# Vedic Astrology Calculation Rules

## Table of Contents

- [Key Differences from Western Astrology](#key-differences-from-western-astrology)
- [Ayanamsa: Concept and Major Systems](#ayanamsa-concept-and-major-systems)
- [Average Planetary Velocities (for estimation)](#average-planetary-velocities-for-estimation)
- [27 Nakshatras — Complete List](#27-nakshatras--complete-list)
- [Vimshottari Dasha System](#vimshottari-dasha-system)
- [Navagraha (Nine Planets)](#navagraha-nine-planets)
- [Twelve Houses (Bhavas)](#twelve-houses-bhavas)

---

## Key Differences from Western Astrology

| Dimension | Western Astrology (Tropical) | Vedic Astrology (Jyotish / Sidereal) |
|-----------|------------------------------|--------------------------------------|
| Zodiac system | Tropical zodiac (0° Aries = Vernal Equinox) | Sidereal zodiac (fixed stars as reference) |
| Difference (Ayanamsa) | — | ~23°–24°, increasing ~50.3" per year |
| Number of signs | 12 | 12 (same signs, different positions) |
| Lunar Mansion system | Not used | 27 Nakshatras (Lunar Mansions) — core tool |
| Rising sign | Ascendant | Lagna (same concept, sidereal zodiac) |
| Number of planets | 10 (including Pluto) | 9 Grahas (no Uranus / Neptune / Pluto) |
| Outer planets | Uranus, Neptune, Pluto | Replaced by Rahu (North Node) + Ketu (South Node) |
| Timing system | Progressions / Solar Return | Vimshottari Dasha (120-year cycle) |
| Predictive focus | Psychological tendencies, personality | Event timing, karmic pathways |
| House system | Placidus / Koch, etc. | Whole Sign Houses (primary) |

**Practical conversion**: Tropical longitude − Ayanamsa ≈ Sidereal longitude (~23–24° less)

---

## Ayanamsa: Concept and Major Systems

**Ayanamsa** (Sanskrit: "portion of movement") is the angular difference between the tropical zodiac and the sidereal zodiac, caused by the precession of the equinoxes. It increases by approximately 1° every 72 years.

### Comparison of Major Systems

| System | Value in 2000 | Value in 2025 (approx.) | Notes |
|--------|--------------|------------------------|-------|
| **Lahiri (Chitrapaksha)** | 23°51' | 24°13' | Official Indian government standard; most widely used |
| Raman | 22°23' | 22°46' | B.V. Raman system |
| Krishnamurti (KP) | 23°51' | 24°13' | Same base as Lahiri; used in the KP system |
| Fagan-Bradley | 24°02' | 24°25' | Common in Western sidereal astrology |
| True Chitrapaksha | 23°51' | ~24°13' | Precision system based on Spica's actual position |

**This Skill uses the Lahiri system by default.**

### Quick Estimation Formula

```
Lahiri Ayanamsa ≈ 23.85° + (year − 2000) × 0.0139°

Example: 2025 Ayanamsa ≈ 23.85° + 25 × 0.0139° ≈ 24.20°
```

Sidereal longitude = Tropical longitude − Ayanamsa (add 360° if the result is negative)

---

## Average Planetary Velocities (for estimation)

| Planet | Average daily motion | Full zodiac cycle |
|--------|---------------------|------------------|
| Sun | ~0.9856°/day | 365.25 days |
| Moon | ~13.176°/day | 27.32 days (sidereal month) |
| Mercury | ~1.38°/day (average, including retrograde) | 88 days |
| Venus | ~1.20°/day (average) | 225 days |
| Mars | ~0.524°/day | 687 days |
| Jupiter | ~0.0831°/day | 11.86 years |
| Saturn | ~0.0334°/day | 29.5 years |
| Rahu / Ketu | ~−0.0529°/day (retrograde) | 18.6 years |

**Note**: Mercury, Venus, and Mars have significant retrograde periods. The figures above are rough averages — precision is limited near boundary dates.

