A globe that shows where it is day and where it is night, this second, computed entirely from your device's own clock. One idea, executed honestly: no map service, no sunrise API, no data feed at all — just the astronomy of the Sun's position, done in a few dozen lines, and read against your own local time.
What it demonstrates
That a live, apparently "connected" visualisation can be completely offline and still be exactly right. The terminator you see is not an image someone drew; it is the set of places where the computed solar altitude is zero. Because the same solar-position function feeds the globe, the sub-solar readout, and every city's day/night state, they can never disagree.
The techniques, honestly named
A standard solar-position model. Solar declination comes from the Spencer Fourier series and the sun's timing from the NOAA equation of time. From those, the sub-solar point sits at the declination in latitude and at the longitude of local solar noon — which sweeps 15° west per hour and passes Greenwich near 12:00 UTC. In early July it lands near 23° N, by the Tropic of Cancer, exactly where it should be after the June solstice.
Day and night from one equation. A place is in daylight when the Sun's altitude there is above the horizon, in civil twilight down to −6°, and in night below that. Altitude is the usual spherical relation of latitude, declination and hour angle — the single formula behind the whole page.
An orthographic globe, drawn per pixel. Each pixel of the disc is back-projected to a latitude and longitude, its solar altitude computed, and it is shaded from ocean-blue day through a warm twilight band to deep-blue night — with a graticule, limb darkening and an atmosphere rim. The view is centred on your own time-zone meridian.
Real time zones, not offsets. Each city clock is its true local time from the browser's own IANA time-zone database via Intl.DateTimeFormat, so daylight saving and odd offsets are correct for free.
Honest accuracy. The model is good to a fraction of a degree — ample for a terminator, and the page says so rather than implying survey precision.
The globe is redrawn about once a minute (the terminator moves a quarter-degree in that time), the clocks tick each second, and the whole thing pauses when the tab is hidden. devicePixelRatio is capped at 2. Nothing here needs the network after the fonts load.
Recreate it
The full recreation prompt — the exact solar-declination and equation-of-time formulas, the sub-solar and altitude math, the orthographic per-pixel globe, the city list with time zones, and the accessibility and performance bar — is in the showroom vault.