Day and night recur regularly and continuously, just like the seasons. This takes place according to the laws of our solar system. During the course of one day, the Earth makes one rotational movement from the West to the East.
It rotates once around its own axis in 23 hours, 56 minutes and 4.1 seconds, although we can feel nothing of this rotation. Every hour, the Earth's_axis rotates through about 1000 kilometers in Europe and about 1700 kilometers at the equator.
This rotation determines the change from day to night and vice versa. In the part that is turned towards the Sun, it is day and the other side has night. In a particular part of the Earth that is just emerging out of the night_shadow, the Sun rises in the east in the morning. While the Earth continues to rotate, the Sun keeps climbing higher, till it has reached its zenith at mid-day.
Then it drops to the horizon again, till it sets in the West and simultaneously rises in the other half of the Earth. But the Sun does not actually rise and set; it merely shines on a certain area of the rotating Earth.
The transition period between full daylight and complete darkness is called the daybreak or the twilight. Thus, there is the pre-morning daybreak and the evening twilight. Its duration depends on the inclination of the (apparent) solar path to the horizon.
The steeper the path, the faster the Sun rises and sets. The inclination and hence the duration of the dawn or twilight depends on the geographical latitude of the point of observation.
Thus, the duration of dawn or twilight in the tropical regions is very short, whereas in Europe, it lasts for about 2 hours. The cause of dawn or twilight is scattering of Sunlight between the upper air layers and the lower atmospheric layers. We talk of an astronomical dawn or twilight when the level of the Sun is about 18 degrees below the horizon.
