Three types of twilight
Darkness does not fall immediately after sunset. The length of twilight depends on the location and the time of year. At higher latitudes, the phases of twilight last longer because the sun rises and sets at a shallower angle in relation to the horizon.
Civil twilight
Twilight occurs when Earth’s upper atmosphere scatters and refracts sunlight which illuminates the lower atmosphere. Civil twilight is the brightest form of twilight. It occurs when the Sun is less than 6 degrees below the horizon. 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.
Nautical twilight
Nautical twilight occurs when the 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. The term dates back to the time when sailors used the stars to navigate the seas. During this time, most stars can be easily seen with naked eyes, and the horizon is usually also visible in clear weather conditions.
Astronomical twilight
Astronomical twilight occurs when the Sun is between 12 degrees and 18 degrees below the horizon. During astronomical twilight, most celestial objects can be observed in the sky. However, the atmosphere still scatters and refracts a small amount of sunlight, and that may make it difficult for astronomers to view the faintest objects. Before astronomical dawn and after astronomical dusk, it is astronomical nighttime, when no indirect sunlight is visible and even faint celestial objects can be seen, weather permitting.
https://www.timeanddate.com/astronomy/different-types-twilight.html