Have you ever wondered how your zodiac sign is determined? In Western astrology, your zodiac sign (specifically your "Sun sign") is calculated based on the position of the Sun relative to the celestial sphere at the exact moment of your birth. Here, we look at the mathematical divisions and dates that make up the tropical zodiac.
The Ecliptic Circle
From our perspective on Earth, the Sun appears to travel along a circular path in the sky over the course of a year. This path is known as the ecliptic. Historically, ancient Babylonians divided this 360-degree circle into 12 equal sectors of 30 degrees each, named after constellations visible along that path.
Tropical vs. Sidereal Zodiac
There are two primary zodiac systems used today:
- Tropical Zodiac: Used by Western astrology, this system locks the zodiac signs to Earth's seasonal cycles. The astronomical start of spring (the Vernal Equinox) defines the first point of Aries (0 degrees). This system is what our calculator uses.
- Sidereal Zodiac: Used in Vedic astrology, this system tracks the actual physical locations of the constellations in the night sky. Because of the precession of the equinoxes (a slow wobble in Earth's axis), the tropical and sidereal systems have drifted apart by about 24 degrees over the past two millennia.
The Date Boundaries
Because Earth's orbit is slightly irregular, the exact day and time the Sun crosses into a new zodiac sector varies slightly from year to year. Standard tropical zodiac sign transitions generally occur on or near the following boundaries:
- Aries: March 21 – April 19
- Taurus: April 20 – May 20
- Gemini: May 21 – June 20
- Cancer: June 21 – July 22
- Leo: July 23 – August 22
- Virgo: August 23 – September 22
- Libra: September 23 – October 22
- Scorpio: October 23 – November 21
- Sagittarius: November 22 – December 21
- Capricorn: December 22 – January 19
- Aquarius: January 20 – February 18
- Pisces: February 19 – March 20
Our age calculator uses a standard calendar date mapping to determine your sign automatically from your birth month and day.