Hint:We know that there are two types of motion of the earth. One is the rotation and other is the revolution. Rotation refers to the movement of earth about its own axis. The time taken by the body to complete one rotation is called time period. The time period for the rotation is 24 hours. The earth and the other planets go around the sun in an elliptical orbit and such motion of earth is called revolution.
Complete answer: The phenomenon of day and night occurs on account of rotation of the earth. We know that earth moves about its own axis in a circular motion and about the sun it goes in an elliptical orbit with the sun situated at one of the foci. So, the position of the sun is fixed and also since the time taken by the body to complete one revolution is 465 days which is very very big in comparison to 24 hours, the earth can also be assumed to be constant at its place.
The earth is a sphere and because of its rotation at a time one part of the earth which faces the sun receives sunlight and another part which is directly opposite to it does not receive any light. Thus, the phenomenon of day and night takes place.
Note:Elliptical orbits are complex in comparison to the circular orbits. In a circle there is one center but in the case of an ellipse there are two foci. Also, when the planet is close to the sun its linear velocity increases and when it is away from the sun its velocity decreases and this is the result of Kepler's second law which says the areal velocity of the planet around the sun remains constant. The motion of the planets around the sun are governed by the three laws of Kepler’s.