign first.

Astrology splits the sky into 12 equal zodiac signs, but the Sun's yearly path — the ecliptic — actually crosses 13 official constellations. Between roughly November 29 and December 17 it sits in Ophiuchus, the Serpent Bearer, a constellation no birth chart ever assigns. Stranger still, the Sun spends only about seven days in Scorpius, less than in Ophiuchus, because the IAU's 1930 constellation boundaries carve the sky into wildly unequal slices.