Whole Sign And Equal House Calculations
Both supported methods begin with the Ascendant. However, they define house boundaries differently. The difference is important when reading a natal chart because planets near a cusp may be assigned to different houses depending on the chosen system.
Whole Sign Houses: The Ascendant's entire zodiac sign becomes the first house. The following sign becomes the second house, beginning at 0° of that sign. If your Ascendant is Leo, your second house is Virgo, regardless of the Ascendant's degree.
Equal Houses: The first-house cusp begins at the exact Ascendant longitude. Each successive house cusp is separated by 30°. Therefore, the second-house cusp is the Ascendant's ecliptic longitude plus 30°, wrapped around the 360° zodiac.
Equal House Example
Suppose your Ascendant is 17°25′ Leo. Leo begins at 120° zodiacal longitude, so the Ascendant is at 137°25′. Add 30° to obtain 167°25′, corresponding to 17°25′ Virgo.
Your second-house cusp is therefore 17°25′ Virgo. In Whole Sign houses, Virgo also rules the second house, but the house begins at 0° Virgo instead.
For either method, the second-house sign is the zodiac sign immediately following the Ascendant sign. The main difference is the precise cusp degree and the house boundaries. In other house systems, including Placidus, the second-house cusp may fall in a different zodiac sign.
To explore the mathematical conventions used in astrology, see Astrodienst's explanation of astrological houses.
Why Your Birth Time Matters
The Ascendant depends on the time and geographic location of birth. Earth's rotation causes the eastern horizon to move through the zodiac over the course of a day, and signs do not necessarily rise for equal lengths of clock time.
An inaccurate birth time can change the Ascendant sign or degree and therefore alter house calculations. If your birth time is unknown, avoid treating a guessed Ascendant as an exact result.
Why House Systems Can Disagree
Whole Sign and Equal House methods are straightforward, but quadrant systems use different geometrical rules. Placidus, for example, incorporates diurnal motion and requires birth coordinates and time-dependent astronomical calculations.
Do not interpret this calculator's Equal House result as a Placidus cusp. For astronomical background on celestial coordinates, consult the U.S. Naval Observatory's explanation of celestial positions.