Aurora towns ranked by darkness at the equinoxes

The widest gap between neighbours falls between Seattle and Anchorage, 0.5 h. 7 neighbouring pairs sit within 0.1 h of each other (Buffalo and Portland, Maine; Portland, Maine and Minneapolis; Minneapolis and Duluth), close enough that their order means little.

This ranking runs in the reverse of map latitude. Buffalo, the farthest south, has the most civil night at the equinoxes, and Tromsø, the farthest north, the least. The spread is small, a little over an hour, because day and night are close to equal everywhere at an equinox.

The far north loses the most to twilight. Its sun sets at a shallow angle, so it spends longer between 0 and -6 degrees before civil night begins. Around the March equinox those towns are also losing darkness fast, a caveat SWPC flags for travelers; the equinox guide shows the day-by-day change.

How to use it. The equinox weeks carry SWPC's tendency toward larger storms, so this ranking shows how many dark hours each town offers in that window. With just over an hour between first and last, it should rarely decide between towns on its own. It matters more with the direction of change: in March the far north is losing darkness fast, in September gaining it. The equinox guide lists civil night two weeks either side of each equinox for every town.

Limits. An equinox is an instant, and Glow takes each town's local calendar date for it. Towns in the Americas and in Europe can land on different dates for the same instant, which shifts their figures by one day's change in night length. The equinox guide gives that change per day for each town.

The order does not follow the map. Buffalo ranks above Portland, Maine although it sits farther south on the map; Portland, Maine ranks above Minneapolis although it sits farther south on the map; Minneapolis ranks above Duluth although it sits farther south on the map.

SWPC says larger storms tend to occur near the spring and fall equinoxes. Night length at the equinox is close to 12 hours everywhere in theory, but twilight lasts longer at high latitude, so civil night differs by town.

Glow finds the next March and September equinox instants from the sun's apparent longitude, takes each town's local date for each, and averages civil night, the sun at or below -6 degrees, over the two nights. The note gives both values.

For your own coordinates, use the dark hours calculator. The background is in why the equinoxes help, and every threshold is listed on the methodology page.