Classification of twilight

Due to the different standards for the beginning and end of dawn, it is usually divided into civil twilight, sea dawn and astronomical dawn. On sunny days, when the angle of the sun below the horizon is about 7 degrees, civil dawn begins and dusk ends; At about 12 degrees, the voyage begins at dawn and ends at dusk; At about 18 degrees, astronomical dawn begins and dusk ends. The duration of dusk is the shortest at the equator and increases with the increase of latitude. In the morning, it starts from the sun center point 6 below the horizon and ends at sunrise; At dusk, it starts at sunset and ends at the sun center, which is 6 degrees below the horizon.

The brightest stars, including planets, can be seen in civil twilight; Like Venus, it has long been called "morning star" or "evening star". There is still enough sunshine at this time, so there are many activities that can be continued outdoors without artificial light subsidies. Civil twilight can also be described as the limit of sufficient sunlight, starting in the morning and ending at dusk. In the case of good weather conditions, you can still clearly distinguish the scenery on the earth and see the horizon clearly; Under good atmospheric conditions, only the brightest stars can be seen without moonlight or other lighting. It is the period when the height of the sun center is 6- 12 below the horizon. Generally speaking, in the evening of sailing, it is no longer possible to navigate on the horizon at sea.

At this time, sailors can use famous stars to confirm their horizon, and they can see the horizon as a reference. This cycle ends at dusk or begins at dawn; This is also the time when the light on the horizon makes the scenery difficult to recognize at sunrise or sunset. Usually, the dawn at sunrise refers to before dawn, and the dusk at sunset refers to after dusk.

At the beginning, at dawn, or at dusk, at the end of dusk, under good atmospheric conditions and without other lighting, the general outline of the ground scenery can still be distinguished, but fine outdoor activities are impossible and the horizon is blurred. There are military considerations in Twilight of Navigation, which will be considered and used when planning military operations. Astronomical shimmer is the sunlight scattered from the upper atmosphere to the lower atmosphere when the height of the sun center is below the horizon 12 ~ 18. Usually, the astronomical dusk is that the sky is no longer illuminated by the sun, and celestial bodies that are black enough to be visible to the naked eye can be presented one by one.

Most temporary observers will consider the limit of astronomical low light in the sky. Astronomers can easily observe point light sources like stars, but dim and diffuse targets like nebulae and galaxies can only be observed when the sky is completely dark after astronomical low light reaches the limit.

Conceptually, the brightness of the darkest star visible to the naked eye is 6-the sun can only be seen below the horizon 18, and the sun will disappear below the horizon 18 in the early morning. But because of light pollution, in some areas, especially in big cities, there may not even be a chance to see four-star stars, so there is no need to mention twilight at any time.