How does astronomy define darkness?

In astronomy, there is a special concept called twilight and shadow.

Due to the refraction and scattering of the atmosphere, dawn and darkness have a gradual process, which does not happen instantly. The process of dawn is called morning light, which is divided into three time standards: astronomical morning light, maritime morning light and civil morning light. The process of darkness is called syncope, which is divided into three time standards: astronomical syncope, nautical syncope and civil syncope. These two processes and six time standards are collectively called "Twilight and Shadow".

In other words, civil fading is the civil dark standard defined by astronomy; Navigation ambiguity is ultimately the standard of navigation darkness defined by astronomy; Astronomical ambiguity is the dark standard of astronomical definition. At this time, if the weather is clear, the darkest 6-magnitude star begins to appear.

The time interval between the end of the faint shadow and the beginning of the morning light the next day is the night time defined by astronomy and related standards.

The following is selected from Introduction to the Earth (third edition) edited by Jin Zumeng and others.

The morning light ends with sunrise, and there is a problem that the morning light begins; One problem is that the faint shadow begins with the sun and ends with the sun. The beginning of the morning light and the end of the faint shadow are all based on a certain "low degree" of the sun. According to different needs, LLL can be divided into three grades: civil twilight, sea LLL and astronomical LLL. Their solar "low degree" standards at the beginning of the morning and at the end of the evening are 6, 12 and 18 respectively (Figure? 1)。

-On a clear day, when the center of sun gear falls 6 degrees below the horizon, the intensity of dusk is bright enough for normal outdoor activities, so there is no need for indoor lighting. This time is called civil morning and evening. At any time, about 5% people in the world are in this state. ?

-When the sun is 6- 12 below the horizon, outdoor activities are too dark and indoor work needs lighting; Bright stars have appeared in the sky, but the horizon in the distance is still clear. This period of time is the most suitable time for measuring stars at sea (measuring the height of celestial horizon), so it is called morning light at sea.

-When the sun sets below the horizon 18, the real night comes (or ends). At this time, the darkest star visible to the naked eye began to appear, the sky was completely dark, and the astronomical twilight ended.

The duration of darkness in the morning depends on the time when the sun sets from the horizon to 18. The length of this period can be calculated according to the angle (90-j) between the sun's Sunday circle and the horizon. If the sun sets perpendicular to the horizon, this route is the shortest and the twilight duration is the shortest; The more inclined the sun's Sunday circle is, the longer the dusk lasts. Therefore, the time of twilight and shadow appears increases with the increase of latitude; It also changes slightly with the seasons (because the size of the sun's Sunday circle changes with the seasons), and the dichotomy becomes shorter and longer. ?

Figure 2 shows the general distribution of day, night and dusk time in a day at different latitudes in the northern hemisphere during winter and summer solstice. On the solstice of summer, the place at 60 degrees north latitude is in a state of civil morning twilight all night. The dusk of the previous day was not over, and the dawn of the next day followed, staying up all night, and it was not dark. This strange sky phenomenon in high latitude summer is called white night. In fact, on the summer solstice, the latitude there is higher than 48 degrees. 5N (66.5- 18), there is no real night. The polar winter nights in the north and south are long, mostly white nights. The real night is only about two months a year.