Many peope are puzzled by the difference between Western Tropical Astrology and Vedic Sidereal Astrology. Although both systems use the same 12 signs (Aries, Taurus, Gemini, etc.) modern calculations show a difference in the date ranges of the signs. For example, by the Tropical Zodiac, the month of Aries is approximately March 21 - April 20. The sidereal dates for Aries month are approximately April 14 - May 14.

In Jyotish, or Vedic astrology, the difference between the tropical and sidereal Zodiacs is called ayanamsha. Ayanamsha is a Sanskrit word that combines ayana, which means "movement", and amsa, which means "component". The current ayanamsha is 23 degrees.

This 23 degree difference arises from a phenomenon called Precession of the Equinox.

The precession of the equinoxes is the westward shift of the equinoxes along the ecliptic, Earth's orbital plane. The Earth's axis of rotation precesses because the Earth isn’t a perfect sphere. Gaia wobbles as She hurls through space at 107,000 k8ikometers per hou

The gravitational pull of the Sun and Moon on the Earth's equatorial bulge causes the Earth's axis to move. The Earth resists this pull, similar to how a spinning top resists being pulled over.

The equinoxes precess about 50 minutes of arc per year, or about 1° per century. This means that it takes about 25,868 years for the equinoxes to precess completely around the ecliptic.

Precession of the Equinoxes was discovered by the 2nd-century-BC astronomer Hipparchus. Isaac Newton explained the physical basis for the precession of the equinoxes in his Principia. 

What does this mean for you? Western Tropical Astrology is still a great way to plan Earth-based activities such as gardening, planting, and harvest which depend on the Equinox and Solstices. If your spiritual path celebrates Gaia’s natural yearly cycles, the Tropical Zodiac defines the shortest and longest days of the year (Solstice) and the times of equal day and night (Equinox). For accurate astrological predictions and understanding astrological personality types, the astronomically accurate Sidereal Zodiac is best.