Provenance
Credits and sources
Nothing on this map is invented. Every star, boundary and predicted time comes from a published catalogue or a published method, named below with its licence and, where the positions are computed, with the measured distance from JPL Horizons.
Sky data
| Layer | Source | Licence |
|---|---|---|
| Star positions, magnitudes, colours, distances 8,920 stars to magnitude 6.5 | HYG stellar database v4.2, David Nash (Astronexus) | CC BY-SA 4.0 |
| Faint stars, packed binary tier 74,559 stars, magnitude 6.5–9.0, 874 KB | HYG stellar database v4.2 | CC BY-SA 4.0 |
| Proper names and Bayer/Flamsteed designations Keyed by Hipparcos number; from the Kostjuk HD–DM–GC–HR–HIP cross index (VizieR IV/27A) | d3-celestial, Olaf Frohn | BSD-3-Clause |
| Constellation stick figures 88 figures, the IAU set drawn by Alan MacRobert for Sky & Telescope | Stellarium sky culture “modern_iau” | CC BY-SA 4.0 |
| Asterisms 61 figures that are not constellations — the Plough, the Summer Triangle, the Teapot | Stellarium sky culture “western” | CC BY-SA 4.0 |
| Constellation boundary lines IAU boundaries (Delporte 1930) as drawn, J2000; upstream is Davenhall & Leggett, VizieR VI/49 | d3-celestial, Olaf Frohn | BSD-3-Clause |
| Which constellation a position falls in 357 ordered rows in B1875 — the frame the boundaries were drawn in, and the only one where they are exactly horizontal and vertical | Roman (1987), “Identification of a Constellation From Position”, VizieR VI/42 | CDS, free to use with citation |
| Region label anchors 89 anchors — Serpens counts twice, because it is two areas of sky | d3-celestial, Olaf Frohn | BSD-3-Clause |
| Deep-sky objects 1,412 objects to magnitude 10, filtered from the 15,809-object catalogue; upstream is the Saguaro Astronomy Club database v8.1 | d3-celestial, Olaf Frohn | BSD-3-Clause |
| Milky Way Equirectangular plate carrée in equatorial coordinates, mapped to the inside of the sky sphere | NASA SVS Deep Star Maps 2020 | NASA imagery, not copyrighted |
| ISS orbital elements Fetched live for NORAD catalogue 25544, cached for a day; a bundled element set is used when the fetch fails | CelesTrak, Dr T. S. Kelso | Free public element sets |
The ISS row is the only catalogue fetched at runtime. Everything else above, typefaces included, is a static file from this domain. Privacy has the detail.
Two tables are this repository's own rather than upstream: 77 observing cities with their IANA time zones, and 10 annual meteor showers with peak date, radiant and zenithal hourly rate. Those rates are the published long-term maxima under a perfect sky, not a forecast for a particular night.
How the positions are computed
The engine carries no ephemeris file: Sun, Moon and planets are computed from published series for the instant on the clock. Every figure below is a test expectation in this repository, with its reference query recorded beside it.
| Quantity | Method | Measured against |
|---|---|---|
| Star places | HYG J2000, precessed to the date on the clock | — |
| Precession | IAU 1976 | — |
| Sun | Meeus, Astronomical Algorithms, ch. 25 | — |
| Moon | Meeus ch. 47, full truncation, on Terrestrial Time, with nutation | JPL Horizons DE441, 60 epochs: 8.1″ worst in longitude, 4.0″ in latitude, 45 km in distance |
| Planets | JPL “Approximate Positions of the Major Planets”, Table 1, corrected for light time | JPL Horizons, 4 epochs: worst residual 24″ Mercury, 38″ Venus, 52″ Mars, 194″ Jupiter, 298″ Saturn, 66″ Uranus, 52″ Neptune |
| Planet brightness | Astronomical Almanac phase-angle series; Saturn carries its rings | Within 0.1 mag of Horizons, Uranus and Neptune within 0.02. Two deliberate disagreements: Saturn runs 0.10–0.24 mag brighter because Horizons reports the globe without the ring system, and Mercury past 150° of phase reads 0.38 mag faint where the series is extrapolating |
| Twilight, rise, set, transit | Bisected altitude crossings | Agrees with an independent analytic solver to 0.1 s |
| Refraction | Bennett (1982) and Sæmundsson (1986); opt-in, never silent | Within 0.1′ of the standard table |
| Air mass | Kasten & Young (1989) — stays finite at the horizon | — |
| ΔT and leap seconds | Espenak & Meeus polynomials before 1972; IERS Bulletin C after | −2.8 s at 1900.0, +13.4 s at 1750.0, +120 s at 1600.0 |
| ISS passes | SGP4 propagation of the current element set | — |
For scale: 298″ is about a tenth of the Moon's width, so Saturn is drawn within a fifth of a Moon-diameter of where JPL puts it. Well inside naked-eye and binocular pointing; well outside what a telescope at high power would accept.
Software and type
| Library | Version | What it does | Licence |
|---|---|---|---|
| three.js | 0.183.2 | WebGL renderer; stars are GPU point sprites | MIT |
| satellite.js | 5.0.0 | SGP4 orbital propagation | MIT |
| earcut | 3.0.2 | Triangulates constellation regions for the highlight fill | ISC |
| Astro | 6.1.1 | Static site build; every page here is pre-rendered HTML | MIT |
| Tailwind CSS | 4.2.2 | Utility layer under the design tokens | MIT |
| TypeScript | 6.0.3 | Strict mode throughout | Apache-2.0 |
| Vitest | 4.1.2 | Runs the astronomy tests quoted above | MIT |
| Font Awesome Pro (Sharp) | 7.3.1 | Icons, inlined as SVG at build time | Commercial licence; the glyphs are not redistributable |
Three typefaces, all under the SIL Open Font License 1.1 and bundled rather than fetched from a CDN: Inter for the interface, JetBrains Mono for anything measured, and Lora for prose.
Books and papers
- Meeus, J. — Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998).
- Standish, E. M. — Keplerian Elements for Approximate Positions of the Major Planets (JPL Solar System Dynamics).
- Roman, N. G. — “Identification of a Constellation From Position”, PASP 99, 695 (1987).
- Delporte, E. — Délimitation Scientifique des Constellations (IAU, 1930).
- Kasten, F. & Young, A. T. — “Revised optical air mass tables and approximation formula”, Applied Optics 28, 4735 (1989).
- Bennett, G. G. — “The calculation of astronomical refraction in marine navigation”, Journal of Navigation 35, 255 (1982).
- Sæmundsson, Þ. — “Atmospheric refraction”, Sky & Telescope 72, 70 (1986).
- Espenak, F. & Meeus, J. — Five Millennium Canon of Solar Eclipses (NASA TP-2006-214141).
- Bortle, J. E. — “The Bortle Dark-Sky Scale”, Sky & Telescope, February 2001.