This is the time at which the sun is 6 degrees below the horizon in the morning. Civil dawn is defined as that time at which there is enough light for objects to be distiguishable and that outdoor activities can commence. This is the time at which the sun is 12 degrees below the horizon in the morning. Nautical dawn is defined as that time at which there is just enough sunlight for objects to be distiguishable. This is the time at which the sun is 18 degrees below the horizon in the morning.
Astronomical dawn is that point in time at which the sun starts lightening the sky.
This is the time at which the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities. This is the time at which the sun is 12 degrees below the horizon in the evening. At this time, objects are no longer distinguishable. This is the time at which the sun is 18 degrees below the horizon in the evening.