WhatTheStar

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

LayerSourceLicence
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, 438 KB HYG stellar database v4.2 CC BY-SA 4.0
Stars to eleventh magnitude 837,270 stars that the zoom shows as binoculars would, served in patches of sky as you zoom in; those already in the catalogue above left out Tycho-2 Catalogue (Høg et al. 2000, A&A 355, L27), via CDS ESA; free for use with the citation
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 62 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 1,367 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
Galaxy and nebula shapes Position angles for 131 of those objects, and sizes where the catalogue above gives no minor axis or no size at all, matched by NGC, IC, Messier and PGC designation OpenNGC, Mattia Verga CC BY-SA 4.0
Double and multiple stars 49 systems, each pair's last measured separation, position angle and component magnitudes; companions the star catalogue lacks are drawn from these Washington Double Star Catalog, Mason et al. (USNO), via VizieR B/wds US Naval Observatory; free for use with the citation
Binary star orbits 9 visual orbits — α Centauri, Castor, Porrima, 70 Ophiuchi and others — that move the companion with the clock Sixth Catalog of Orbits of Visual Binary Stars (ORB6), Hartkopf, Mason & Worley USNO / Georgia State University; free for use with the citation
Variable stars 43 variables' classes, ranges and light-curve shapes; 28 keep a clock the map runs. Clocks are GCVS's where recent, AAVSO VSX's for 11 pulsators with a newer epoch, and the Kraków up-to-date elements (Kreiner 2004) for 6 eclipsing binaries General Catalogue of Variable Stars, Samus et al. (GCVS v5.1), via VizieR B/gcvs Sternberg Astronomical Institute / INASAN; free for use with the citation
Cepheid colours B−V at maximum and minimum light for six Cepheids, fitted to their published UBVRI photometry Berdnikov (1997), Photoelectric observations of Cepheids, VizieR II/217 CDS, free to use with citation
Milky Way Equirectangular plate carrée in equatorial coordinates, mapped to the inside of the sky sphere: 4k for the whole sky, and 8k in 45° tiles that stream in as you zoom NASA SVS Deep Star Maps 2020 NASA imagery, not copyrighted
The Moon's face Albedo map on the Moon's disc in the sky and on the globe in the view from space, turned to the Moon's IAU pole; reduced to 1024 × 512 greyscale NASA SVS CGI Moon Kit (Lunar Reconnaissance Orbiter LROC colour mosaic) NASA imagery, not copyrighted
Deep-sky pictures 166 galaxies, nebulae and globular clusters — the Messier objects and the catalogue's other named showpieces — cut in their true projection and scale through CDS's hips2fits, stars removed so the map's own stars show through. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166, from photographic data obtained with the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope; 3 come from Pan-STARRS1 (Institute for Astronomy, University of Hawaii, and partners) DSS2 colour and Pan-STARRS1 DR1, via CDS hips2fits DSS: © Caltech, AAO/UKSTU and AURA, by agreement; HiPS © CNRS/Unistra (ODbL)
The sky survey at deep zoom The whole sky photographed, under the stars below a 15° field when you turn Sky survey on: the Digitized Sky Survey's red and blue plates (POSS-II, SERC and AAO), coloured and cut into HEALPix tiles by CDS, fetched from its server as you look DSS colored HiPS (CDS/P/DSS2/color), CDS, Strasbourg DSS: © Caltech, AAO/UKSTU and AURA, by agreement; HiPS © CNRS/Unistra (ODbL)
Observing locations 8,803 places with their IANA time zones: every GeoNames city of 50,000 people or more, plus a hand-kept list of smaller towns and observing sites GeoNames gazetteer; zones from the timezone-boundary-builder polygons CC BY 4.0
The globe Coastlines, the Antarctic and Greenland ice sheets, and the Antarctic ice shelves on the Earth seen from space, from the 1:110m land and glaciated areas and the 1:50m ice shelves Natural Earth Public domain
The Earth by day The daylit globe seen from space: a cloud-free true-colour mosaic of MODIS imagery with shaded relief, one for each month of 2004 — Stöckli R., Vermote E., Saleous N., Simmon R., Herring D. (2005), The Blue Marble Next Generation, NASA Earth Observatory NASA Earth Observatory, Blue Marble Next Generation Public domain
Sea ice on the globe The Arctic and Antarctic pack ice on the Earth seen from space, growing and shrinking through the year: each month the ice at least half of 2015–2024 had — Fetterer F., Knowles K., Meier W. N., Savoie M., Windnagel A. K., Stafford T. (2025), Sea Ice Index, Version 4, NSIDC, doi:10.7265/a98x-0f50 NSIDC Sea Ice Index (G02135) Free to use, cited
Sky brightness where you are The Bortle class the sky slider suggests for a location, and the night lights on the globe seen from space: the World Atlas of Artificial Night Sky Brightness, reduced to one class per 0.05° cell — Falchi F., Cinzano P., Duriscoe D., Kyba C. C. M., Elvidge C. D., Baugh K., Portnov B. A., Rybnikova N. A., Furgoni R. (2016), Science Advances 2:e1600377, and the dataset at GFZ Data Services, doi:10.5880/GFZ.1.4.2016.001 The New World Atlas of Artificial Night Sky Brightness (Falchi et al. 2016) Non-commercial use, cited; the atlas files themselves are not redistributed
Satellite orbital elements Fetched live for the ISS (NORAD 25544), Tiangong (48274) and Hubble (20580), each cached for a day; a bundled element set is used for the ISS when the fetch fails CelesTrak, Dr T. S. Kelso Free public element sets
Cloud forecast Hourly total cloud cover for the dark hours on Tonight, fetched only once you turn it on, for your position rounded to a quarter of a degree; sky-condition words from the US National Weather Service’s definitions in oktas Open-Meteo, from the national weather services’ models CC BY 4.0
Geomagnetic activity for the aurora The planetary Kp index, observed over the past week, estimated for the last hours and forecast three days ahead, fetched live for the aurora the map draws; the auroral oval round the WMM-2025 and IGRF dipole poles NOAA Space Weather Prediction Center US Government work, public domain

