(One wonders what the rationale for this might be.
Civil twilight is also used in some jurisdictions to determine whether or not a crime has been committed at night or not, as the penalties for nocturnal malfeasances may be more severe than those committed during the day. By definition the twilight means the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun’s rays from the atmosphere. Here in the United States, the FAA defines night, and the additional regulations pertaining to nighttime aircraft operations, as the period between the end of Civil Twilight and the beginning of morning Civil Twilight. There are 3 stages of dusk, each marking the end of 1 of the 3 phases of evening twilight: Civil dusk the geometric center of the Sun's disk is 6 degrees below the horizon. Civil Twilight is not only an astronomical event – it is also important in fields as diverse as aviation and law. As with dawn, scientists define dusk as specific moments in time based on the solar elevation angle, which is the position of the Sun in relation to the horizon.
There are 3 stages of dusk, each marking the end of 1 of the 3 phases of evening twilight: 1.
The accepted definition of Civil Twilight today is that it begins at Sunset (which by definition is an instantaneous moment in time) and that it lasts until the geometric center of the Sun is 6º below the horizon. As with dawn, scientists define dusk as specific moments in time based on the solar elevation angle, which is the position of the Sun in relation to the horizon.