The types of zodiac system: tropical, sidereal and astronomical

The question "what is my zodiac sign" is in fact not a simple one: there are three main frames of reference, and the sign changes depending on which of them is chosen. Let's sort out the tropical, the sidereal and the astronomical.

Tropical zodiac — the zodiac of the seasons

The tropical zodiac begins at the point of the vernal equinox — the moment when the Sun crosses the celestial equator moving north. It is tied to the calendar and to the seasons:

• 0° of Aries — the vernal equinox (~March 20);
• 0° of Cancer — the summer solstice (~June 21);
• 0° of Libra — the autumnal equinox (~September 23);
• 0° of Capricorn — the winter solstice (~December 22).

The Western horoscope is always "synced" with the seasons: the sign of Aries always falls on the astronomical spring.

Sidereal zodiac — the zodiac of the stars

The sidereal zodiac is measured off the fixed stars. The starting point is related to a particular star (often to Spica, the star of Libra). Since the stellar background is fixed and the equinox points "drift" because of the precession, the sidereal zodiac is shifted relative to the tropical.

The value of this shift is called ayanamsha. Today it makes about 24°. Therefore a sign shown as "Sun in Aries" in the tropical system in the sidereal one most often turns out to be Pisces.

Astronomical zodiac — the zodiac of real constellations

The astronomical approach gives up the equal sectors. It uses the real boundaries of the constellations, approved by the International Astronomical Union (IAS). It changes the picture:

• the Sun passes unequal periods in the constellations: about 45 days in Virgo, and only 7 in Scorpio;
• the 13th constellation is added — Ophiuchus (between Scorpio and Sagittarius);
• the dates are set by the astronomical ephemerides, and not by the scheme of the calendar.

Comparison table of the three systems

How to choose the suitable kind

If in doubt, start out with the tropical one: it is the standard of most applications. For the Vedic tradition, switch on the sidereal mode and choose the proper ayanamsha.

Recap

There are three kinds of zodiac: tropical (seasons), sidereal (stars, shift ~24°) and astronomical (real constellations). On lunar.day the systems can be switched on any page of the lunar calendar, and in the sidereal mode over forty variants of the ayanamsha are available.

---

Also read: what is the zodiac, tropical against sidereal, what ayanamsha is.