What does "day" or "daylight" refer to in aviation terms?

In aviation, "day" or "daylight" refers specifically to the period defined as civil twilight, which encompasses the time when the center of the sun is between 6 degrees below the horizon and the horizon. This period allows for sufficient natural light for flight operations without the need for artificial lighting.

Civil twilight is crucial for pilots because it indicates the time when visibility is adequate for flying, even though the sun has not completely risen or has just set. This definition is essential for complying with visual flight rules (VFR), which depend on adequate light conditions. Understanding this timeframe helps pilots in planning their flights and ensuring safety during operations that require daylight.

The other options define different periods that are not relevant to aviation's specific usage of the term "day" or "daylight":

• The time between sunset and sunrise is more accurately described as nighttime, not daylight.

• The moment when the sun is highest in the sky refers primarily to solar noon, which is also not the standard aviation definition for daylight.

• The duration of daylight saving time pertains to time adjustments rather than defining the light conditions essential for flight operations.