Jathagam Guide - Part 0: What is Jathagam & How Machines Calculate It

ஜாதகம் வழிகாட்டி - பாகம் 0: ஜாதகம் என்றால் என்ன? கணினி எப்படி கணக்கிடுகிறது?

Welcome to the foundation article of this entire series! Before learning what each planet means or how to read a chart, we must first understand — what is Jathagam? And in today’s world, how does software actually compute one?

WHAT IS JATHAGAM? — ஜாதகம் என்றால் என்ன?

The Simple Definition

Jathagam (ஜாதகம்) — also called Janma Kundali, Birth Chart, or Horoscope — is a sky map frozen at the exact moment of your birth.

It records:

• Exactly where each planet was in the sky when you were born
• Which zodiac sign was rising on the eastern horizon (Lagna)
• How the 27 Nakshatras and 12 Rashis were positioned relative to the Moon and Sun

This snapshot is then used to understand a person’s personality, strengths, challenges, timing of life events, and karma from past lives.

Why Exactly Three Things Are Needed

To generate a Jathagam, three inputs are mandatory:

Jathagam vs Western Horoscope — Key Difference

This is why a person who is a “Scorpio” in Western astrology may be a “Libra” in Tamil Jathagam. Neither is wrong — they use different coordinate systems.

THE CALCULATION ENGINE — கணினி எப்படி கணக்கிடுகிறது?

The Big Picture — What the Algorithm Does

When you enter your birth details into a Jathagam app, here is the complete computation pipeline:

flowchart TD
    A["📥 Input: Date + Time + Place"] --> B["🗓️ Step 1: Convert to Julian Day Number (JDN)"]
    B --> C["🌍 Step 2: Apply Timezone → Universal Time (UT)"]
    C --> D["🔭 Step 3: Swiss Ephemeris → Tropical Planetary Longitudes"]
    D --> E["📐 Step 4: Apply Ayanamsa → Sidereal Longitudes"]
    E --> F1["🌙 Moon Longitude → Nakshatra + Rasi + Tithi"]
    E --> F2["☀️ Sun Longitude → Tamil Month + Uttarayana/Dakshinayana"]
    E --> F3["⬆️ Local Sidereal Time → Lagna (Ascendant)"]
    E --> F4["🪐 All 9 Planets → Rasi + Bhava placement"]
    F1 --> G["📊 Generate Rasi Chart + Navamsa Chart"]
    F2 --> G
    F3 --> G
    F4 --> G
    G --> H["⚠️ Detect Dosham (Manglik, Kala Sarpa, etc.)"]
    G --> I["✨ Detect Yogam (Raja, Dhana, Gaja Kesari, etc.)"]
    G --> J["⏳ Calculate Vimshottari Dasha Periods"]
    H --> K["📄 Final Jathagam Output"]
    I --> K
    J --> K

Let us now walk through each step in detail.

Step 1 — Julian Day Number (JDN)

The first thing any astrology engine does is convert your calendar date and time into a Julian Day Number (JDN) — a single continuous count of days since January 1, 4713 BCE (noon).

Why Julian Day? Because calendar systems change — Gregorian, Julian, proleptic — but the JDN is universal. All planetary position databases (including NASA’s JPL DE430 and Swiss Ephemeris) index their data using JDN.

Formula (simplified for Gregorian dates):

1
JDN = 367×Y − INT(7×(Y + INT((M+9)/12))/4)
2
      + INT(275×M/9) + D + 1721013.5 + (UT/24)

Where Y = Year, M = Month, D = Day, UT = Universal Time in decimal hours.

Example:

Birth: 15 August 1990, 10:30 AM IST, Chennai
IST = UT + 5:30 → UT = 10:30 − 5:30 = 05:00 UT
JDN ≈ 2448118.708

This single number now precisely identifies the moment of birth in astronomical time.

Step 2 — Timezone to Universal Time (UT)

Every birth time must be converted to Universal Time (UT / UTC) before astronomical calculations.

Step 3 — Planetary Longitudes via Swiss Ephemeris

Once the JDN is known, the engine feeds it into a planetary position library.

📌 The industry standard is Swiss Ephemeris (SE) — an open-source, high-precision astronomy library originally developed by Astrodienst AG (Switzerland), based on NASA’s JPL DE431 planetary dataset.

