This is the time of day which marks the beginning of twilight before sunrise or a period for sunrise. The time is usually characterized by weak sunlight when the sun is still insignificant under the horizon. Types of dawn are also divided into astronomical, nautical and civil. Astronomical dawn starts when the sky ceases to be dark. The sun is 18 degrees under the horizon at morning hours. Both astronomical dawn and dusk are usually dark even in absence of clouds.
Nautical twilight starts at time when there is enough light for sailing. It is easier to tell between the horizon and the sea. However, artificial light may be required to carry out activities outdoors. This time is usually dark and is considered as night. Civil dawn occurs when the sun is 6 degrees below the horizon in the morning. Enough light is available to distinguish objects but the sky is slightly dark.
This is the darkest moment of twilight, precisely, the very end of astronomical twilight. This happens after sunset and before nightfall. This is preceded by intermediate twilight where there is enough light under the sky. The human eye can read texts without relying on artificial light.
Civil dusk occurs when the center of the sun is 6 degrees below the horizon in the evening. Nautical dusk is time of day when the sun’s center is 12 degrees below horizon in the evening. Stars in the sky can be seen with naked eyes. This term is very common among sailors and the military. For instance, the morning and evening nautical twilight is used by United States military to plan tactical operations. Astronomical dusk is when the geometrical center of the sun is 18 degrees immediately below the horizon. This is the onset of darkness and light is no longer in the sky.
The duration of an equinox depends on latitude. Longer twilights are experienced longer on higher latitudes than equatorial and tropical regions. Astronomical twilight during sunrise resembles the one experienced at sunset during summer months. This happens when the angle between the sun and the horizon is less than 18 degrees also known as solar elevation angle. Higher altitudes may also experience an extended nautical twilight.