Zodiac System
The difference between tropical and sidereal zodiacs, and how HOSHIDOU calculates the sidereal one.
The Zodiac system setting determines the reference frame used to define the positions of the 12 zodiac signs. Choose between tropical and sidereal.
HOSHIDOU calculates the sidereal zodiac with the Lahiri ayanamsa, and you cannot choose another. The current gap from the tropical zodiac is about 24°, and it grows slowly.
Tropical
The tropical zodiac anchors the start of Aries to the spring equinox (vernal point). Because it is based on the cycle of seasons (the equinoxes and solstices), the sign boundaries gradually drift away from the actual constellations over time.
The vast majority of Western astrology uses the tropical system, and it is the default in HOSHIDOU. Nearly all English-language astrology books and online resources assume tropical positions.
Sidereal
The sidereal zodiac is anchored to the actual positions of the fixed stars. It is the system used in Vedic astrology (Jyotish).
Unlike the tropical zodiac, sign definitions are locked to the stars and are not affected by the precession of the equinoxes. The same birth details produce different sign placements in tropical and sidereal.
How to switch
Open Display settings with the gear on a chart screen, Home, or Ephemeris. On the Calc & tables tab, under Calculation, choose Tropical or Sidereal for the Zodiac system. The change takes effect right away.
If you practice Western astrology, leave this on Tropical. Sidereal is for Vedic (Jyotish) practitioners and anyone interested in star-based interpretation.
Note: ayanamsa
Ayanamsa is the angular difference between the tropical and sidereal zodiacs. Because the spring equinox point shifts gradually due to the precession of the Earth’s axis, the gap between the two systems changes over time.
When you select sidereal, HOSHIDOU works out that gap with the Lahiri method and converts tropical positions into sidereal positions.
Related pages
- Zodiac Signs — what each of the 12 signs means under either system.
- Calculation Presets — switch zodiac, house, and planet settings together.