Swiss Ephemeris computes, for any JDN:

• The ecliptic longitude of each planet (0° to 360°)
• The speed (degrees/day) of each planet
• Whether the planet is in retrograde (moving backwards relative to Earth)

These longitudes are initially in the Tropical zodiac — measured from the Vernal Equinox point (0° Aries), which is a seasonal reference tied to the Sun’s position on the spring equinox.

Rahu and Ketu are not physical planets — they are the lunar nodes: the two points where the Moon’s orbit crosses the Sun’s ecliptic. Rahu is the north node (ascending), Ketu is the south node (descending). They are always exactly 180° apart and always retrograde.

Gulika (குளிகை) — a sub-planet of Saturn — is a calculated point (not an observed body) derived from Saturn’s position and the weekday. It is traditionally used in Tamil astrology for Dosham assessment.

Step 4 — Applying Ayanamsa (CRITICAL STEP)

Main Ayanamsa Systems:

Most Tamil Jathagam apps default to Lahiri Ayanamsa, which is the official standard recognised by the Government of India’s Rashtriya Panchang.

WHAT EACH VALUE IS BASED ON — எது எதிலிருந்து கணக்கிடப்படுகிறது?

The One-Page Mental Model — Before the Numbers

Before diving into the formulas, pause here. This is the most important conceptual clarity you need. Four of the five core Jathagam values each come from a completely different source — and mixing them up is the most common source of confusion.

1. Lagna — Earth’s Rotation (Not a Planet)

1
Lagna = The zodiac sign crossing the eastern horizon at your birth moment

Lagna is not based on any planet. It is based purely on Earth rotating on its own axis.

Imagine standing at your birth location and looking due East. The zodiac wheel is tilted across the sky. The exact degree of the zodiac that is intersecting the eastern horizon at that moment — that is your Lagna.

flowchart LR
    A["🌍 Earth rotating\n(once every 24 hours)"] --> B["Eastern horizon sweeps\nthrough all 360° of zodiac"]
    B --> C["The degree touching\nthe East horizon at birth\n= Lagna"]
    C --> D["Changes every\n~2 hours per Rasi"]

Key properties of Lagna:

• Requires exact birth time + exact birth location — both are mandatory
• Shifts every ~2 hours (all 12 Rashis cycle in 24 hours)
• Two twins born 2 hours apart in the same hospital can have different Lagnas
• It defines the 1st house — all other 11 houses radiate from it
• This is why a 15-minute birth time error can completely change the chart

2. Rasi — Moon’s Position in the Zodiac

1
Rasi = Which 30° segment of the zodiac the Moon occupies

The zodiac (360°) is divided into 12 Rashis of 30° each. Find the Moon. See which 30° block it sits in. That is the Rasi.

1
Moon longitude = 128.45°
2
128.45° ÷ 30° = 4.28
3
Floor(4.28) = 4 → 5th Rasi → Simha (சிம்மம்) 🦁

Why the Moon? Because in Tamil Jathagam the Moon represents the mind and emotional identity — so the Moon’s zodiac position becomes your “sign”. This is why a Tamil person says “நான் கடக ராசி” (I am Kataka Rasi) — they are naming their Moon’s Rasi, not the Sun’s sign as in Western astrology.

3. Nakshatra — Moon’s Position, but More Precise

1
Nakshatra = Which 13°20′ segment of the zodiac the Moon occupies

Nakshatra uses the same Moon longitude as Rasi — but cuts the zodiac into 27 smaller pieces instead of 12.

1
Each Nakshatra = 360° ÷ 27 = 13°20′ = 13.333°
2

3
Moon longitude = 128.45°
4
128.45° ÷ 13.333° = 9.63
5
Floor(9.63) = 9 → 10th Nakshatra → Magha (மகம்) ⭐

Rasi vs Nakshatra — the key difference:

Think of it this way: Rasi is the city, Nakshatra is the neighbourhood. Both describe where the Moon is — but Nakshatra is more precise. Every Rasi contains exactly 2¼ Nakshatras (= 30° ÷ 13.333°).

