Star Chart Poster
The sky exactly as it stood over one place, on one night.
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About the Star Chart Poster converter
Give it a date, a time and a place, and it draws the sky that stood over that spot — the real one, from a real star catalogue, with the constellations where they actually were.
It is the night sky as it stood when someone was born, or when two people met, or on a wedding day. The result is a vector file you can print at any size, from a postcard to a wall.
A custom star map is normally something you buy from a shop that renders it on a server and posts you a print. This draws the same thing in your browser, for nothing, and hands you the file.
Where the stars come from
Positions are from the Yale Bright Star Catalogue, about 9,000 stars down to magnitude 6.5 — roughly everything a human eye can pick out on a genuinely dark night. Each one is drawn at a size set by its brightness and tinted by its colour index, so hot stars come out faintly blue and cool ones faintly orange.
The whole catalogue is built into the page. Nothing is fetched from an observatory, no account is involved, and no request leaves your browser at any point.
How a date becomes a sky
Your date and time are converted to a Julian date, then to local sidereal time using your longitude — which is simply the astronomer's way of asking which direction the Earth happens to be facing at that moment.
That turns each star's fixed catalogue position into an altitude and azimuth above your particular patch of ground. Positions are also precessed to your date: the Earth's axis drifts about fifty arcseconds a year, so a chart for 1975 would otherwise be drawn a third of a Moon-width out of place.
Time zones are the one thing the page cannot work out for you. Knowing the offset for an arbitrary place on an arbitrary past date needs megabytes of historical rules, so the offset is a field you can see and change, starting from your own clock. It matters: an hour out rotates the entire sky by fifteen degrees.
Why it looks the way it does
The projection is stereographic, with the zenith at the centre and the horizon as the outer circle. That is the traditional choice for star charts because it is conformal — constellations keep their shape towards the edge rather than smearing sideways, which is what lets you match the paper against the actual sky.
North is at the top and east is on the left. This looks backwards on a table and is correct when the chart is held overhead, which is how a chart of the sky is meant to be read.
There are two looks: a night version with a deep blue field and coloured stars, and an ink version that is black on off-white for cheaper and sharper printing.
Printing it
The download is SVG, which is vector: the same file prints crisply at ten centimetres or at a metre, because there are no pixels to run out of. Give that to a print shop if they will take it.
A PNG is offered as well, rendered at 150 pixels per inch at the chosen paper size, for the shops and photo services that will only accept raster files.
Thirty by forty centimetres fits a standard frame and suits a birthday or wedding print. Fifty by seventy makes a proper wall piece, with enough room for the fainter stars to read as a night sky rather than a scatter of dots.
Frequently asked questions
Are the star positions real?
Yes. They come from the Yale Bright Star Catalogue, a standard astronomical reference, and are precessed to the date you choose. This is not a decorative pattern of dots — it is where those stars were.
Why do I have to set the time zone myself?
Because working out the UTC offset for any place on any past date needs historical time zone and daylight saving rules, which run to megabytes and would have to be downloaded. The field starts on your own clock, which is right for tonight where you are, and is clearly visible so it can be corrected for anywhere else.
Does it show planets or the Moon?
Not yet. Planets move against the fixed stars and need their own orbital calculations, which is a separate job from drawing a catalogue. The chart currently shows stars and constellation figures.
Why is east on the left?
Because a star chart is meant to be held up towards the sky rather than laid on a table. Turn it over in your mind and the directions come out right — this is the standard convention for sky maps.
What size should I print it at?
Anything. The SVG is vector, so it stays sharp at any size. Thirty by forty centimetres suits a standard frame; fifty by seventy makes a proper wall piece with room for the fainter stars to show.
Is anything uploaded?
No. The star catalogue is part of the page, the drawing happens in your browser, and the file is produced on your own device. Nothing about the date or place you enter is sent anywhere.