South glass is the sunniest in 74 of 88 cities, north glass in 13.

Why south, and why north

The sun crosses the sky on the side of the equator. North of the tropics that is the south; south of them, the north. A pane facing that way sees the sun all day, every day, and the further from the equator, the more it has over the pane facing away — at Melbourne (37.8°S) the north face gets 4.7 times the hours of the opposite one.

Below 37°, the sunny side switches for part of the summer

In early summer the sun rises far north of east and hangs in the north-east for hours before it crosses due east; it does the same in the north-west before it sets. At Las Vegas (36.2°N) those hours add up to more than the south face gets in between, so for 15 days around midsummer — June 13 to June 28 — the north-facing window is the sunnier one, in hours if not in warmth. Nearer the equator the spell lengthens: 191 days in Nairobi, where the sun passes overhead twice a year and stands on the north side of the sky in between. At the equator itself the two halves are equal, and east and west — half of every day each — match them almost exactly. Above about 37° the switch never happens: the south face keeps the sun every day of the year.

Hours are not warmth

The full-sun column weighs each minute by how squarely the sun meets the glass and how much air it crosses. At high latitude the winter sun comes in low and head-on, so a south room there is warmest in the months with the fewest hours. That is the passive-solar rule of thumb, and here it is as a number for every city.

Method

For every city in Sol's fleet, the sun's position (a compact NOAA-style algorithm, good to about 0.1°) is sampled every four minutes on every day of 2026. A vertical pane faces one of the eight compass points. A minute counts as direct sun when the sun is above an open, flat horizon and within 90° of the facing. Full-sun hours weigh each minute by the cosine of incidence and a clear-sky beam (Kasten–Young air mass, Meinel attenuation). No clouds, no obstructions, no reflections: the numbers are what the geometry allows, an upper bound that every real window sits below. "Sunniest" is the facing with the most hours; where the four cardinals lie within 5% of each other the city is marked even. Coordinates are the city centres in the fleet. The code that produced every number is on the page: window-tools.js.