4. Tithi — The Angle Between Moon and Sun

1
Tithi = How far the Moon has moved ahead of the Sun
2
      = (Moon longitude − Sun longitude + 360°) mod 360°
3
      ÷ 12° gives the Tithi number

Tithi is not the Moon alone and not the Sun alone — it is the gap between them.

flowchart LR
    S["☀️ Sun\n118.20°"] -->|"Moon has moved\n10.25° ahead"| M["🌙 Moon\n128.45°"]
    M --> T["Gap = 10.25°\n10.25 ÷ 12 = 0.85\n→ Tithi 1 (Prathama)"]

Why this matters: The entire Tamil calendar — festival dates, auspicious days, Ekadashi fasting days, Karthigai Deepam — is built on Tithis. The Moon must reach a specific angle from the Sun for each of these.

5. Yoga — Sun + Moon Combined

1
Yoga = (Sun longitude + Moon longitude) ÷ 13.333°
2
→ 27 Yogas, same span as Nakshatras

Unlike Rasi, Nakshatra, and Tithi — Yoga uses both Sun and Moon longitudes added together. Each of the 27 Yogas has a character — some auspicious (Siddha, Amrita), some inauspicious (Vyatipata, Vaidhriti). This is the 4th element of the daily Panchangam (பஞ்சாங்கம்).

The Complete “What Comes From Where” — At a Glance

flowchart TD
    subgraph EARTH["🌍 Earth's Rotation"]
        L["Lagna — Eastern horizon\nChanges every ~2 hours"]
    end
    subgraph MOON["🌙 Moon Longitude"]
        R["Rasi — ÷ 30°\n12 signs, coarse"]
        N["Nakshatra — ÷ 13.333°\n27 stars, fine"]
    end
    subgraph GAP["🌙↔️☀️ Moon − Sun Angle"]
        T["Tithi — ÷ 12°\n30 lunar days"]
    end
    subgraph BOTH["☀️+🌙 Sun + Moon"]
        Y["Yoga — ÷ 13.333°\n27 Panchangam Yogas"]
    end
    subgraph SUN["☀️ Sun Longitude"]
        M["Tamil Month — ÷ 30°\n12 solar months"]
    end

DERIVING JATHAGAM VALUES — கணக்கீட்டு விவரங்கள்

Nakshatra — Moon’s Star Position

The 27 Nakshatras divide the full 360° zodiac into 27 equal segments:

1
Each Nakshatra = 360° ÷ 27 = 13°20′ = 13.3333°

Algorithm:

1
Nakshatra Index = Sidereal Moon Longitude ÷ 13.3333°
2
(Use the integer part + 1 for the Nakshatra number)

Worked Example:

Sidereal Moon Longitude = 128.45°
128.45 ÷ 13.3333 = 9.63
Integer part = 9 → 9 + 1 = 10th Nakshatra
→ Magha (மகம்) ⭐

Each Nakshatra is further divided into 4 Padas (quarters) of 3°20′ each — giving 108 Padas total, which corresponds to the 108 beads of a Japa Mala.

Rasi — Moon Sign (Janma Rasi)

The 12 Rashis divide 360° into 12 equal segments of 30° each:

1
Rasi Index = Sidereal Moon Longitude ÷ 30°
2
(Use integer part + 1 for Rasi number)

Worked Example:

Sidereal Moon Longitude = 128.45°
128.45 ÷ 30 = 4.28
Integer part = 4 → 4 + 1 = 5th Rasi
→ Simha / Simham (சிம்மம்) 🦁

Tithi — Lunar Day

A Tithi is a lunar day, calculated not from the clock but from the angle between the Moon and the Sun:

1
Moon–Sun Angle = Sidereal Moon Longitude − Sidereal Sun Longitude
2
(If negative, add 360°)
3

4
Tithi Number = (Moon–Sun Angle) ÷ 12°
5
(Integer part + 1 gives the Tithi)

Example:

Moon = 128.45°, Sun = 118.20°
Angle = 128.45 − 118.20 = 10.25°
10.25 ÷ 12 = 0.854 → Integer 0 + 1 = 1st Tithi → Prathama (பிரதமை)

There are 30 Tithis in a lunar month — 15 in the Shukla Paksha (waxing Moon) and 15 in the Krishna Paksha (waning Moon).

Tamil Month — From Sun Longitude

The Tamil Solar Calendar month is determined by the Sun’s sidereal longitude:

1
Tamil Month Index = Sidereal Sun Longitude ÷ 30°
2
(Integer part + 1 = month number)

Uttarayana / Dakshinayana:

• When Sun crosses 270° (enters Thai) → Uttarayana begins (January 14, Thai Pongal) — Sun’s northward journey, auspicious
• When Sun crosses 90° (enters Aadi) → Dakshinayana begins — Sun’s southward journey

Lagna — The Ascendant

The Lagna (Ascendant) is the zodiac sign rising on the eastern horizon at the moment of birth. It changes every ~2 hours and is highly location-specific.

