Star Map by Date
Enter a date — any date — and see the sky as it stood that night over any place on Earth.
Pick a date and see its sky
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 night sky is a calendar. Because Earth orbits the sun, the stars rise about four minutes earlier every night; over a year the sky at a fixed hour makes one complete turn. A January midnight and a July midnight share almost no constellations — the date genuinely changes the picture.
This generator converts your date to a Julian Date and sidereal time, then computes the position of every catalog star for that night. Time and place matter too: the same date over Sydney and over London gives two different skies, which is why the form asks for all three.
Picking the right moment
- Date — the calendar date at the place, not in your current timezone.
- Time — defaults to 21:00, a natural “evening sky” hour. For a specific event, use the event’s hour.
- After midnight? A night that ran into the small hours belongs to the next calendar date — a common mix-up worth double-checking.
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
How far back or forward can the date go?
The time math is valid for any Gregorian calendar date. Star positions are epoch J2000; within a few centuries of the year 2000 the drift is invisible at print size. For dates many centuries away, precession would start to matter — this v1 deliberately ignores it.
Does the time of day matter?
Yes — the sky rotates 15° per hour, so an evening chart and a 3 a.m. chart of the same date look substantially different. If you only care about the date, the 21:00 default is a good representative evening sky.
Is the sky the same on that date every year?
For the fixed stars, almost exactly — the same date and clock time repeats to within about a degree year over year. (The moon and planets don't repeat, but this chart doesn't draw them.)