The expression “born with the Sun in Aries” (Taurus, Gemini, Cancer, etc.) is an artifact that has endured for thousands of years. In 1,000 BC if a baby was born at dawn on the first day of spring and you looked out at the sky at the same time, you would see the Sun rising in (in front of) the constellation Aries, the Ram. And it would be literally true to say this newborn was “born with the Sun in Aries”.
Today, if you are born between March 20th and April 20th people still say you are “born with the Sun in Aries” even though the Sun appears to be in the constellation Pisces.
There are a small number of astrologers who still link the astrological signs with the constellations (this is known as sidereal astrology), but almost all modern astrologers link the beginning of the first sign of the Zodiac, Aries, to the vernal equinox. In other words, the astrological signs are linked to the earth and its seasons rather than to the constellations. So, the astrological sign Aries begins on the first day of spring rather than when the Sun appears in the Aries constellation; Taurus follows Aries on April 20th, Gemini follows Taurus on May 21st, and so forth.
The Precession of the Equinoxes
Confusion about what astrological sign we are “born under” is caused by a real astronomical phenomenon called the precession of the equinoxes. This phenomenon, related to the alignment of the vernal equinox (first day of spring) as it moves backward through the constellations of the Zodiac, is created by the slow rotation of the earth’s axis in space. As the earth spins it “wobbles” and the wobbling movement causes its axis to trace a cone shape (see Earth’s axis extending into a North and South Pole – diagram on left). As the North Pole moves, it points to different stars.
Currently, the North Pole is pointing to our pole star, Polaris. In 3,000 BC it pointed to Thuban, and in 14,000 AD our pole star will be Vega. It will take about 26,000 years before the Earth’s North Pole completes its rotation and points to Polaris again.
The Earth’s wobbling movement causes the alignment at the equinoxes to regress through the astrological constellations along the band the ecliptic. For example, in 3,000 BC, on the first day of spring (the vernal equinox) the Sun rose in front of the constellation Taurus, the Bull. On the first day of spring in 1,000 BC the Sun rose in front of Aries, the Ram. On the first day of spring in 1,000 AD the Sun rose in front of the constellation Pisces, and on the first day of spring in 3,000 AD the Sun will rise in Aquarius (see diagram on right).