Method
How the sky is calculated
A custom star map is not a stock illustration. Three numbers pin a unique view of the sky: a local date, a time, and a place on Earth.
Inputs
You pick a city. We resolve latitude, longitude, and timezone. Date is required. Time defaults to 22:00 in that timezone — an evening sky — unless you set an exact minute. Valid years are 1900 through 2100. Times skipped by a daylight-saving change are rejected. If a local time occurs twice when clocks move back, we use the earlier occurrence.
Library
Solar-system positions, moon illumination, and precession use astronomy-engine (MIT), the same family of algorithms cited by several digital star-map tools. It is not Stellarium, and we do not vendor Stellarium’s GPLv2 code.
Catalog
Stars are a Hipparcos / Yale Bright Star subset, magnitude 6.5 and brighter — about 8,800 naked-eye and near-naked-eye stars. Names come from IAU proper-name lists where they exist. The packed catalog we ship is derived from the d3-celestial Hipparcos export (BSD-3-Clause).
Projection
The hemisphere above you is flattened with a stereographic projection centred on the zenith. The outer edge is the horizon. If your head is north, west is to the right — astronomical convention, not a mirrored gift layout. Constellation stick figures are the 88 IAU traditional line sets from the d3-celestial export (BSD-3-Clause), the same family of data Gift The Stars and OwnStarMap cite. They are not Stellarium (GPLv2) or H.A. Rey art. The Milky Way glow is NASA’s Deep Star Maps 2020 celestial layer (Hipparcos/Tycho stars omitted so it sits under our catalog), tinted to the poster. If that texture is missing we fall back to d3-celestial density contours, including dust-lane holes. Credit: NASA / GSFC SVS; Gaia DR2: ESA / Gaia / DPAC.
Moon and planets
The moon uses NASA LRO WAC color (CGI Moon Kit, public domain) sampled onto a sphere, lit from the Sun angle, with libration, bright-limb position angle, and a southern-hemisphere flip. The dark side keeps faint earthshine. The Moon template fills the plate with that surface and draws the sky on top — it is a keepsake look, not a phase portrait. Mercury, Venus, Mars, Jupiter, and Saturn appear only if they are above the horizon. The Sun is shown on daylight and dusk plates.
Limits
We do not model atmospheric refraction beyond a simple horizon correction, terrain, light pollution, or weather. Proper motion of individual stars is not applied. This is a keepsake of the calculated sky, not a planetarium or a research reduction.
Compare in Stellarium
Set Stellarium to the same date, time, and coordinates. Orion or the Southern Cross should point the same way. The moon phase should match. If a naked-eye planet is up, it should be on both views. That comparison is the product.