What exactly is twilight?

Pink clouds over sharp snowy peaks reflected in a lake.
Photographer Yuri Beletsky wrote: “We were absolutely thrilled to witness truly amazing burst of colors over the mountains in Torres del Paine national park in Chilean Patagonia. The colorful moment lasted just few minutes, but it was quite spectacular indeed. At some point we had a feeling that the whole scene was flooded with soft red glow coming from the sky. I hope you’ll enjoy the view!”

You can define twilight simply as the time of day between daylight and darkness, whether that’s after sunset, or before sunrise. It’s a time when the light from the sky appears diffused and often pinkish. The sun is below the horizon, but its rays are scattered by Earth’s atmosphere to create the colors of twilight.

We have twilight because Earth has an atmosphere. Some light scatters through small particles in the atmosphere – so there’s still some light in the sky even after the sun has gone down.

This time of day is important for a lot of reasons to many people – for example, astronomers, who are waiting for true darkness to fall so they can begin their observations. So some more strict definitions have evolved on the subject of twilight.

Pink along horizon behind tall stick-like trees reflected in a lake.
Image via joiseyshowaa.
Tops of snowy mountains white, middle pink, lower slopes in shadow, darkening blue sky.
Alpenglow at twilight. Image via Lucy Bee.

Civil twilight. It starts as soon as the sun dips below the western horizon. There’s enough light to see, but people turn on their lights to drive a car, and the streetlights are starting to come on. Civil twilight officially ends when the sun is six degrees below the horizon.

Nautical twilight. It begins when it’s fairly dark outside. By definition, nautical twilight ends when a distant line of a sea horizon stops being visible against the background of the sky – about when the sun is 12 degrees below the horizon. And even then some people still call it twilight.

Astronomical twilight. It ends when all traces of sky glow are gone. By definition, astronomical twilight ends when the sun is 18 degrees below the horizon. Then astronomers can begin to observe the stars, assuming no clouds are in the way!

Part of Earth showing fading colors from light side to dark side.
This image of twilight on Earth viewed from space is a single digital photograph from June of 2001 via the International Space Station orbiting at an altitude of 211 nautical miles. The sun illuminates the scene from the right. The cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet’s nurturing atmosphere. Image via ISS Expedition 2 Crew, Gateway to Astronaut Photography of Earth, NASA.

If you could see twilight from outer space, you’d find that it isn’t marked by a sharp boundary on Earth’s surface. Instead, the shadow line on Earth – sometimes called the terminator line – is spread over a fairly wide area on the surface and shows the gradual transition to darkness we all experience as night falls.

The image above – from the International Space Station (ISS) – shows twilight from an altitude of 211 nautical miles. Cloud tops reflect reddened sunlight filtered through Earth’s troposphere, the lowest layer of our planet’s atmosphere.


Yellow sky on horizon, deep blue hills below, many big birds standing in lake in foreground.
Image via Joe Randall.

Streaming pink clouds reflected in lake with ducks.
After sunset. Image via Lorie Vignolle-Moritz.
Pink stripes above, yellow below, reflected in sea cove.
Before sunrise. Image via Lorie Vignolle-Moritz.
Cloudy pink sky reflected in lake with dark posts sticking out of the water.
Image via Ailee Bennett Farey.
Glorious orange sky behind receding blue hills, man on tractor in foreground.
Image via Cynthia Koeppe.
Puffy clouds against pink sky reflected on incoming waves.
Twilight at Waimanalo Beach, Oahu, Hawaii, via Chantel Dunlap.
Clouds lit from below against darkening blue sky reflected in lake with boat.
Guwahati, Northeast India. Image via Indrajit Dutta.
Pink and blue striped clouds over shallow lake with standing birds.
Image via Catherine Fisher.
Pink on horizon fading into lavender with many spiky fir trees silhouetted.
Twilight from Mount Shasta via Robert Holzman.
Striped pink to gray clouds reflected in water with several sailboats.
Twilight at Newport, Rhode Island via Dennis Chabot.
Clouds brightly lit, gold, pink, from underside.
Image via Stu Spencer.
Stunning view red, yellow, dark blue stripes across horizon with rocks in sea in foreground.
Twilight at Marina di Pisa, Italy, via EarthSky Facebook friend Hubert Kosmowski.
Purple sky with pink clouds over the West Philippine Sea.
Twilight in the Philippines, via EarthSky Facebook friend Jv Noriega.
Easter morning sunrise, pink sky, man kneeling in water with arms held out.
Twilight in the U.K., via EarthSky Facebook friend Adrian Strand.

Bottom line: You can define twilight simply as the time of day between daylight and darkness, whether that’s after sunset, or before sunrise. Astronomers recognize three kinds of twilight, which are explained in this post.

March 25, 2019

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