What is the definition of night in aviation terms?

In aviation terms, night is defined as the period between the end of evening civil twilight and the beginning of morning civil twilight. This definition is important because it determines the legal and operational requirements for pilots and aircraft operations. Civil twilight is the time when there is enough natural light for most outdoor activities without artificial light, specifically the time when the center of the sun is at 6 degrees below the horizon.

Understanding this definition helps pilots know when they need to adhere to specific regulations, such as using navigation lights and operating under instrument flight rules if necessary. It establishes a clear boundary that doesn't rely purely on the visibility conditions or the time of day in a straightforward sense, but on the more precise, light-based measures defined by civil twilight.

Other choices could lead to confusion; for instance, defining night as the period from sunset to sunrise would include times of substantial ambient light, and could mislead pilots regarding their responsibilities. Similarly, tying night to visibility thresholds or defining it based on dusk would not align with regulatory standards established in aviation, potentially compromising safety.