What Did the Sky Look Like on a Certain Date?
Exactly — not approximately. The positions of the stars are fully deterministic, so the sky over any place on any date can be recomputed, star by star.
See the sky for your date
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.
Three inputs pin down a sky: the date, the time of night, and the place. Earth’s rotation is a precise clock and its orbit a precise calendar, so given those three facts the altitude of every catalog star follows from arithmetic — the same arithmetic whether the date is last Tuesday or fifty years ago.
This generator answers the question for the fixed stars: which stars and constellations were up, how high, and in which compass direction, drawn as a circular all-sky chart you can also caption and print.
What it can and cannot tell you
- It shows: all naked-eye stars (to magnitude 5.0), the constellation figures, their exact placement over your horizon.
- It does not show: the moon's phase or position, the planets, comets or satellites. Those wander against the stars and need ephemerides — if your question is “was there a full moon?”, a planetarium app such as Stellarium answers it; this tool deliberately sticks to the stars.
- Dates before 1582: the form takes Gregorian calendar dates; historical Julian-calendar dates must be converted first.
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
Can it show the moon and planets?
No — v1 draws fixed stars and constellations only. The moon and planets move against the star background and require ephemeris calculations, which are on the backlog. For those, use a planetarium app (e.g. Stellarium) — for the stars themselves, this chart is exact.
How can you know the sky from decades ago?
Because nothing about it was random: star positions change imperceptibly over centuries, and Earth's rotation is modeled to high precision. Recomputing a 1970 sky is the same arithmetic as computing tonight's.
Was the same sky visible everywhere on Earth that night?
No — latitude decides which stars can rise at all (Crux for the south, the Plough's circumpolar sweep for the north), and longitude plus time decide which are up right now. That's why the place is a required input.