Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference.
Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.
Nautical dusk is the moment when the geometric center of the Sun is 12 degrees below the horizon in the evening.
When the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime.
When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight.
Such statutes typically use a fixed period after sunset or before sunrise (most commonly 20–30 minutes), rather than how many degrees the sun is below the horizon.
Examples include the following periods: when drivers of automobiles must turn on their headlights (called lighting-up time in the UK); when hunting is restricted; when the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions.