About StarMapGen

StarMapGen is a free star map generator: enter a place, a date and a moment, and it renders the night sky that stood above them — accurate enough to check against the real sky, beautiful enough to frame.

Why we built it

Custom star maps had become a $15–60 commodity while the underlying maths stayed public-domain and 100-years-old. Planetarium software, astronomy textbooks and the Yale Bright Star Catalog are freely available; the only barrier was packaging. We removed it: the generator runs entirely in your browser, computes real positions, and lets you download a high-resolution result for free.

How it works, technically

• Stars: the Yale Bright Star Catalog (fifth edition), filtered to the naked-eye limit — over 5,000 plotted stars with magnitudes and B–V colour indices.
• Rotation: your local date-time becomes universal time, then Greenwich Mean Sidereal Time, then local sidereal time — the standard formulas used in every astronomy textbook.
• Projection: zenith-centred stereographic, horizon on the rim, east on the left, matching planisphere convention.
• Moon and planets: Paul Schlyter’s low-precision orbital method — accurate to a couple of arcminutes, invisible at poster scale.
• Milky Way: rendered along the true galactic equator via the standard galactic-to-equatorial rotation.
• Cities: a built-in list of 6,200+ world cities with coordinates from the GeoNames project.

Nothing is uploaded: the catalog downloads to your browser once, and all computation and drawing happen locally. We collect no personal data beyond standard, privacy-respecting analytics and advertising described in the privacy policy.

What’s with the free high-resolution download?

It costs us nothing to give: your browser does all the work. The site is supported by unobtrusive advertising — the trade is “look at an ad” instead of “pay $29 for the same pixels”. If you print one of our maps and hang it, the mission is accomplished.

Roadmap

On the list: double-star and Messier-object overlays; more poster themes and layouts; constellation-name localisation; an embeddable widget. Requests are welcome — tell us what you need.