Civil twilight refers to the period when there is enough natural light for most outdoor activities without the need for artificial illumination, bridging the gap between night and full daylight.
This specific phase of twilight is defined by the sun's position relative to the horizon. It begins in the morning, or ends in the evening, when the geometric center of the sun is 6 degrees below the horizon.
Understanding Civil Twilight
Civil twilight is the brightest of the three main twilight phases (civil, nautical, and astronomical). During this time, the horizon is clearly visible, and the sky is bright enough that most larger stars and planets are not yet visible, or are just beginning to appear.
- Morning Civil Twilight: This period begins when the geometric center of the sun is 6 degrees below the horizon and ends at sunrise. As the sun approaches the horizon, the sky brightens considerably, making it possible to discern objects clearly without artificial light.
- Evening Civil Twilight: This period begins at sunset and ends when the geometric center of the sun descends to 6 degrees below the horizon. As the sun dips lower, natural light diminishes, and artificial lighting may become necessary for outdoor activities towards the end of this phase.
Key Characteristics of Civil Twilight
| Aspect | Description |
|---|---|
| Sun's Position | The geometric center of the sun is between 0 degrees (sunrise/sunset) and 6 degrees below the horizon. |
| Visibility | The horizon is clearly defined, and there is sufficient natural light for most outdoor activities, such as reading or walking, without artificial aid. |
| Sky Appearance | The sky is bright; the brightest stars and planets may become visible only towards the very end of evening civil twilight or the beginning of morning civil twilight. |
| Common Perception | Often referred to colloquially as "dawn" (morning) or "dusk" (evening). |
Practical Implications
Understanding civil twilight is important for various activities and regulations:
- Outdoor Activities: It's generally safe and easy to engage in outdoor sports, photography, and other leisure activities during civil twilight without needing artificial lights.
- Aviation Regulations: Many aviation rules regarding visual flight rules (VFR) and night flying are tied to the official civil twilight times, defining when aircraft must have their navigation lights on.
- Street Lighting: Municipalities often program streetlights to turn on or off based on the start and end of civil twilight to optimize energy consumption while ensuring public safety.
- Photography: The soft, diffused light during civil twilight, often called "golden hour" (though this is a slightly different concept, civil twilight precedes/follows it closely), is highly prized by photographers for its unique quality.
In essence, civil twilight is the brightest phase of twilight, characterized by enough natural light for most daily outdoor activities, marking the transition between night and day.
