# Space Observatory — data, scope and sources

The observatory is a learning atlas. Its all-sky map has **5,044 measured bright-star positions** and line patterns for **all 88 IAU constellations**. Its Solar System, nearby-star and deep-sky tours are curated examples, not a complete catalog of the universe. The observer tool estimates a selected star, constellation centre or deep-sky object's geometric altitude and azimuth for an Earth location and UTC time, with a 24-hour altitude track. It uses approximate sidereal time based on [US Naval Observatory guidance](https://aa.usno.navy.mil/faq/GAST) and [equatorial-to-horizon transformations](https://aa.usno.navy.mil/faq/alt_az). UTC approximates UT1; catalog directions remain J2000 without precession, nutation, atmospheric refraction or terrain corrections. This is an educational horizon estimate, **not telescope-pointing guidance or a visibility prediction**. Solar-system objects are excluded because their positions are illustrative rather than current ephemerides.

## Local sky catalog

`data/catalog.json` is normalized from [Olaf Frohn's d3-celestial](https://github.com/ofrohn/d3-celestial), commit `7e720a3de062059d4c5400a379146a601d9010e0`, downloaded 21 October 2026. The project's BSD 3-Clause license is retained in `data/LICENSE-d3-celestial.txt`; original source and format notes are retained beside the data.

- `stars.6.json` → `stars.source.json`: XHIP, *An Extended Hipparcos Compilation*, Anderson & Francis (2012), [VizieR V/137D](https://cdsarc.cds.unistra.fr/viz-bin/cat/V/137D). Apparent visual magnitude and B−V colour index are catalog measurements; colour rendering is an illustration of that index.
- `starnames.json` → `starnames.source.json`: names/designations assembled by d3-celestial from the catalogs acknowledged in `data/UPSTREAM-README.md`. The normalized catalog uses proper names or Bayer/Flamsteed/HIP identifiers. It does not invent names or distances for unnamed stars.
- `constellations.json` and `constellations.lines.json`: IAU names, reference label positions and d3-celestial line patterns. Serpens Caput and Serpens Cauda are combined into the single official constellation **Serpens**, with both line sets retained.
- `dsos.bright.json` and `messier.json`: upstream Saguaro Astronomy Club and H. Frommert/SEDS compilations, retained as coordinate cross-checks for curated objects.

All catalog coordinates are **J2000 equatorial directions**. Right ascension is normalized from signed GeoJSON longitude to **0–360 degrees**; declination stays **−90–+90 degrees**. It is not converted to degrees from hours a second time. Reproduce the normalized catalog with `node apps/space-observatory/data/build-catalog.cjs`.

At load time `ObservatoryModel.normalizeCatalog` validates the fetched catalog (malformed rows are dropped; an unusable response leaves the curated fallback in place). Upstream constellation `ra`/`dec` values are **label anchors**, not centres: Orion's anchor sits at +13° although its figure is centred near +2°. The app therefore computes each constellation's displayed position, focus point and horizon estimate as the **mean unit vector of its distinct line-pattern vertices** (seam-safe at 0h), keeping the anchors as `labelRa`/`labelDec`. This is the centre of the drawn figure, which can differ by several degrees from the centre of the official IAU boundary (median 4.7°, largest for Serpens with its two parts). Verified in `tests/audit.test.cjs` against approximate IAU region centres for Orion, Crux, Cassiopeia, Ursa Major and Puppis.

The normalized `stars` array contains `{id, hip, name, ra, dec, mag, bv, constellation}`; `bv` and `constellation` may be null. The `constellations` array contains `{id, name, ra, dec, lines}` with each line a sequence of `[ra, dec]` pairs. Lines crossing zero right ascension must be drawn on the sphere or wrapped correctly on a flat map.

An official constellation is a **bounded sky area**, not a physical star system. Stick figures vary by map and are not official boundaries. The stars in a pattern usually lie at different distances. See [IAU: The Constellations](https://www.iau.org/IAU/Astronomy-FAQs/Constellations.aspx).

## Curated objects and educational scale

`data.js` exposes `ObservatoryData.OBJECTS`, `SOURCES`, constants and a spherical direction helper. Each object carries its own primary-source URL. Facts are original short paraphrases.

