When the geometric center of the sun is positioned 12 degrees below the horizon, it marks the beginning or end of Nautical Twilight. This specific angular distance is significant for navigation and astronomical observations.
Understanding Nautical Twilight
Nautical Twilight is a distinct phase of twilight that occurs both in the morning, before sunrise, and in the evening, after sunset. It is precisely defined by the sun's position relative to the horizon.
- Sun's Position: During Nautical Twilight, the sun's geometric center is located 12 degrees below the horizon.
- Visibility: Even under moonless conditions, the horizon remains visible, although the general sky becomes quite dark. This visibility is crucial for certain activities.
- Purpose: The term "Nautical Twilight" highlights its importance for sailors and navigators. It's during this time that the horizon is still discernible enough to take reliable readings of well-known stars, which are essential for celestial navigation.
Why 12 Degrees Below the Horizon Matters
The specific angle of 12 degrees is not arbitrary; it represents a key point where the balance between a visible horizon and observable stars is optimal for practical purposes:
- Celestial Navigation: For sailors, the ability to see both the natural horizon and celestial bodies (like specific stars) allows for accurate measurements with a sextant. This enables them to determine their position at sea.
- Dim but Visible: At 12 degrees below the horizon, enough ambient light is scattered in the atmosphere for the horizon line to be distinguishable against the darker sky, while brighter stars and planets become clearly visible.
- Distinction from Other Twilight Phases: This phase differentiates itself from Civil Twilight (when the sun is 6 degrees below the horizon and there is still enough light for outdoor activities) and Astronomical Twilight (when the sun is 18 degrees below the horizon and the sky is considered fully dark for astronomical observations).
This precise measurement allows for consistent and reliable navigational practices across different locations and times of the year. For further details on the definitions of twilight, you can refer to resources like the National Weather Service.
