Midnight at the June solstice, facing north. The marked body is the Sun at its lowest, under the northern horizon. Walk north a stop at a time: at each it sits higher, and the card keeps its depth. At Tromsø's latitude it never sets.
That night's Sun at three latitudes, drawn on your sky, then unrolled into height by your clock. Twilight is graded by depth: civil to 6°, nautical to 12°, astronomical to 18°. Only below that is night truly dark, and none of the three dips so far.
Now your own year, a night per column by your clock: sunset at the top, sunrise at the bottom, each shade a deeper twilight. The darkest is true night. Where it pinches shut on the dotted line, the Sun's lowest, your nights never get dark.
The Sun's lowest point each night, for you and three others. Tromsø's climbs over the horizon in summer, and its noon sinks under in winter. Quito's never comes near: true night all year. Ushuaia's is the mirror: no true night in December.
The rule: the solstice Sun's midnight depth is 90° − latitude − 23.4°. North of 48.6° that is under 18°: no true night. North of 65.7°: no sunset.