“In certain latitudes there comes a span of time approaching and following the summer solstice, some weeks in all, when the twilights turn long and blue.” — Joan Didion, Blue Nights
| Sunset | — | −0° | |
| Civil end (dusk) | — | −6° | |
| ◆ Bluest moment | — | −9°* | |
| Nautical end | — | −12° | |
| Walk interval | — → — | — | |
| Blue hour span | — min | 6° arc |
At this latitude the blue hour is never the same length twice. In late June it stretches to roughly thirty-four minutes — Didion’s long, blue twilights. At the equinoxes it contracts to twenty-six; in December, to twenty-four.
The app centers a walk on the midpoint, so the shoulder season walks are the shortest — and, some argue, the most saturated.
Today’s duration: — min · Annual Δ ≈ 11 min
The blue hour is not a metaphor. It is the window between the end of civil twilight and the end of nautical twilight — the interval during which the sun sits between six and twelve degrees below the geometric horizon.
Direct sunlight, by that point, has left the observer entirely. What remains is indirect light: photons that enter the atmosphere tens of kilometers above, scatter against air molecules, and arrive at the ground having lost their longer wavelengths along the way.
“The blue of the glass on a clear day at Chartres,” Didion wrote — the color of a sky that has exhausted all its other colors.
Molecules of nitrogen and oxygen scatter short (blue) wavelengths roughly sixteen times more strongly than long (red) ones. During the day this blue is washed out by direct white light. At nautical twilight, direct light is gone, scattered red light no longer reaches us through the longer atmospheric path, and the scattered blue comes through alone.
The published twilight times assume an ideal, sea-level horizon. At a Midwestern inland site — trees, bluffs, river valleys, the 450-foot relief above the Mississippi — the effective horizon is higher than the geometric one, and the bluest moment arrives earlier. The offset shown above (Δ, in minutes) is a local calibration, not an astronomical constant.
Long enough to leave the house and return; short enough that the whole interval fits inside even December’s foreshortened blue hour. The walk is centered, not frontloaded — you want the deepest blue at minute ten, not minute one.