Algorithm:

1
1. Compute Greenwich Sidereal Time (GST) from JDN
2
2. Add birth place's longitude offset → Local Sidereal Time (LST)
3
3. Convert LST to the ecliptic degree rising on the horizon
4
   (using the birth place's geographic latitude for obliquity correction)
5
4. Apply Ayanamsa → Sidereal Lagna longitude
6
5. Lagna Rasi = Sidereal Lagna longitude ÷ 30° (integer + 1)

This is why place of birth is mandatory — the same moment of birth in Chennai vs New York will produce completely different Lagnas.

Bhava Placement — Assigning Planets to Houses

Once all 9 planet longitudes (sidereal) and the Lagna longitude are known:

flowchart LR
    A[Lagna Longitude] --> B[House 1 = Lagna Rasi]
    B --> C[House 2 = Next Rasi]
    C --> D[House 3 ...]
    D --> E[... up to House 12]
    F[Each Planet's Sidereal Longitude] --> G[Compute Planet's Rasi]
    G --> H[Place in the matching Bhava/House]

In the Equal House system (used in most Tamil Jathagam):

• House 1 = Lagna Rasi and the 30° segment containing the Lagna degree
• Houses 2–12 = the 11 successive Rashis

Each planet falls into whichever 30° Rasi segment contains its longitude, and that determines its house.

CHARTS GENERATED — எந்த வரைபடங்கள் உருவாகும்?

Rasi Chart (ராசி சக்கரம்)

The Rasi Chart (D-1 chart) is the primary birth chart — it shows:

• All 9 planets placed in their Rasi (zodiac sign)
• The Lagna (Ascendant) marked
• Their house positions (Bhava) from the Lagna

This is the chart most people see first and the one used for all basic readings.

Tamil Chart Format (South Indian style): A fixed grid of 12 boxes. Rasis are fixed in position — Mesha always top-middle-left, going clockwise. The Lagna and planets are written inside the appropriate boxes.

Navamsa Chart (நவாம்சம் / D-9)

The Navamsa Chart is the second most important chart — used for:

• Marriage compatibility (primary use)
• Strength of planets — a planet weak in Rasi but strong in Navamsa still performs well
• After age 35, Navamsa gains more weight than the Rasi chart

How it is calculated:

Each Rasi (30°) is divided into 9 Navamsa segments of 3°20′ each. A planet’s Navamsa position is determined by which 3°20′ segment it falls in within its Rasi, mapped to a new Rasi sequence.

1
Navamsa Index = (Sidereal Planet Longitude mod 30°) ÷ 3.3333°
2
→ Integer gives the Navamsa pada (0–8)
3
→ Mapped to 12 Rashis in a fixed cycle

DOSHAM DETECTION — தோஷம் கண்டறிதல்

How Software Detects Dosham

Once planetary house positions are known, Dosham detection is straightforward rule-based logic:

1. Manglik Dosham (செவ்வாய் தோஷம்)

1
IF Mars (Chevvai) is in House 1, 2, 4, 7, 8, or 12
2
  → Manglik Dosham detected
3

4
(Some systems also check Navamsa chart for confirmation)

Exceptions in software: Many apps apply traditional exceptions — if Mars is in Aries, Scorpio, Capricorn, or Cancer (its own / exalted signs); or if Jupiter aspects the 7th house — the Dosham may be partially cancelled. These are programmed as conditional rules.

2. Kala Sarpa Dosham (காலசர்ப்ப தோஷம்)

1
IF all 7 planets (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn)
2
   are within the 180° arc from Rahu to Ketu (in Rahu's forward direction)
3
   AND no planet is in the remaining 180° arc
4
→ Kala Sarpa Dosham

flowchart LR
    R["🐍 Rahu"] -->|"All planets inside this arc"| K["☠️ Ketu"]
    K -->|"Empty arc"| R

12 types of Kala Sarpa are named based on which houses Rahu and Ketu occupy (Ananta, Kulika, Vasuki, etc.).

