Twilight: Dawn, Dusk, and Earth's Atmosphere

Twilight=Dawn & Dusk

noun: twilight

1. the soft glowing light from the sky when the sun is below the horizon, caused by the refraction and scattering of the sun's rays from the atmosphere, darkness.

"it was scarcely visible in the evening twilight"

synonyms: half-light, semidarkness, dimness, gloom

2. the period of the day when twilight takes place, between daylight and darkness

"a pleasant walk in the woods at twilight"

synonyms: dusk, early evening, evening, close of day; dawn, daybreak

3. a period or state of obscurity, ambiguity, or gradual decline.

"he was in the twilight of his career"

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Twilight is the time between day and night when there is light outside, but the Sun is below the horizon. We can define twilight as the time of day between daylight and darkness, whether that’s before sunrise or after sunset. The light from the sky appears diffused and often pinkish  –  because the sun is below the horizon, but its rays are scattered by Earth’s atmosphere to create the colors.

The transition from twilight to darkness is not a global (or constant) phenomenom, because some areas of the world are so close to the poles that the sun NEVER drops far enough for darkness. Here's a plot of the sun's angle relative to locations on earth. The top of the graph is the North Pole; the bottom is the South Pole.

Since I live about 30° North I am used to having twilight, which means interesting sunrises and sunsets. 

Because I have always lived very close to the Pacific Ocean, I am used to my sunsets occurring over the water (I find it disorienting to be on the East Coast of an island or continent, because I rarely see sunrise out of the ocean).

Technically, there are three types of twilight:

1. Civil twilight
2. Nautical twilight
3. Astronomical twilight

The morning twilight is often called dawn, while the evening is also known as dusk. 

Astronomical terms & definitions

Astronomers define the three stages of twilight – civil, nautical, and astronomical – on the basis of the Sun's elevation which is the angle that the geometric center of the Sun makes with the horizon.

CIVIL TWILIGHT

Civil twilight occurs when the Sun is less than 6 degrees below the horizon. 

• Civil dawn occurs when the geometric center of the Sun is 6 degrees below the horizon in the morning.
• Civil dusk is the moment when the geometrical center of the Sun is 6 degrees below the horizon in the evening.

Civil twilight is the brightest form of twilight. There is enough natural sunlight during this period that artificial light may not be required to carry out outdoor activities. Only the brightest celestial objects can be observed by the naked eye during this time. Several countries use this definition of civil twilight to make laws related to aviation, hunting, and the usage of headlights and street lamps.

NAUTICAL TWILIGHT

Nautical twilight occurs when the geometrical center of the Sun is between 6 degrees and 12 degrees below the horizon. This twilight period is less bright than civil twilight and artificial light is generally required for outdoor activities.

• Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning.
• Nautical dusk occurs when the Sun goes 12 degrees below the horizon in the evening.

Nautical twilight was defined when sailors used the stars to navigate the seas, and most stars could be easily seen with the naked eye. In addition to being important to navigation, nautical twilight also has military implications. The United States Navy uses nautical twilight, called begin morning nautical twilight (BMNT) and end of evening nautical twilight (EENT) as it plans tactical operations.

ASTRONOMICAL TWILIGHT

Astronomical twilight occurs when the center of the Sun is between 12 degrees and 18 degrees below the horizon.

• Astronomical dawn is the time when the geometric center of the Sun is at 18 degrees below the horizon. Before this time, the sky is absolutely dark.
• Astronomical dusk is the instant when the geographical center of the Sun is at 18 degrees below the horizon. After this point, the sky is no longer illuminated.

In the morning, the sky is completely dark before the onset of astronomical twilight, and in the evening, the sky becomes completely dark at the end of astronomical twilight. Any celestial bodies that can be viewed by the naked eye can be observed in the sky after the end of this phase.

Or watches that indicate these times?