Star Chart for a Specific Date and Location
Not a decorative approximation: this chart is computed star-by-star for the date, time and coordinates you enter, with the math open to inspection.
Generate your star chart
Free, in your browser — no account, nothing uploaded. Free downloads carry a small watermark; a clean high-resolution print export is planned as a paid upgrade.
The pipeline is classical positional astronomy:
- Local time → UTC → Julian Date (Fliegel–Van Flandern algorithm).
- Julian Date → Greenwich sidereal time → local sidereal time (IAU 1982 polynomial).
- Per star: J2000 right ascension/declination + your latitude + sidereal time → altitude and azimuth.
- Projection: everything above the horizon is mapped stereographically onto a disc — zenith at centre, horizon at the rim.
The chart includes the 1,630 stars of the Yale Bright Star Catalogue down to magnitude 5.0 with brightness-scaled, colour-tinted dots, 89 IAU constellation figures clipped at the horizon, compass points, an optional altitude grid, and name labels for the brightest stars visible from your spot.
Accuracy, stated plainly
- Star positions are epoch J2000.0; precession and proper motion are ignored — a sub-arcminute effect over decades, invisible in print.
- Atmospheric refraction is ignored (it lifts stars near the horizon by up to ~0.5°).
- The moon, planets and deep-sky objects are not drawn — fixed stars and constellation figures only.
- The engine passes 44 automated checks against textbook values (J2000 epoch, GMST, pole altitude = latitude, zenith culmination, projection orientation).
How the map is computed
Your date, time and place are converted to a Julian Date and then to local sidereal time — the astronomer’s clock for “which way is the sky facing.” Each of the 1,630 stars in the Yale Bright Star Catalogue (every star brighter than magnitude 5.0, i.e. everything a good naked eye can see) is transformed to its altitude and azimuth at that exact moment, and everything above the horizon is projected onto the circular chart: zenith at the centre, horizon at the rim, north at the top — and east on the left, because a star map is read looking up, which mirrors east and west compared to a ground map.
The 89 constellation figures are drawn with the same math and clipped at the horizon. Star dots are scaled by real brightness (magnitude) and tinted by star colour. Everything runs in your browser — nothing is uploaded, there is no account, and the page works offline once loaded.
Common questions
Which projection does the chart use?
Stereographic by default — it is conformal, so constellation shapes stay true even near the horizon. A linear azimuthal option (radius proportional to zenith distance) is one click away.
Why are east and west swapped compared with a normal map?
Because you read a star chart looking up. Hold a ground map overhead and its east/west flip; star charts are drawn pre-flipped so the chart matches the sky when raised. North stays at the top, east goes left.
Can I check the result against something?
Yes — enter tonight, your city, and the current hour, then compare with the actual sky or any planetarium app. Polaris should sit at an altitude equal to your latitude, due north.