Astrology · behind the calculation
Tropical or sidereal, Placidus or equal houses
A birth chart is not a fixed object dropped from the sky. Behind every chart sit two calculation choices: which zodiac (tropical or sidereal) and which house system. They don’t change who you are, but they move where your planets land. Here’s what each one does, no jargon and no pretending there’s a single truth.
The two zodiacs
The tropical zodiac holds onto the seasons: 0° Aries is the spring equinox. It’s the frame of Western astrology, the one your Sun sign belongs to as you know it. It speaks of the cycle of light, not of the stars.
The sidereal zodiac holds onto the actual constellations. It’s the frame of Vedic astrology, in India. Because of the precession of the equinoxes (Earth slowly wobbles on its axis), the two zodiacs have drifted apart by about 24° over the centuries. Your “Aries” Sun in tropical can become “Pisces” in sidereal. Neither is wrong: they answer two questions, the seasons on one side, the fixed stars on the other.
The house systems
The houses are the twelve territories of your life (work, love, home…), laid onto your chart from the horizon and the meridian of your place and time of birth. That’s why the time matters so much. There are several ways to carve up those twelve territories. Here are the four we calculate:
- Whole sign
- The oldest, the most readable. One sign, one house. The 1st house starts at 0° of your rising sign, then each following sign is a whole house. Clean, sturdy everywhere on Earth, even near the poles. This is the system of Hellenistic astrology, rediscovered and very much in favour today.
- Equal houses
- Twelve 30° slices, starting from the exact degree of the rising sign. You keep the precise degree of the Ascendant as the cusp of the 1st house, then add 30° each time. Finer than whole sign on the Ascendant, while staying simple and stable.
- Porphyry
- Unequal houses, with no approximation. You divide the arc between the Midheaven and the Ascendant into three equal parts, then the arc between the Ascendant and the Imum Coeli the same way. The houses become unequal (they follow the sky’s asymmetry), but the calculation stays exact and geometrically transparent.
- Placidus
- The most used in the world, built on time. Each house matches a fraction of the TIME a degree takes to rise and then culminate. Very widespread, very detailed, but it warps at high latitudes and becomes undefined beyond the polar circles (there, we fall back on whole sign). Our Placidus calculation is checked to a hundredth of a degree against the references of the craft.
What we calculate, and why we show it to you
At Nohemia, positions are computed with a precision ephemeris (astronomy-engine, to the arcminute), and our Placidus calculation is checked to a hundredth of a degree against the references of the craft. In the app, you choose your zodiac and your house system, and a small i explains each option, museum-label style. No black box: you see what you’re reading, and why.
And we stay honest: without an exact birth time, the houses and the Ascendant are only estimates. The planets within the signs stay reliable.
Frequently asked questions
Tropical or sidereal: which one is right?
Neither is wrong, they answer two different questions. The tropical zodiac is anchored to the seasons (the spring equinox = 0° Aries): it’s the standard of Western astrology, the one your usual Sun sign belongs to. The sidereal zodiac is anchored to the actual constellations; it’s used in Vedic (Indian) astrology and shifts positions by about 24°, because of the precession of the equinoxes. Most Western readings use the tropical zodiac.
Why does my Ascendant change with the house system?
It doesn’t. The Ascendant (the degree rising in the east) and the Midheaven are identical in every quadrant system. What changes are the cusps of the intermediate houses (2, 3, 5, 6…): the same planet can fall in the 2nd house in one system and the 3rd in another. That’s why it helps to know which one you’re reading.
Why does the birth time matter so much?
Because the houses, the Ascendant and the Midheaven depend on the horizon and the meridian at the exact moment of your birth, and those turn by about 1° every 4 minutes. An approximate time shifts the whole scaffolding of the houses. The planets’ positions within the signs move far more slowly and stay reliable even without an exact time.
Which system should I pick if I’m starting out?
Whole sign: one sign, one house, nothing to memorise. When you want more nuance, Placidus is the most common in modern readings. In the app, you switch systems with a single tap and see the difference on your own chart.
The 12 houses, one by one · Understand your chart