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What exactly is twilight?

Twilight is the time of day between daylight and darkness. Astronomers, the experts on nighttime, recognize three kinds of twilight.

Easter morning sunrise via EarthSky Facebook friend Adrian Strand.

Easter morning sunrise via EarthSky Facebook friend Adrian Strand.

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.

Before sunrise. Image via Robin Reilly

Before sunrise. Image via Robin Reilly

Alpenglow at twilight. Image via Lucy Bee

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 6 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!

After sunset. Photo: Loire Vignolle-Moritz

After sunset. Image via Loire Vignolle-Moritz

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

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.

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Marina di Pisa, Italy. Image via Hubert Kosmowski

Marina di Pisa, Italy. Image via Hubert Kosmowski

Image via Joe Randall

Image via Joe Randall

Imagr via Dave Pair

Image via Dave Pair

Twilight Over the West Philippine Sea

Image Credit: Jv Noriega.

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, surely the experts on nighttime, recognize three kinds of twilight, which are explained in this post.

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