---

## 27 Nakshatras — Complete List

Each Nakshatra spans 13°20' (360° ÷ 27 = 13.333...°).

| # | Sanskrit Name | Sidereal Range (Lahiri, approx.) | Ruling Planet | Dasha Years |
|---|--------------|----------------------------------|---------------|-------------|
| 1 | Ashwini | Pisces 6°40' – Aries 0° (sidereal) | Ketu | 7 |
| 2 | Bharani | Aries 0° – 13°20' | Venus | 20 |
| 3 | Krittika | Aries 13°20' – 26°40' | Sun | 6 |
| 4 | Rohini | Taurus 0° – 13°20' (sidereal) | Moon | 10 |
| 5 | Mrigashira | Taurus 13°20' – Gemini 0° (sidereal) | Mars | 7 |
| 6 | Ardra | Gemini 0° – 13°20' | Rahu | 18 |
| 7 | Punarvasu | Gemini 13°20' – 26°40' | Jupiter | 16 |
| 8 | Pushya | Cancer 0° – 13°20' | Saturn | 19 |
| 9 | Ashlesha | Cancer 13°20' – 26°40' | Mercury | 17 |
| 10 | Magha | Cancer 26°40' – Leo 10° (tropical ref.) | Ketu | 7 |
| 11 | Purva Phalguni | Leo 0° – 13°20' (sidereal) | Venus | 20 |
| 12 | Uttara Phalguni | Leo 13°20' – 26°40' | Sun | 6 |
| 13 | Hasta | Virgo 0° – 13°20' (sidereal) | Moon | 10 |
| 14 | Chitra | Virgo 13°20' – 26°40' | Mars | 7 |
| 15 | Swati | Libra 0° – 13°20' (sidereal) | Rahu | 18 |
| 16 | Vishakha | Libra 13°20' – 26°40' | Jupiter | 16 |
| 17 | Anuradha | Scorpio 0° – 13°20' (sidereal) | Saturn | 19 |
| 18 | Jyeshtha | Scorpio 13°20' – 26°40' | Mercury | 17 |
| 19 | Mula | Scorpio 26°40' – Sagittarius 10° (tropical ref.) | Ketu | 7 |
| 20 | Purva Ashadha | Sagittarius 0° – 13°20' (sidereal) | Venus | 20 |
| 21 | Uttara Ashadha | Sagittarius 13°20' – 26°40' | Sun | 6 |
| 22 | Shravana | Capricorn 0° – 13°20' (sidereal) | Moon | 10 |
| 23 | Dhanishtha | Capricorn 13°20' – 26°40' | Mars | 7 |
| 24 | Shatabhisha | Aquarius 0° – 13°20' (sidereal) | Rahu | 18 |
| 25 | Purva Bhadrapada | Aquarius 13°20' – 26°40' | Jupiter | 16 |
| 26 | Uttara Bhadrapada | Pisces 0° – 13°20' (sidereal) | Saturn | 19 |
| 27 | Revati | Pisces 13°20' – 26°40' | Mercury | 17 |

> **Note**: Sidereal ranges above are approximate Lahiri values and shift slightly each year as the Ayanamsa increases. Use the sidereal longitude (tropical − ~24°) to look up the Nakshatra.

---

## Vimshottari Dasha System

### Planetary Period Table (total: 120 years)

| Planet | Mahadasha Years | Corresponding Nakshatras (starting Nakshatras) |
|--------|----------------|------------------------------------------------|
| Ketu | 7 | Ashwini, Magha, Mula |
| Venus | 20 | Bharani, Purva Phalguni, Purva Ashadha |
| Sun | 6 | Krittika, Uttara Phalguni, Uttara Ashadha |
| Moon | 10 | Rohini, Hasta, Shravana |
| Mars | 7 | Mrigashira, Chitra, Dhanishtha |
| Rahu | 18 | Ardra, Swati, Shatabhisha |
| Jupiter | 16 | Punarvasu, Vishakha, Purva Bhadrapada |
| Saturn | 19 | Pushya, Anuradha, Uttara Bhadrapada |
| Mercury | 17 | Ashlesha, Jyeshtha, Revati |

**Fixed sequence**: Ketu → Venus → Sun → Moon → Mars → Rahu → Jupiter → Saturn → Mercury → (back to Ketu)

### Dasha Calculation Method

**Step 1**: Identify the Moon's Nakshatra and its ruling planet (= the birth Dasha planet)

**Step 2**: Calculate the fraction of the Dasha already elapsed at birth

```
Degrees traversed within Nakshatra = Moon's sidereal longitude − Nakshatra start longitude
Nakshatra span = 13.333°

Fraction elapsed = degrees traversed / 13.333°

Remaining years in birth Dasha = planet's Dasha years × (1 − fraction elapsed)
```