3. Naga Dosham (நாக தோஷம்)

1
IF Rahu or Ketu is in House 1, 2, 5, 7, or 9
2
→ Naga Dosham considered

4. Rahu–Ketu Dosham

1
IF Rahu or Ketu conjoins the Moon, Sun, or Lagna lord
2
→ Various shadow planet afflictions detected

YOGAM DETECTION — யோகம் கண்டறிதல்

How Software Detects Yogam

Yogams are auspicious planetary combinations — also detected as rule-based algorithms after house placement is complete.

1. Gaja Kesari Yogam (கஜகேசரி யோகம்)

1
IF Jupiter is in House 1, 4, 7, or 10 from the Moon
2
  (i.e., in Kendra from Moon)
3
→ Gaja Kesari Yogam

This is one of the most common and celebrated Yogams — grants intelligence, fame, and prosperity.

2. Raja Yogam (ராஜ யோகம்)

1
IF the lord of a Kendra house (1, 4, 7, 10)
2
   conjoins or mutually aspects the lord of a Trikona house (1, 5, 9)
3
→ Raja Yogam formed

The 1st house lord counts as both Kendra and Trikona, making it especially powerful.

3. Dhana Yogam (தன யோகம்)

1
IF lords of Houses 1, 2, 5, 9, or 11
2
   are in mutual conjunction, aspect, or exchange
3
→ Dhana Yogam (wealth-generating combination)

4. Pancha Mahapurusha Yogam (பஞ்ச மகாபுருஷ யோகம்)

Five separate Yogams, one for each of the five non-luminary planets:

Each grants extraordinary qualities of that planet — Ruchaka gives military prowess, Hamsa gives wisdom, Malavya gives beauty and luxury, etc.

THE SOFTWARE ENGINES — எந்த மென்பொருள் பயன்படுத்தப்படுகிறது?

What Powers Jathagam Apps Today

Swiss Ephemeris is the gold standard — accurate to within 1 arc-second for dates between 2400 BCE and 3400 CE. It is used by virtually all professional Jathagam software worldwide.

Popular Jathagam apps (Tamil / South Indian focus) include:

• Astro-Vision (AstroVision Futurepoint)
• Kundli Pro / Kundli Chakra
• Jagannatha Hora (free, by P.V.R. Narasimha Rao)
• Shri Jyoti Star
• Various web services: Astrosage, Prokerala, Mpanchang

All of these use Swiss Ephemeris (or equivalent) + Lahiri Ayanamsa as their default engine.

Full Calculation Summary

flowchart TD
    subgraph INPUT["📥 Input"]
        I1[Date of Birth]
        I2[Time of Birth]
        I3[Place of Birth]
    end

    subgraph STEP1["⚙️ Core Computation"]
        S1["JDN = Date + Time converted to Julian Day"]
        S2["UT = Local Time − Timezone Offset"]
        S3["Swiss Ephemeris → Tropical Longitudes of 9 Planets"]
        S4["Ayanamsa (~24.13°) subtracted → Sidereal Longitudes"]
    end

    subgraph STEP2["📐 Derived Values"]
        D1["Moon ÷ 13.333 → Nakshatra"]
        D2["Moon ÷ 30 → Rasi (Janma Rasi)"]
        D3["(Moon−Sun) ÷ 12 → Tithi"]
        D4["Sun ÷ 30 → Tamil Month"]
        D5["LST + Latitude → Lagna"]
        D6["All Planets → Bhava (House) Placement"]
    end

    subgraph STEP3["📊 Charts"]
        C1["Rasi Chart (D-1)"]
        C2["Navamsa Chart (D-9)"]
    end

    subgraph STEP4["⚠️✨ Analysis"]
        A1["Dosham Detection (Manglik, KalaSarpa, Naga...)"]
        A2["Yogam Detection (Raja, Dhana, GajaKesari...)"]
        A3["Vimshottari Dasha Periods"]
    end

    INPUT --> STEP1
    STEP1 --> STEP2
    STEP2 --> STEP3
    STEP3 --> STEP4
    STEP4 --> OUT["📄 Final Jathagam"]

QUICK REFERENCE — விரைவு குறிப்பு

Formulas at a Glance

நன்றி! வணக்கம்!

May this understanding of the ancient science — now computed with modern precision — bring you clarity and wisdom. The stars do not compel; they incline. The final choice is always yours.

ஓம் சாந்தி சாந்தி சாந்திஹி!
Om Shanti Shanti Shantihi!