What Is Twilight? Civil, Nautical, and Astronomical Twilight Explained

What twilight means

Twilight is a phenomenon that can be observed every day as long as the Sun rises and sets. It is not a brief moment immediately before sunrise or after sunset, but a period that lasts for a certain amount of time.

This happens because even after the Sun moves below the horizon, its light continues to scatter in different directions through air molecules, dust, and other particles in the atmosphere. Twilight is divided into three stages—civil, nautical, and astronomical twilight—according to how far the Sun has moved below the horizon. This angle is referred to here as the Sun’s altitude.

Civil twilight

The first stage of twilight is civil twilight. It occurs when the Sun is closest to the horizon while still below it, with the center of the Sun positioned between 0 and 6 degrees below the horizon. Because the Sun is still relatively close to the horizon, civil twilight is the brightest of the three stages and is the first type of twilight we experience after sunset.

Before sunrise, civil twilight appears as the sky gradually becomes brighter, eventually leading to sunrise and full daylight. After sunset, the process is reversed: the sky gradually darkens until civil twilight ends. Civil twilight often lasts around 20 to 40 minutes before sunrise or after sunset, although its duration varies depending on the location and season.

Nautical twilight

The second stage of twilight is nautical twilight. While civil twilight occurs when the center of the Sun is between 0 and 6 degrees below the horizon, nautical twilight occurs when it is between 6 and 12 degrees below the horizon. Because the Sun is farther below the horizon, the sky is darker than it is during civil twilight. However, in areas with little light pollution, the outline of the horizon may still be visible, and bright stars can also be seen.

The term “nautical twilight” comes from the way sailors navigated in the past. During this period, they could observe both the horizon at sea and bright stars in the sky, allowing them to calculate the position of their ship.

Astronomical twilight

The final stage is astronomical twilight. As the last stage of twilight, it is the darkest of the three. It occurs when the center of the Sun is between approximately 12 and 18 degrees below the horizon. During this period, the sky already appears almost completely dark.

It is called astronomical twilight because the small amount of sunlight still scattered through the atmosphere can affect observations of faint stars, galaxies, and nebulae. Once astronomical twilight ends and the Sun moves more than 18 degrees below the horizon, astronomical darkness generally begins, providing better conditions for observing faint celestial objects.

However, even after all stages of twilight have ended, the sky is not always completely dark. Its actual brightness may vary depending on atmospheric conditions, cloud cover, moonlight, and light pollution from nearby cities.

Quick comparison

Twilight in Everyday Life

The yellow and orange colors we often see around sunrise and sunset are part of the natural phenomenon known as twilight. In the past, people did not have the advanced technology or instruments we use today to measure brightness, so the practical ways in which each stage was used in daily life became reflected in the names of the three types of twilight.

Now that we know civil, nautical, and astronomical twilight occur around both sunrise and sunset, it may be worth observing the sky the next time the Sun approaches or moves below the horizon.