The satellite elements, the cloud forecast and the Kp index are fetched at runtime, and so are the sky survey's tiles, from CDS, once you turn Sky survey on. Everything else above, typefaces included, is a static file from this domain. Privacy has the detail.

One table is this repository's own rather than upstream: 10 annual meteor showers with peak date, radiant, zenithal hourly rate and population index, from the International Meteor Organization’s Meteor Shower Calendar. Those rates are the published long-term maxima under a perfect sky, not a forecast for a particular night; Tonight turns each into what you would count, ZHR · sin(radiant altitude) · rLM − 6.5, with the shower’s own r and your sky’s limiting magnitude.

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.

QuantityMethodMeasured against
Star places HYG J2000, moved by its Hipparcos proper motions to the year the map is opened, then precessed to the date on the clock Meeus Example 21.b (θ Persei to 2028.9) to 0.001 s and 0.01″
Precession IAU 2006 (P03; Capitaine, Wallace & Chapront 2003) General precession 5028.796″ a century, within 0.05″ of the IAU 2006 rate
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 VSOP87 (Bretagnon & Francou 1988), truncated, over 1000–3000; JPL’s approximate Keplerian elements outside it; corrected for light time and aberration JPL Horizons DE441, every 10 days over 1900–2100: worst residual 0.23″ Mercury, 0.57″ Venus, 1.04″ Mars, 0.62″ Jupiter, 0.47″ Saturn, 1.73″ Uranus, 2.45″ Neptune
Planet brightness Mallama & Hilton (2018) for Mercury, Venus, Uranus and Neptune; the Astronomical Almanac’s phase-angle series for Mars, Jupiter and Saturn; Saturn carries its rings Mercury, Venus, Uranus and Neptune within 0.02 mag of Horizons, the rest within 0.1. One deliberate disagreement: Saturn runs 0.10–0.24 mag brighter because Horizons reports the globe without the ring system
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
Satellite passes SGP4 propagation of the current element set; brightness from range and phase against each satellite’s standard magnitude Geometry held to the latitude limits the inclinations imply
Zodiacal light and gegenschein Analytic fit to Leinert et al. (1998) Table 17, dimmed by the same Bortle, twilight and moonlight terms as the stars Within 20 % of the table everywhere past 20° from the Sun (8.5 % rms)
Binary star orbits Kepler's equation and the Thiele–Innes projection, Meeus ch. 57, on the ORB6 elements; position angles turned to the equinox of date for the card ORB6's own ephemerides for 2025–2029, all 9 orbits: within 0.05° and 0.001″ — the rounding of the published table; Meeus Example 57.a to 0.05°
Variable stars Phase on the clock's heliocentric Julian Day (light time across the Earth's orbit, up to ±8.3 min); light curves modelled from GCVS's rise time or eclipse length Algol: the Kraków elements time AAVSO VSX's 2016 minimum to 5 min (GCVS's 1983 elements would run 4 h early in 2026). δ Cephei: 0.08 mag rms against Berdnikov's 1996 photometry, on a 0.89 mag cycle
Surface brightness Integrated magnitude spread over the catalogue ellipse, μ = m + 2.5 log₁₀(2827.4 a′b′) (Clark 1990), against the Bortle class's zenith sky M 33 at 23.0 mag/arcsec² from the catalogue's own 5.72 and 68.7′ × 41.6′
Galaxy orientation OpenNGC position angle and axis ratio, turned onto the screen every frame with the sky On-screen north within 0.6° of the parallactic angle (Meeus eq. 14.1); M 31 and M 82 within 3° of the RC3

For scale: the Moon is about 1,900″ across, so the worst planet above, Neptune at 2.45″, is drawn within a seven-hundredth of a Moon-width of where JPL puts it — finer than the steadiest seeing will show you. The stars are the coarser part now: they are drawn at their mean places, without nutation or the annual aberration of starlight, each worth up to about 20″, so a planet passing close to a star can be shown a few tens of arcseconds off the true gap. Well inside any finder or eyepiece; not good enough to plate-solve against.

Software and type

LibraryVersionWhat it doesLicence
three.js 0.186.1 WebGL renderer; stars are GPU point sprites MIT
satellite.js 7.1.0 SGP4 orbital propagation MIT
earcut 3.2.4 Triangulates constellation regions for the highlight fill ISC
postprocessing 6.39.5 Bloom, tone mapping and dithering after the sky is drawn Zlib
three-atmosphere 0.19.1 Bakes the colours of twilight from precomputed atmospheric scattering MIT
Astro 7.3.5 Static site build; every page here is pre-rendered HTML MIT
Tailwind CSS 4.3.3 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

Built by Jesper Kiledal