Herong's Notes on Astrology and Horoscope - Version 2.11, by Dr. Herong Yang
Zodiac Coordinate Systems - Starting Points
This section describes what are zodiac coordinate systems and starting points.
In Western and Indian astrology, the zodiac circle is divided into 12 equal zodiac signs. But where is the starting point?
Ancient astrologers used two coordinate systems to define the zodiac sign starting point:
1. Tropical zodiac coordinate system is used in Western astrology. In tropical zodiac, the starting point of zodiac, the first point of Aries, is defined as the vernal equinox, when the Sun is cross the celestial equator from south to north.
Vernal equinox, also called spring equinox, usually happens on March 21 each year. This is why Aries sign starts on March 21 in Western astrology.
2. Sidereal zodiac coordinate system is used in Indian astrology, Jyotisha. In sidereal zodiac, the starting point of zodiac, the first point of Aries, is defined using two fixed stars, Aldebaran and Antares, on two opposites sides of the ecliptic circle. Aldebaran is defined as the middle point of the Taurus sign; while Antares is defined as the middle point of the Scorpius sign. The rest of the zodiac signs are defined acoordingly.
As you can see, the sidereal zodiac coordinate system is a fixed coordinate system. But the tropical zodiac coordinate system is a relative coordinate system. In ancient time, about 2000 years ago, the tropical zodiac coordinate system matches well with the sidereal zodiac coordinate system.
However, due the precession of Earth's axis, the tropical year is about 20 minutes and 24 seconds shorter than the sidereal year. This makes the vernal equinox moving backward 1 degree every 71.57 years. But this small difference can be accumulated into a big gap over time. As of year 2007, this gap is about 24 degrees.
This 25-degree difference is about 24 calendar days. This is why sidereal zodiac Aries sign starts on April 14 in 2007, which is 24 days after the tropical zodiac Aries sign start date, March 21.
The following table shows you the dates when Sun enters different zodiac signs in tropical zodiac coordinate and sidereal zodiac coordinate systems:
Sun Sign Tropical Zodiac Sidereal Zodiac Aries Mar 21 - Apr 19 Apr 14 - May 14 Taurus Apr 20 - May 20 May 15 - Jun 14 Gemini May 21 - Jun 21 Jun 15 - Jul 16 Cancer Jun 22 - Jul 22 Jul 17 - Aug 16 Leo Jul 23 - Aug 22 Aug 17 - Sep 16 Virgo Aug 23 - Sep 22 Sep 17 - Oct 17 Libra Sep 23 - Oct 22 Oct 18 - Nov 16 Scorpio Oct 23 - Nov 21 Nov 17 - Dec 15 Sagittarius Nov 22 - Dec 21 Dec 16 - Jan 14 Capricorn Dec 22 - Jan 19 Jan 15 - Feb 12 Aquarius Jan 20 - Feb 18 Feb 13 - Mar 14 Pisces Feb 19 - Mar 20 Mar 15 - Apr 13