> ## Documentation Index
> Fetch the complete documentation index at: https://docs.jyotiguide.com/llms.txt
> Use this file to discover all available pages before exploring further.

# How Jyoti Guide Converts a Birth Moment to Sidereal Sky

> Learn how the engine converts a birth moment into precise sidereal positions — from clock time through ayanāṁśa subtraction to Lagna and houses.

Before any astrology can happen, the app must answer a purely astronomical question: where was everything in the sky at that exact instant, seen from that precise spot on Earth? The engine follows a clear, step-by-step pipeline so that every number in your chart traces back to a transparent calculation rather than an opaque black box. The astronomy follows standard treatments such as V. P. Jain's *Elements of Astronomy* — the same chain of steps a careful astrologer once worked through by hand using an ephemeris.

## The conversion pipeline

<Steps>
  <Step title="Enter local clock time and place">
    You supply a **local clock time** and a **birthplace**. The place fixes a time zone, and the engine uses that to convert your entry to **Universal Time (UT)** — the global reference clock. For example, `2026-06-01 10:30` in `Asia/Kolkata` (UTC+05:30) becomes `2026-06-01 05:00 UT`. All subsequent calculations run against this single universal instant.

    <Info>
      The app handles historical time-zone quirks — war-time offsets, zones that have shifted over the decades — through its built-in place database. When the civil time zone for a historic birth is uncertain, use the **Enter coordinates** option to set the UTC offset explicitly.
    </Info>
  </Step>

  <Step title="Compute tropical longitudes">
    From the universal instant, the engine calculates each body's **tropical longitude** — its position measured eastward from the March-equinox point. The Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn each follow well-established orbital models. The lunar nodes **Rāhu** and **Ketu** are computed as the points where the Moon's orbit crosses the ecliptic.

    <Info>
      **Mean vs. true nodes.** Rāhu and Ketu can be expressed as the *mean node* (a smoothed average motion) or the *true node* (the instantaneous crossing point). Jyoti Guide uses the **mean node** by default — the most common convention in *jyotiṣa* — and places Ketu exactly 180° opposite Rāhu. You can switch this in **Chart settings**.
    </Info>
  </Step>

  <Step title="Subtract the ayanāṁśa">
    To move from the tropical frame to the **sidereal** (fixed-star) frame, the engine subtracts the **ayanāṁśa** — the precessional gap described in [Why sidereal?](/introduction/why-sidereal) — from every longitude. The result is each body's position against the actual constellations. Switching the ayanāṁśa setting (Lahiri, Raman, Pushya) changes this single subtraction and therefore shifts the whole chart together. See [Ayanāṁśa options](/reference/ayanamsa) for the values each system uses.
  </Step>

  <Step title="Derive the Lagna (rising sign)">
    The houses are anchored to the **Lagna** — the degree of the zodiac rising over the eastern horizon at birth. Computing it requires the **latitude and longitude of the birthplace** and the **local sidereal time**, because the horizon's angle depends on where on Earth you stand. The Lagna sweeps through all twelve signs roughly once every 24 hours, changing sign approximately every two hours. This is why a precise birth time is essential.
  </Step>

  <Step title="Assign whole-sign houses">
    With the Lagna known, the engine assigns houses. Jyoti Guide uses the **whole-sign house system** by default: the sign containing the Lagna degree becomes the entire 1st house, the next sign the 2nd house, and so on. Signs and houses align one-to-one, which keeps the chart clean and is the oldest and most widespread house system in classical *jyotiṣa*.
  </Step>

  <Step title="Resolve nakṣatra and pada">
    Finally, each body's sidereal longitude is resolved into a **nakṣatra** (lunar mansion) and **pada** (quarter). These feed directly into period calculations and divisional charts.
  </Step>
</Steps>

## What the output looks like

At the end of the pipeline every body has a sidereal longitude expressed as a sign, a degree within that sign, a house, and a nakṣatra–pada pair. Everything else in the app — strength scores, daśā periods, transits, compatibility — builds on these primary positions.

```text theme={null}
Lagna   1st   Cancer / Karka       17°06′40″   Ashlesha · 1
Sun     11th  Taurus / Vrishabha   16°30′36″   Rohini · 2
Moon     5th  Scorpio / Vrischika  25°43′28″   Jyeshtha · 3   (debilitated)
…
```

<Tip>
  **Continue reading:** now that you know how the positions are built, learn about the twelve signs and nine planets that interpret them — [Rāśis & Grahas](/concepts/rashis-and-grahas).
</Tip>