The Colours of a Clear Dusk

A few minutes after a clear sunset, turn your back on it. Low in the east there's a band of dusky pink sitting on a band of slate blue, both rising slowly. It's worth turning round for, and it's on the print too.

What are the three twilights?

Three bands of the Sun's height below the horizon, each 6° deep.

Civil twilight runs from sunset until the Sun is 6° below the horizon. There's still light to read by outdoors, and the brightest stars and planets come out one by one. Nautical twilight runs on to 12° below: the name is the sailor's, because until then the sea horizon is still visible enough to measure a star's height against it with a sextant. Astronomical twilight runs to 18° below, while the last trace of glow fades from the horizon. Past that, the sky is as dark as it gets, and the Milky Way comes in.

How long each one lasts depends on where you are. Near the equator the Sun drops almost straight down and dusk is quick; farther from it the Sun slants into the horizon and dusk is long.

Why does the sky go gold, then rose, then blue?

Because the lower the Sun gets, the more air its light crosses, and air scatters the blue out first.

ScatteringRayleigh: strength ∝ 1/λ⁴. Air mass at the horizon ≈ 38 times the zenith's.

Air scatters short, blue wavelengths far more than long, red ones, which is why a clear day is blue overhead. Near sunset the light reaching you has crossed nearly forty times as much air as at noon, so most of its blue has been scattered away on the way, and what's left is gold and orange. It lights the haze and any clouds on the Sun's side, and the colour deepens toward red as the Sun goes down.

The deep blue overhead after sunset isn't scattering alone. In 1953 the American physicist E. O. Hulburt showed that ozone high in the atmosphere, absorbing in the orange and red, is what keeps the twilight zenith blue rather than a pale grey.

What are the Belt of Venus and the Earth's shadow?

The pink band and the blue-grey band opposite a sunset: the last sunlight on the high air, and the planet's own shadow beneath it.

Once the Sun is down, the Earth casts its shadow onto the atmosphere on the far side of the sky. It shows as a slate-blue band along the eastern horizon in the evening, and the western one at dawn. Above it, air that's still in sunlight catches the reddened light and scatters some of it back toward you: that rosy band is the Belt of Venus. As the Sun sinks the shadow climbs, its edge softens, and by the end of civil twilight it has dissolved into the darkening sky.

The name is old and a little romantic, after the girdle of the goddess. It has nothing to do with the planet, which, as an evening star, shines on the other side of the sky, over the sunset.

How does a sunset print show them?

The As Seen ground paints the sky's colour for your minute, so a sunset minute brings the glow, the Belt and the shadow with it.

As Seen is computed from the Sun's height and bearing at the minute you set. After sunset the warm band stays on the Sun's side; opposite it, the Earth's shadow rises as a grey-blue band with the pink Belt of Venus along its top. By the time the Sun is 18° down the sky is night, and from there only the stars change.

The view decides how much of it you get. A Dome or Portrait takes in the whole sky at once, so the glow and the Belt sit together on opposite sides of the ring. A Panorama is a window: facing the Sun, it holds the glow, and the Belt is behind you.

For exampleA summer wedding evening: set the minute about twenty minutes after sunset. The Sun is a few degrees down, the glow is at its warmest, the Belt of Venus is up, and the first stars are arriving.

One honest note. The colour is a model, tuned by eye against photographs of clear twilight skies; it isn't a photograph of your evening, and it doesn't know about clouds, haze or streetlights. On a clear evening it's close. On a grey one, it's the evening you'd have had if the sky had cleared.

Why don't summer nights get dark in the north?

Because near midsummer the Sun never sinks 18° below the horizon there.

Lowest Sun, June solsticeh = φ + 23.44° − 90°, at local midnight, refraction ignored.

At midnight on the June solstice the Sun is as low as it gets, and how low depends only on your latitude. North of about 48.6° it never reaches 18° below, so a midsummer night never quite becomes night.

Paris misses true night by a third of a degree. If your wedding was in June in any of these cities, the As Seen sky at midnight keeps a trace of twilight low in the north, where the Sun is hiding, because that's how it was.

Can I watch it before I choose?

Yes: the chart on the front page plays tonight's dusk for you.

Press Watch Tonight's Sunset on the chart and it sets the clock 25 minutes before sunset and plays one minute of sky every second, so the whole evening goes by in a couple of minutes, wherever you've set it. When you find the minute you love, the studio will take it from there. About the Stars has the colour model in more detail, and if you're still deciding on a date, The Sky You Were Born Under covers finding the minute.