What is Nautical Twilight?

Nautical twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 12° below the rational horizon.

In other words we can say:

Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon.

During nautical twilight the illumination level is such that the horizon is still visible even on a Moonless night allowing mariners to take reliable star sights for navigational purposes, hence the name.

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