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OwnStarMap

Astronomy method

How our star chart is calculated

This page documents the renderer as it works today. It separates verifiable calculations, visual choices, and phenomena that the chart does not model.

Method reviewed on August 5, 2026 · Documentation maintained by Théo Marie

Data verified in the repository

01

The source file contains 8,921 source records, including one Sun record. The renderer skips that record before plotting stars.

02

Stars use fixed catalog coordinates and source magnitudes up to 6.5. The number actually plotted changes with the geometric horizon and selected shape.

03

The constellation file contains 88 conventional line sets. It does not contain official IAU boundary polygons.

Implemented calculation

For a selected date, time, and place, the renderer performs these steps in order:

  1. 1

    Civil time to UTC

    Local civil time is converted to UTC with the applicable time zone and daylight-saving rules.

  2. 2

    Sidereal time

    Julian Date feeds Greenwich Mean Sidereal Time; longitude then produces Local Sidereal Time.

  3. 3

    Position in the local sky

    Fixed right ascension and declination are transformed into horizontal coordinates for the observer’s latitude.

  4. 4

    Selection and projection

    Points above the geometric horizon and inside the selected shape are kept, then placed with a stereographic projection.

What the chart does not model

These limitations matter most for dates far from the catalog epoch, moving objects, and real-world observing conditions.

  • The calculation does not apply precession or proper motion to the fixed catalog coordinates.
  • It does not plot the Moon or planets, and the Sun record in the source is excluded.
  • Atmospheric refraction, terrain, weather, and light pollution are not simulated.
  • When a local wall time is skipped or repeated during a daylight-saving transition, the current renderer uses a deterministic fallback instead of asking which instant was intended.
  • Constellation lines are graphic conventions, not official IAU boundary polygons.
  • The Milky Way is a procedural artistic layer, not a measured sky dataset.

Public, reproducible evidence

The manifest publishes checksums, record counts, the implemented method, and its limitations. You can also inspect both input files directly.

Primary references

Inspect the result before ordering

The free preview uses the same placement engine as the final chart. Choose a place and moment, then inspect the rendering yourself.

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