The Eath's Movements (Why day and night is there?)

Being a planet of the sun, the earth revolves around the sun. Besides this it also rotates on its own axis. Thus the earth has two motions, the rotation and the revolution.

Rotation of the Earth

The earth rotates on its axis from west to east. The rotation of the earth is completed in about 24 hours and it is called the daily motion of the earth. This motion is responsible for the occurrence of day and night. One rotation of the earth is completed when a given heavenly body crosses the observer's meridian two times in succession. The period of rotation is calculated with reference to a star and with reference to the sun. When it is calculated with reference to a star, it is called a sidereal day and when it is calculated with reference to the sun, it is called a solar day.The solar day is a time period of 24 hours and the duration of a sidereal day is 23 hours 56 minutes. This difference of four minutes between a solar day and a sidereal day is due to the fact that the position of the earth keeps changing with reference to the sun due to the earth's revolution around it while with reference to a star at infinity it will remain unchanged. Thus a sidereal day is the actual time taken by the earth for a rotation of exactly 360 degrees on its axis.As a result of the apparent motion of the sun, the appearance of the night sky as seen from the earth changes from day to day. The stars rise every day 4 minutes earlier than the preceding day and every night one degree of celestial sphere is added to the eastern sky while one degree is lost from the western sky.

Revolution of the Earth