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North pole at the center. Distance from center is linear in co-latitude, so the equator is a circle of radius R⊕ and the south pole / ice wall is the rim at 2 R⊕.
The dome is a flattened hemisphere: pole height runs from 100 km (the usual “rockets hit the firmament” figure) up to a true hemisphere on the rim. The ice wall itself is only a few kilometres tall. Stars are white pixels on the dome, parked above the ground point where that star is at zenith right now: latitude = declination, longitude = RA − GST.
As the clock runs, GST advances and every star slides around the pole to stay over its current zenith longitude. The Sun does the same on the solar declination parallel. At 12:00, noon is longitude 0°. Stand under Sun puts you on that ground point so looking up is looking at ☉.
Northern stars pile up near the pole because high declination maps to a small radius. Nothing sets. The nearest-overhead name stays true by construction. The distribution does not.
Trails are the last 24 hours, sampled every 6 minutes. That is one solar day, a little longer than the sidereal day, so a star's line overlaps itself by about a degree instead of closing cleanly. Only stars brighter than the magnitude limit are drawn. The sun, moon, and planets are drawn too. The lines live in the sky, so the panel and the other readouts do not wipe them, and looking around does not either.
Light, FE: the sun is a flat disk facing the earth. Straight under it is full day. Brightness falls as the fourth power of distance, because the disk is foreshortened and the ground is lit at an angle. The sky follows that same pool of light. The brightness slider only multiplies that pool. Turn it up and a wider patch of the disk goes to full day. The light is still a spot under a downward-facing disk, so it never becomes the true sky. True: ground and sky match clear-sky sun on a round earth. Off leaves the dome dark.
Mountains are regional high points: the most prominent summit in a neighborhood, kept only if it stands at least 1,800 km from one already shown. The line is the true height above the disk. A summit is drawn, and named, only when that line is taller than a point on the screen. From Raleigh the one you can actually see as a line is Mitchell, west, about a third of a degree tall. The set is Everest, Aconcagua, Denali, Kilimanjaro, Simón Bolívar, Orizaba, Elbrus, Vinson, Tomort, Puncak Jaya, Klyuchevskaya, Ras Dashen, Rainier, Toubkal, Mauna Kea, Kinabalu, Cameroon, Yushan, Kerinci, Erebus, Fuji, Aoraki, Gunnbjørn, Somoni, Mont Blanc, Lautaro, Mitchell, Kosciuszko, and Thabana Ntlenyana. Eye altitude is your height above sea level, up to a cruise altitude. In planetarium mode the same peaks sit on a round earth of mean radius and disappear below the horizon when a round earth would hide them.
Planetarium moves every star, the sun, the moon, and the planets from the flat dome onto the direction they really have for this observer. It eases there rather than jumping. Looking straight up is the same either way. That is the point. Overview turns off while Planetarium is on: the flat map and the real sky are not one picture.
Sim speed is how fast the clock runs against the wall clock. 1.0× is real time. The Real time button also locks the clock to now; any other speed lets go of that lock. 0 holds the clock.
Shadow puts a short pole in the open part of the view. The top of the plate is the way you are facing, and looking up does not turn it. The rim-colored triangle points north. Blue is the real shadow for this place and time. Gold is the shadow the flat-dome sun would cast. Both bars start at the foot of the pole.
Drag to look · scroll to zoom · Yale BSC5
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What this covers
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CC0 above applies to the original program text of this project: the
page structure, interface, simulation logic (the flat-disk mapping, the
dome, ground light, and label placement), and the particular way those
pieces were assembled.
It does not apply to third-party catalogs, line art, libraries, or
search results the page loads or queries. Those remain under their own
terms. Credit them if you redistribute; that is politeness and, in a
few cases, required.
The catalogs ship two ways, and neither way changes who owns them.
The web app loads them as separate files, bsc-stars.json and const.json.
The single-file download, flatearthsky.html, inlines those same files.
Third-party material
--------------------
Yale Bright Star Catalog (BSC5)
Star names, positions, and magnitudes.
A historical astronomical catalog. Credit: Yale University Observatory
/ Hoffleit & Warren. This project does not claim ownership of the
catalog.
IAU constellation boundaries
Official constellation boundary data of the International Astronomical
Union (Delporte 1930, J2000 form as commonly redistributed). The IAU
remains the authority on those boundaries.
Constellation stick figures
Traditional Western asterism polylines as packaged by Olaf Frohn
(d3-celestial), BSD-3-Clause:
https://github.com/ofrohn/d3-celestial
Those figures are not an IAU standard.
Three.js (r160)
Scene, camera, and controls. MIT License.
https://github.com/mrdoob/three.js
Bundled into the web app. The single-file download loads it from a
CDN at runtime and does not bundle it.
Astronomy Engine (astronomy-engine 2.1.19)
Sun, Moon, and planet positions, magnitudes, and lunar phase fraction.
Copyright (c) 2019-2023 Don Cross. MIT License.
https://github.com/cosinekitty/astronomy
Bundled into the web app. The single-file download loads
astronomy.browser.min.js from a CDN at runtime and does not bundle it;
the MIT notice is in that file. Rise/set routines from the engine are
not used for placement.
OpenStreetMap Nominatim
Find a place sends the text you type to the public Nominatim service
(nominatim.openstreetmap.org). Returned place names and coordinates
are © OpenStreetMap contributors, ODbL. This project does not store
or redistribute that data.
React and TanStack Start
The web app's page shell. Both MIT. They are not part of the CC0
program text. The single-file download does not include them.
The simulation was written in conversation with Grok (xAI). Neither
party is trying to get a Big Mac out of it. If a lawyer appears anyway,
CC0 is the answer for the original text, and the list above is the
answer for everything else.