- Solar System sizes, orbital periods and average distances: [NASA planetary fact sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/), [JPL planetary physical parameters](https://ssd.jpl.nasa.gov/planets/phys_par.html), and NASA's individual body pages linked in each record. Radii are rounded mean radii. Orbits may be shown schematically; initial phases are illustrative, not today's positions. Planet distances are **from the Sun**; the Moon's distance is **from Earth**. These must not be labeled Earth-to-planet distances.
- The Sun's `distanceAU:1` is its average Earth distance. Its `distanceLy:0` exists only to place our star at the origin of a Sun-centred nearby-star model. It is not a claim that sunlight takes no time to reach Earth.
- Nearby stars: [ESA Hipparcos closest-star table](https://www.cosmos.esa.int/web/hipparcos/closest-stars), [NASA Sirius A and B](https://science.nasa.gov/asset/hubble/the-dog-star-sirius-and-its-tiny-companion/), [NASA Summer Triangle](https://science.nasa.gov/image-article/apod-2015-june-27-stars-of-a-summer-triangle/), and the per-object links. Nearby-map distances are rounded; not all nearby stars are included. Sun-centred and Earth-centred stellar distances differ negligibly at the precision displayed. Stellar motion is not propagated beyond J2000.
- Betelgeuse: [NASA's educational profile](https://science.nasa.gov/universe/what-is-betelgeuse-inside-the-strange-volatile-star/) uses about 700 light-years. Its distance is uncertain and published estimates differ; the app labels this as a rounded estimate.
- Galaxies, nebulae and clusters: [NASA Hubble Messier catalog](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/) and individual records. In particular: M31 about 2.5 million ly, M33 about 3 million ly, M51 about 31 million ly, M42 about 1,500 ly, M1 about 6,500 ly, M45 about 445 ly. Distances depend on measurement methods and may be revised.
- Magellanic Clouds: [NASA's Swift ultraviolet survey](https://www.nasa.gov/centers-and-facilities/goddard/nasas-swift-produces-best-ultraviolet-maps-of-the-nearest-galaxies/) gives approximately 163,000 and 200,000 ly.
- Omega Centauri: [NASA Hubble core image metadata](https://science.nasa.gov/asset/hubble/core-of-omega-centauri/) gives approximately 16,000 ly.
- Sombrero: the later [NASA Hubble image record](https://science.nasa.gov/asset/hubble/sombrero-galaxy-2/) uses approximately 30 million ly; older NASA pages use 28 million ly. The app deliberately rounds to 30 million.
- Sagittarius A*: [ESO's EHT report](https://www.eso.org/public/news/eso2406/) uses about 27,000 ly. The Milky Way marker uses the **galactic centre's direction and distance**, not a fictitious distance to our own galaxy. We live inside the Milky Way. It should not be plotted as another galaxy centred on the Sun.

Symbols and illustrated models are enlarged for readability. They are not telescope images or true angular sizes. Light-travel time in years can be read from a distance in light-years for these nearby-universe examples; this is not a spacecraft travel estimate.

## Attribution retained with the data

Copyright (c) 2015, Olaf Frohn. Source conversion and line modifications: d3-celestial. Original measurements and catalog compilation: the astronomy catalogs above and those credited in `data/UPSTREAM-README.md`. No NASA, ESA, IAU or ESO endorsement is implied.

## Local telescope photographs

These are real astronomical observations used in the inspector, separate from the illustrative map artwork. Preview renditions were downloaded on 21 October 2026 without further image editing; the four files total about 341 KB. Colours in astronomical composites may map different filters to visible colours and do not necessarily match unaided-eye appearance. Use is educational and informational under [NASA's Images and Media Usage Guidelines](https://www.nasa.gov/nasa-brand-center/images-and-media/), with source credits retained. The chosen images have no third-party copyright restriction identified on their NASA source records. No endorsement is implied.

| Local file | Observation and credit | Source and downloaded rendition |
| --- | --- | --- |
| `data/images/andromeda.jpg` | 2025 Hubble mosaic, 1200 × 281. NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); image processing: Joseph DePasquale (STScI). The jagged border marks mosaic coverage, not the galaxy's physical outline. | [NASA M31 source](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-31/); [preview asset](https://assets.science.nasa.gov/dynamicimage/assets/science/missions/hubble/galaxies/andromeda/Hubble_M31Mosaic_2025_42208x9870_STScI-01JGY8MZB6RAYKZ1V4CHGN37Q6.jpg?w=1200&h=281&fit=crop&crop=faces%2Cfocalpoint). |
| `data/images/orion-nebula.webp` | Hubble Orion mosaic, 900 × 900. NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team. | [NASA M42 source](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-42/); [preview asset](https://science.nasa.gov/wp-content/uploads/2023/04/orion-nebula-xlarge_web-jpg.webp?w=900). |
| `data/images/whirlpool.webp` | Hubble M51 and NGC 5195, 900 × 626. NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA). | [NASA M51 source](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-51/); [preview asset](https://science.nasa.gov/wp-content/uploads/2023/04/m51-and-companion_0-jpg.webp?w=900). |
| `data/images/pleiades.jpg` | Palomar Observatory Sky Survey / Digitized Sky Survey ground-based colour composite, 900 × 649. NASA, ESA and AURA/Caltech. This is the full cluster photograph associated with a Hubble distance study, **not a Hubble photograph** or the close-up of dust near Merope on the Messier page. | [NASA image record](https://science.nasa.gov/asset/hubble/hubble-refines-distance-to-the-pleiades-star-cluster/), linked from the M45 source page; [preview asset](https://assets.science.nasa.gov/dynamicimage/assets/science/missions/hubble/releases/2004/06/STScI-01EVVEYWX1TA3MGBK5F6EFQVGQ.tif?w=900) (NASA's image service returns JPEG). |