Check what the scene claims.

The sky

Sun, Moon and its phase, the planets and the stars are placed by ephemeris for that instant, that latitude and that longitude. Nothing is a picture of a sky: point at any of it and it names itself — “Venus, mag −4, 8° above the horizon”.

What is drawn from the catalogue stops at magnitude 6.5, and that number is a fact about a witness rather than about the sky: it is how faint a dark-adapted human eye goes. Put something else in front of the same night and the threshold moves with it — see the instrument.

The catalogue follows the same threshold. A naked-eye sky costs 400 kB, down to magnitude 7.5; 57 688 more stars, down to magnitude 9, are fetched only by a recording whose own optics reach that far. Past magnitude 9 it is the data that stops, and the tool reports the limit rather than drawing a sky emptier than the photograph held.

The Milky Way and the zodiacal light are integrated along the line of sight rather than painted as a texture — which is why they move correctly with the season, the hour and the observer's latitude instead of merely being in the right place once.

What else was up there

Each of these is a candidate explanation, so each must answer the same question before it is drawn: could it have been visible from there, then?

Comets appear at their own apparition and nowhere else. The catalogue is generated from JPL Horizons and the orbit is propagated, so a comet is either there on that date or it is not.

Meteor showers come with their radiant and their hourly rate for that night, over the sporadic background that never stops.

Satellites get the harder question, because being lit is not the same as being seen: the Earth's shadow is computed for the date and hour to say whether one could have caught the Sun at all. Without period orbital elements no individual pass is drawn for historical dates — but how many objects were in orbit on that date is known, and stated.

Ice and water

22° and 46° haloes, sundogs, the parhelic circle, tangent, circumzenithal and circumhorizontal arcs: not one of those angles is stored. Each is derived from the refractive index of ice and the geometry of the crystal, which is what makes the whole display move together and stay consistent with the Sun's own height.

Rainbows and moonbows are ray-traced through a spherical drop for the same reason — the order of the colours, the gap between the two bows and the light inside the primary come out of the trace rather than being drawn in.

The weather that day

Cloud cover, cloud base, rain, snow, hail, storms and their thunder, and wind are read from ERA5, the ECMWF reanalysis: hourly, worldwide, from 1940 on. They are keyframed along the observation, so a sky that cleared during those four minutes clears in the reconstruction.

Cloud attenuates every celestial body rather than merely covering it, which is what makes a Moon behind thin cloud read as a Moon behind thin cloud.

The exact query is kept in the recording. That is the part that matters: the claim stays checkable decades later, by someone who does not trust this tool.

The ground

Real relief and aerial imagery are fetched around the witness, along with the decor that got in the way: buildings, trees, streetlights, vehicles, windows and other witnesses — with their lights, their flash rates and their tracks.

How high the witness stood is looked up from where they stood. The ground under those coordinates comes from a real elevation model, so the altitude on the form is a height above sea level whose floor is the ground itself: nobody can be placed under it. And it is not a detail — at 1500 m the horizon really is 1.2° lower than it is at sea level, which is enough to decide whether something was above it.

The instrument

An eye is not a lens. Naked-eye viewing maps an angle to an angle; a camera maps it to f·tan θ, with a sensor, a focal length, an aperture and an exposure that draws star trails and dots a flashing light. Switch the device and the whole frame changes — the same sighting through three of them.

Only what that device could actually have been set to is offered: an Instamatic had one aperture and one shutter speed, so there is nothing to choose, and a camera that did not exist yet is flagged against the observation's own date.

And the instrument decides how faint a thing could be recorded at all. That same Instamatic stops at magnitude 4.2 — two short of the witness holding it, which is half of why so many “the sky was full of stars” accounts come with an empty black photograph. A 50 mm at f/2 for twenty seconds reaches 9.7, three magnitudes past that witness. Aperture, shutter and focal length settle it against the sky's own brightness.

A longer pose stops helping once the sky has slid further than the lens can resolve, which is why an hour on a tripod is no deeper than five seconds on one — only longer trails.

Every source is named where its data is reported, with the attribution its licence requires — and can be swapped for another. The picker is the credit. What is still missing, and what each item is waiting on, is on the roadmap.