Twilight in 1, 2, 3 stages

Glittering, colorful city lights seen from a hilltop with blue, pink, orange, and purple twilight sky.
View at EarthSky Community Photos. | Prateek Pandey in Bhopal, India, took this twilight photo on November 13, 2021. Prateek wrote: “The sunset painted the sky in shades of pink.” Thank you, Prateek!

Twilight is that magical time of day when a glow still pervades the air even though the sun is not above the horizon. Twilight happens twice every day, once before the sun rises as the sky is getting light, and again after sunset before the sky is truly dark. Earth’s atmosphere scatters the sun’s rays to create the colors of twilight. On worlds with no atmospheres, such as the moon, it gets instantly dark when the sun sets.

Astronomers recognize three different stages of twilight: civil twilight, nautical twilight and astronomical twilight. All three stages happen twice in a 24-hour period, between day and night and then repeating in reverse order between night and day.

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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!”

Stage 1: Civil twilight

Let’s consider the stages of twilight as occurring after sunset. Keep in mind that they would reverse their order at sunrise. Civil twilight begins the moment the sun slips below the horizon. The official definition of civil twilight is the time from when the sun disappears until the sun’s center is six degrees below the horizon. A measurement of six degrees of sky is a bit more than three fingers held at arm’s length.

During civil twilight, there’s enough light to see, but people turn on their lights to drive a car, and the streetlights are starting to come on. The brightest planets appear during civil twilight.

For mid latitudes, civil twilight lasts a bit longer in summer and winter and is a bit shorter in spring and fall. In spring and fall, the sun rises and sets more directly in the east and west and therefore makes a straighter path downward (or upward), reaching the six degree mark in a shorter period of time. In summer and winter, the sun arcs across the sky, cutting across the horizon at an angle. This angle is more pronounced in summer, which is why civil twilight lasts the longest in summer. Civil twilight in mid latitudes can last, on average, 1/2 hour.

Compare this to tropical regions. At the equator, the length of civil twilight hardly varies. The sun around the equator makes a path across the sky that cuts cleanly down toward the horizon at sunset in a nearly perpendicular fashion, therefore the sun and its rays disappear faster, giving equatorial regions a shorter twilight than higher latitudes. Near the poles, twilight times last much longer.

Pink on horizon fading into lavender with many spiky fir trees silhouetted.
Twilight from Mount Shasta. Image via Robert Holzman.

Stage 2: Nautical twilight

In the evening, nautical twilight takes over where civil twilight ends. The definition of nautical twilight is the time period when the center of the sun is six degrees below the horizon to 12 degrees below the horizon. You can remember the name “nautical” because it ends when the distant line between sea and sky is no longer distinguishable. Also, more bright stars appear during this time, which was important in the early days of navigation. When nautical twilight began, sailors could use the stars as directional cues.

During nautical twilight, terrestrial objects are visible, but you need artificial lights to carry on outdoor activities.

For polar regions, the summer sun does not set below 12 degrees below the horizon. Therefore, these regions have nautical twilight all night long, never reaching astronomical twilight or total darkness. For mid latitudes, nautical twilight can last from about 1/2 hour in spring, winter and fall, to about 45 minutes in summer.

Stage 3: Astronomical twilight

The darkest twilight stage is astronomical twilight. The definition of astronomical twilight is the period of time when the center of the sun is 12 degrees below the horizon to 18 degrees below the horizon. You probably don’t even notice any illumination left in the sky at this time.

For stargazers, this is the time when fainter stars, clusters and other sky objects appear and become good observing targets.

In mid latitudes, astronomical twilight can last about 1/2 hour from fall through spring but up to an hour in summer. Astronomical twilight begins about an hour to 1 1/2 hours after sunset for mid latitudes. So, as a rule of thumb, if you’d like to observe something in the night sky that isn’t particularly bright, you should wait about 90 minutes after sunset before you start observing.

The view from outer space

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.

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 2001. Astronauts on the International Space Station took this photo, which shows illumination from the sun on the right. Image via ISS Expedition 2 Crew/ Gateway to Astronaut Photography of Earth/ NASA.

The image above – from the International Space Station (ISS) – shows twilight from an altitude of 211 nautical miles (390 km). Cloud tops reflect reddened sunlight filtered through Earth’s troposphere, the lowest layer of our planet’s atmosphere.

Photo gallery

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.
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. Image 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.
Striped pink to gray clouds reflected in water with several sailboats.
Twilight at Newport, Rhode Island. Image 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. Image via EarthSky Facebook friend Hubert Kosmowski.
Purple sky with pink clouds over the West Philippine Sea.
Twilight in the Philippines. Image via EarthSky Facebook friend Jv Noriega.
Easter morning sunrise, pink sky, man kneeling in water with arms held out.
Twilight in the U.K. Image via EarthSky Facebook friend Adrian Strand.

Bottom line: Twilight is that magical time between sunlight and darkness. Astronomers, the experts on nighttime, recognize three stages of twilight.

November 28, 2021

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