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. Both downloads are the complete chart: a vector SVG and a 300 dpi PNG, with nothing held back and nothing to buy.
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, and their exact placement over your horizon. The Moon is drawn as well, at its real altitude, azimuth and illuminated phase for that instant — so “was there a full moon?” is answered on the chart itself, and if the Moon had already set it is simply not there.
- It does not show: the planets, comets or satellites. Those wander against the stars and need ephemerides this tool does not compute.
- Dates before 1582: the form takes Gregorian calendar dates; historical Julian-calendar dates must be converted first.
How the map is computed
You give it three things: a date, a time, and a place. For the place, type a town or city name and pick it from the list — 29,919 towns and cities of 18,000 people or more, from GeoNames — or enter an exact latitude and longitude instead. Accents are optional (“malmo” finds Malmö), but type the name on its own: “Roskilde Denmark” finds nothing, and a typo finds nothing rather than the wrong sky.
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. every star a good naked eye can see) is transformed to its altitude and azimuth at that exact moment, and every one of those stars 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?
The Moon, yes. It is computed from the standard truncated ELP-2000/82 lunar series and drawn at its real altitude, azimuth and illuminated phase for the exact moment you enter — and left off the chart when it was below your horizon, which is the honest answer rather than a guess. It is drawn where a person standing at your coordinates would see it, not where the Moon sits as seen from the centre of the Earth — those differ by up to about 1° (two Moon widths) near the horizon, so the correction is what decides borderline "was it up?" cases. The limits that remain: about 0.003° from the lunar series itself, and atmospheric refraction, which lifts objects near the horizon by up to ~0.5° and is not modelled for the Moon or for any star. The planets, comets and satellites are not drawn; they need ephemerides this version does not compute.
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.