What is the definition of nighttime in aviation regulations?

In aviation regulations, nighttime is specifically defined as the period that begins at the end of evening civil twilight and extends until the beginning of morning civil twilight. This definition is crucial because it ensures consistency and safety during flight operations. Civil twilight is defined as the time when the sun is just below the horizon, which allows for some light but not enough to fly as if it were daytime. By using this definition, pilots and aviation operators can plan flights with the understanding of visibility and light conditions impacting safe operations.

Looking at other options, while they provide alternative definitions of nighttime, they do not adhere to the precise criteria established for aviation regulation purposes. The period between sunset and sunrise is broader and may not account for the varying degrees of light that could affect visibility. Similarly, specifying a time frame like one hour after sunset to one hour before sunrise does not encompass the entirety of civil twilight, which is essential for determining operational limits for pilots. Thus, the definition based on civil twilight is the most accurate for guiding pilots’ decisions about flight operations during low-light conditions.