What period begins in the morning and ends in the evening when the center of the sun is 12 degrees below the celestial horizon?

• A. Civil Twilight
• B. Nautical Twilight
• C. Sunset
• D. Sunrise

Answer: (B)

The correct choice represents the period known as Nautical Twilight. This phase occurs when the center of the sun is between 6 degrees and 12 degrees below the horizon. During Nautical Twilight, enough light is present for horizons and larger celestial bodies to be visible, which is particularly beneficial for navigation purposes—hence the name.

This period is valuable for mariners who need to take sightings of stars and other celestial bodies while still having some visibility on the horizon. The defining aspect of Nautical Twilight is that it marks a transitional phase from daylight to darkness, providing conditions suitable for certain types of navigation.

Civil Twilight, by comparison, occurs when the sun is less than 6 degrees below the horizon and is characterized by a greater amount of light, allowing most outdoor activities without artificial lighting. Sunset refers to the moment the sun disappears below the horizon, while sunrise is the moment the sun rises above it. Each of these other periods has distinct definitions related to the sun's position relative to the horizon, but Nautical Twilight specifically stands out as the time when the center of the sun is between 6 and 12 degrees below the celestial horizon.