**Step 3**: Starting from the birth date, add the Dasha periods in sequence until reaching today

```
Example: Born June 15, 1990 — Moon in Rohini (Moon Dasha, 10 years)
Moon approximately at Rohini 7° (elapsed: 7 / 13.33 ≈ 52.5%)
Remaining Moon Dasha at birth = 10 × (1 − 0.525) = 4.75 years → until ~March 1995

Timeline:
Jun 1990 – Mar 1995  Moon Mahadasha (remainder)
Mar 1995 – Mar 2002  Mars Mahadasha (7 years)
Mar 2002 – Mar 2020  Rahu Mahadasha (18 years)
Mar 2020 – Mar 2036  Jupiter Mahadasha (16 years)
...
```

### Antardasha (Sub-Period) Calculation

Each Mahadasha is subdivided into 9 Antardashas (sub-periods) in the same proportional sequence:

```
Antardasha duration = Mahadasha years × sub-planet's Dasha years / 120

Example: Jupiter Antardasha within Rahu Mahadasha (18 years):
18 × 16 / 120 = 2.4 years
```

The Antardasha sequence follows the same order as the Mahadasha, beginning with the Mahadasha planet itself.

---

## Navagraha (Nine Planets)

| Planet | Sanskrit Name | Nature | Domains |
|--------|--------------|--------|---------|
| Sun | Surya | Yang / Fire | Self, father, authority, soul |
| Moon | Chandra | Yin / Water | Mind, mother, emotions, prosperity |
| Mars | Mangala | Yang / Fire | Energy, courage, siblings, land |
| Mercury | Budha | Neutral / Earth | Intellect, communication, commerce |
| Jupiter | Guru / Brihaspati | Yang / Ether | Wisdom, religion, children, expansion |
| Venus | Shukra | Yin / Water | Love, beauty, wealth, the arts |
| Saturn | Shani | Yin / Air | Discipline, karma, delay, service |
| Rahu | Rahu (North Node) | Yin / Air (smoky) | Worldly desire, foreign lands, technology |
| Ketu | Ketu (South Node) | Yin / Fire (smoky) | Spirituality, liberation, past-life karma |

**Rahu and Ketu** are always exactly opposite each other (180° apart), move retrograde, and represent the Moon's ascending and descending nodes.

---

## Twelve Houses (Bhavas)

| House | Sanskrit Name | Core Domains | Notes vs. Western System |
|-------|--------------|-------------|--------------------------|
| 1st | Lagna | Self, body, personality, appearance | Similar to Western 1st house |
| 2nd | Dhana | Wealth, family, speech, food | Jyotish places greater emphasis on lineage and speech |
| 3rd | Sahaja | Siblings, effort, short travel, courage | Stronger emphasis on personal effort and valor |
| 4th | Sukha | Mother, home, inner happiness, vehicles | Similar to Western 4th |
| 5th | Putra | Children, intelligence, creativity, past merit | Greater emphasis on karma and intellectual gifts |
| 6th | Ripu | Enemies, illness, debts, service | Similar to Western 6th (house of difficulty) |
| 7th | Yuvati | Spouse, partnerships, public, trade | Similar to Western 7th |
| 8th | Randhra | Death, transformation, secrets, inheritance, the occult | Similar; stronger longevity emphasis |
| 9th | Dharma | Father, fate, religion, long travel, guru | Stronger emphasis on father and Dharma |
| 10th | Karma | Career, reputation, action, authority | Similar to Western 10th |
| 11th | Labha | Gains, friends, wish fulfillment | Greater emphasis on gains and desires |
| 12th | Vyaya | Loss, liberation, foreign lands, spiritual practice, bed pleasures | Greater emphasis on moksha (liberation) |

**Kendra (Angular houses)**: 1, 4, 7, 10 — Houses of Strength
**Trikona (Trine houses)**: 1, 5, 9 — Houses of Auspiciousness
**Dusthana (Difficult houses)**: 6, 8, 12 — Houses of Challenge
**Upachaya (Growth houses)**: 3, 6, 10, 11 — Houses that improve over time