Twilight

Twilight is the soft light in the sky.

It happens when the sun is below the ground. Sunlight hits the air and scatters. This makes the sky glow.

There are three kinds of twilight. The first is called civil twilight. It is bright enough to see without lamps.

The next kind is nautical twilight. It is darker here. Sailors can see the horizon.

The last kind is astronomical twilight. This is the darkest stage. It is very dark for looking at stars.

Some places have twilight all night long. This can happen in the summer. It is a very pretty time of day.

Twilight is the soft light in the sky. It happens when the sun is below the horizon. Sunlight hits the upper atmosphere and scatters. This makes the sky and the ground glow.

There are three main stages of twilight. The first is civil twilight. This happens when the sun is up to 6° below the horizon. It is bright enough to see objects clearly. Most people do not need lamps during this time.

The next stage is nautical twilight. This occurs when the sun is between 6° and 12° below the horizon. Sailors can still see the horizon to guide them. However, people need artificial lights to see things well.

The last stage is astronomical twilight. The sun is between 12° and 18° below the horizon. The sky is dark enough to see many stars.

In some places, twilight lasts all night. This can happen near the summer solstice. In these areas, the sun never sinks deep enough for total darkness.

Twilight is the special light we see when the Sun is below the horizon. Even though the Sun is hidden, the sky stays lit up. This happens because sunlight hits the upper atmosphere and scatters in many directions. This scattered light shines down on the Earth's surface and lower atmosphere.

As the Sun sinks lower, the sky becomes much dimmer. When the Sun reaches 18° below the horizon, the light is nearly gone. At that point, evening twilight ends and nighttime begins.

Scientists divide twilight into three main stages based on the Sun's position. The first stage is civil twilight, which occurs when the Sun is between 0° and 6° below the horizon. During this time, there is enough light for most outdoor activities without using lamps. You can clearly see the horizon and even the brightest planets like Venus. The next stage is nautical twilight, when the Sun is between 6° and 12° below the horizon. Sailors can still see the horizon to help them navigate by the stars. However, people usually need artificial lights to see objects on the ground clearly.

The third stage is astronomical twilight, which happens when the Sun is between 12° and 18° below the horizon. The sky is quite dark during this period. It is dark enough for astronomers to observe many stars and point sources of light.

However, very faint items like some galaxies might still be hard to see. Once the Sun sinks past 18°, the astronomical twilight ends and it is truly night. This transition is very important for people who study the deep sky.

Different parts of the world experience twilight in different ways. People living near the Equator see twilight twice every single day.

However, near the North and South Poles, things change during the summer. At latitudes greater than about 48°34', twilight can actually last from sunset all the way to sunrise. This happens near the summer solstice, which is June 21st in the North and December 21st in the South. In some northern places, this long twilight is even called a "white night."

Twilight is a very important time for many different groups of people. Photographers and painters love it because it creates a beautiful "blue hour."

During this time, objects often look like dark silhouettes against a bright sky. Animals that are most active during these times are called crepuscular. Even the military uses these stages to plan their work. They use special names like BMNT for the start of morning nautical twilight. These names help everyone know exactly how much light is left in the sky.

Twilight is the period of daylight illumination that occurs when the Sun is below the horizon. This light is produced by diffuse sky radiation. This happens when sunlight from the upper atmosphere is scattered. This scattering illuminates both the Earth's lower atmosphere and the Earth's surface.

As the Sun sinks lower beneath the horizon, the sky becomes dimmer. When the Sun reaches 18° below the horizon, the illumination is nearly zero. At this specific point, evening twilight ends and nighttime begins. Conversely, when the Sun approaches re-emergence and reaches 18° below the horizon, nighttime transitions into morning twilight.

Scientists define twilight using the solar elevation angle, or θs. This is the position of the geometric center of the Sun relative to the horizon. There are three established subcategories of twilight. These stages are civil twilight, nautical twilight, and astronomical twilight. Each stage is determined by how many degrees the Sun is below the horizon.

These stages help astronomers, sailors, and the military understand the level of light available.

Civil twilight is the stage nearest the horizon. It occurs when the Sun's geometric center is between 0° and 6° below the horizon. During this period, enough natural light remains that artificial light is often unnecessary for outdoor activities. Under clear conditions, the human eye can clearly distinguish objects on the ground. The horizon is clearly defined, and the brightest stars and planets are visible. For example, Venus is often seen as the "morning star" or "evening star" during this time. In the United States military, the start of morning civil twilight is called BMCT. The end of evening civil twilight is called EECT.

Nautical twilight occurs when the Sun is between 6° and 12° below the horizon. During this stage, the human eye can see the general outlines of ground objects. However, detailed outdoor operations become difficult. Sailors can still take reliable star sightings because they can distinguish a visible horizon for reference. If the sky is clear and there is no other light, the eye can see outlines. But generally, artificial lighting must be used to see terrestrial objects clearly. The military uses the terms BMNT and EENT to mark the beginning and end of this stage.

Astronomical twilight is the final stage before true night. It occurs when the Sun is between 12° and 18° below the horizon. The sky is dark enough to allow for the observation of point sources of light, such as stars.

However, the sky may still have some glow from light pollution, moonlight, or auroras. Some very faint, diffuse items like certain nebulae or galaxies might still be hard to see. Theoretically, the faintest stars visible to the naked eye become visible during evening astronomical dusk. These stars may become invisible again during astronomical dawn.

Where you live on Earth changes how much twilight you experience. Observers within about 48°34' of the Equator can see twilight twice every day. This includes the period between astronomical dawn and sunrise, and between sunset and astronomical dusk.

At latitudes closer than 8°35' to the Poles, twilight cannot occur on the same day. This is because the angular difference between solar noon and solar midnight is less than 17°10'. In the Northern Hemisphere, observers within 63°26' of the Equator see twilight twice daily from September 1 to March 31. In the Southern Hemisphere, this occurs from March 1 to September 30.

In some parts of the world, twilight can last for a very long time. At latitudes greater than 48°34' North or South, twilight can last from sunset to sunrise near the summer solstice. This happens because the Sun does not sink more than 18° below the horizon. In these regions, complete darkness does not occur even at solar midnight. This includes the United Kingdom and parts of northern Europe. In these northern areas, when civil twilight lasts all night, it is sometimes called a "white night."

Twilight has many connections to culture and biology. Photographers and painters often call it the "blue hour." This refers to the distinctive quality of light and the appearance of silhouettes. Animals that are most active during these periods are described as crepuscular. Even the concept of twilight is used metaphorically. It can imply that something is losing strength or approaching its end, such as the "twilight of someone's life."