Blue Hour: The Twenty Minutes After the Sun Leaves

The sun has gone and yet nothing has switched off. For twenty minutes or so the sky holds a blue that is deeper than daylight and thinner than night, and the first street lamps come on into it rather than against it. Photographers wait for this less for the colour than for the arithmetic: the sky and the lamps are finally close enough in brightness that a single exposure can hold both.

In daylight a lit window is just a small white hole in a photograph. At blue hour it becomes a warm rectangle sitting inside a cool field, and the two colours do each other a favour. The shutter stays open a second or two, so the water goes flat, the clouds smear a little, and traffic turns into a line instead of a car. Nothing in the frame is moving quickly, and the frame agrees to it.

It ends without announcing itself. The blue keeps sinking until it is simply dark, and the exposure that worked five minutes ago leaves a black sky with a few blown-out lamps in it. On a screen that short window turns out to be the useful one: dark enough for white icons to sit quietly, coloured enough that the wallpaper is not merely switched off. It behaves like a room with the lights turned low.