This morning twilight before dawn is classified into three groups based on the quantity of sunlight present in the sky, which is determined by the sun's angular distance (degrees below the horizon) in the morning. There are three types of dawn: astronomical, nautical, and civil. Dawn of the stars is astronomical dawn. Dawn with just darkness visible is nautical dawn. Civil twilight ends when sunrise occurs, regardless of what time it is.
Astronomical dawn: In this case, the sun is more than 10 degrees below the horizon and its appearance changes from dim to dark as it rises. The sky is completely clear, there are no clouds in sight or audible. The air is still and calm. Animals begin to stir, birds start singing, and insects emerge to greet the new day. Human beings also wake up feeling refreshed after a good night's sleep under clear skies. During a total lunar eclipse, the moon becomes red because all the earth's atmosphere is blocking out the light from the sun. This type of full moon is called a "red" moon because of this coloration.
Nautical dawn: At this time, the sun is between 5 and 10 degrees below the horizon and its appearance does not change from dim to dark as it rises. The sky is partially cloudy with some stars visible through breaks in the clouds. The air is still but not as calm as at astronomical dawn; waves may be seen on a lake or ocean shoreline.
When the sun is 18 degrees below the horizon in the morning, astronomical dawn begins. Astronomical twilight immediately follows until the sun is 12 degrees below the horizon. During this time, only visible light from the sun reaches the earth. Any objects on the sky during this period will appear to glow from the light they reflect off of the moon or planets.
At night, when astronomical dusk begins, the sun is about 12 degrees above the horizon. This is when it starts to get dark outside again after being illuminated all day long. At this time, only infrared and longer wavelength radiation from the sun reaches the earth. Anything on the sky at night will not be illuminated by sunlight.
Civil twilight begins when the sun is 6 degrees below the horizon. During this time, you can see shadows under trees, across lawns, and inside buildings. After civil twilight, darkness falls quickly because there is no more illumination from the sun.
Astronomical dawn, astronomical dusk, and civil twilight start at different times throughout the year due to their locations in relation to the equator. For example, in the northern hemisphere summer, astronomical dawn occurs later in the morning because the sun is higher in the sky than in the winter.
Civil twilight begins at 6 degrees below the horizon in the morning and ends at daybreak. It starts at sunset and finishes when the sun is 6 degrees below the horizon in the evening. Civil dawn occurs when the sun's center is 6 degrees below the horizon in the morning. The length of civil twilight varies from country to country and from season to season. In general, it lasts about an hour more in summer than in winter because light travels faster across cold distances.
Astronomical twilight ends when there is no longer a discernible difference between night and day from a location within the sky that does not experience light pollution. The time span varies depending on your distance from the celestial equator (the plane of the earth's orbit around the sun), but generally speaking, it takes about an hour and a half for the sky to go completely dark at the beginning of astronomical twilight and about an hour and a half for it to become fully light again at the end.
It is important to remember that these are only guidelines. There are many factors that can affect how long twilight lasts, such as the altitude of your location or whether it is daylight saving time.