The Geochron (https://www.geochron.com/about/what-is-it/) is a mechanical map+clock created in 1965 that shows it.
An over-simplified explanation is that the outer layers of the atmosphere persist continuously, and can be used for very long distance communication, but only reflect radio waves at relatively low frequencies (<10MHz). The lower layers that become ionized during the day absorb some of the radio waves, reducing overall efficiency and range, but also allow operation at higher frequencies which would be unusable at night.
And you can get very efficient long-distance propagation along the terminator at dawn and dusk: https://www.qsl.net/w2vtm/grayline.html
Gnome2 had a map like this in its calendar/time drop-down when you add cities to it:
It's one of the main reasons I still use Mate. No other desktop environment keeps it handy like this. I try them for a bit, then always go back to Mate.
To arrange meetings and sync with remote teams, I'm looking up timetables on a small Go utility.. You can check it in action in the Go Playground .
The page could be modified to present a gradient, since it only requires calculating the sun's altitude and solar time, which is not that complicated.
Meaning: you get full visual acuity with just a little light. Further increases often don't even register.
In numbers: typical overcast daylight is around 1,000 lux. Full sunlight is above 100,000 lux.
e.g. https://www.nrel.gov/gis/images/solar/solar_ghi_2018_usa_sca... but on the daily.
Would love to figure out how to incorporate a world dot map like this, but showing the two current golden hours (morning and evening) instead.
I've gone back and forth on replacing this with a browser-based location request so the user is aware what's going on -- the tradeoff being the friction of clicking the allow button.
My location resolved a province over, so I suspect it is at least somewhat IP-based (or IP + some other magic)
You can actually just link the image to your desktop and depending on settings will auto update throughout the day. Sometimes you need to write a script
Huh, I wanted to draw something exactly like this to put as hero image of my company page, except with ISS orbit instead of the terminator. I really like this "world map out of dots" style.
You might want to take a look at the Natural Earth  datasets and GeoPandas , they're great if you're dealing with any kind of geographical data.
Want another challenge? Try one with the azimuthal equidistant projection. Would be quite interesting to watch the comparison.
Can't find it any more though - probably been removed.
You're right, the weird shape is due to the projection of "earth" in the X-Y plane, if you chose a different projection it would appear differently. The calculations make sure that the result is ±1 accurate all year long.
The truth is that even the definition of 'daylight' and 'night-time' is not straightforward. The most common definitions include the Civil, Nautical and Astronomical Twilights , depending on the sun's position, so it should actually be a gradient.