UFOCheck what the scene claims.
The observer supplies the phenomenon. Everything else is looked up in a named record or computed from physics — and where neither can answer, the tool says so instead of drawing something plausible. This is that, part by part, with the numbers it actually applies.
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”.
The light of the sky
The sky itself is not a colour gradient. Sunlight and moonlight are traced through a spherical atmosphere at fifteen wavelengths across what an eye sees, the way light really reaches an observer: scattered once by the air on its way, then again and again, dimmed by what stands in front of it.
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?
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.
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 (for the last five days, which it has not caught up with, the forecast model's own analysis). They are keyframed along the observation, so a sky that cleared during those four minutes clears in the reconstruction.
The ground
Real relief and aerial imagery are fetched around the observer, along with the decor that got in the way: buildings, trees, streetlights, vehicles, windows and other observers — with their lights, their flash rates and their tracks.
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.
A picture of the place
Everything above is computed, and a reader looking at the result has no way to tell a faithful reconstruction from a plausible one. A photograph of the same place does.
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.