SunZenith

Official zenith for sunrise and sunset. Occurs when the sun goes below or rises above the horizon

Occurs when the sun dips 6 degrees below the horizon. During civil twilight, the atmosphere reflects enough light to allow outdoor activities.

Occurs when the sun dips 12 degrees below the horizon. The horizon is generally still visable, but many of the brighter stars are visible during nautical twilight (between civil and nautical sunset). This enables navigation by stars at sea and is the namesake for it.

Occurs when the sun dips 18 degrees below the horizon. Often hard to differentiate between astronomical twilight and night but astronomers often have difficulty seeing fainter stars during this time. Thus astronomical twilight.