What Causes Day and Night

Earth is one of the nine planets of the solar system and it orbits around the Sun at a speed of 30 kms/sec. It takes one year for the Earth to complete one cycle or revolution around the Sun.

Earth's mass is approximately around 5.98 x 10²⁴ kg and the average density is about 5520 kg/m³. Earth's diameter is 12,756 kms. Earth is the fifth largest planet and the only one known to support life. It has one satellite Moon. The Moon is around 1/4th of the Earth's diameter.

Eratosthenes, a Greek scholar, was the first person to determine the circumference of the earth. He calculated the circumference by comparing the shadow of the Sun (at noon) in the deep wells in Syene and Alexandria. In his calculation, he assumed the rays of the Sun are parallel. He stated the circumference of the Earth to be around 250,000 stadia (an ancient Greek measure - one stadia is equal to a modern furlong or 660 ft). He also calculated the distance of the earth from the Sun and the moon and the tilt of the Earth’s axis.

The Earth's circumference at the equator is around 40,000 kms and the planet takes 24 hours to complete one rotation on its own axis. The Earth spins on its own axis at the speed of 1670 kms/hr. The speed of the rotation of the planet Earth is calculated by dividing the circumference of the Earth at the equator by the total number of hours in a day.

What causes Day and Night

Day and night is the result of the rotation of the Earth on its own axis. The axis of the Earth is an imaginary line that passes through the North and the South poles. The Earth takes approximately twenty-four hours to complete one rotation. The Sun is the source of light.

At the time of the rotation, it is day on the side of Earth that faces the Sun; while it is night on the other side of Earth not facing the Sun. The position of the Sun is fixed and it does not change.

The Sun rises in the east because the Earth is moving towards the east. Later, in the mid noon the Sun is in the south because Earth is moving towards the south and gradually towards the west and the Sun sets.

The length of the day is measured from the time the Sun rises to the time the Sun sets. Another interesting fact about Earth is that because Earth tilts on its axis, it is day in the southern hemisphere and night in the northern hemisphere and vice versa.

The axis of the Earth is always tilted at an angle of 23.5 degrees. The rotation of the Earth on its own axis not only results in day and night but also creates the four seasons (summer, winter, autumn and spring). The summers and winters are the result of the tilt of the axis of the Earth.

The length of the day and night varies in each season. For instance, the daylight is longer in summer and the night is shorter. In winter, the nights are longer and the days are short. It is the length of the day and night that determine the temperature of the seasons and not the revolution of the Earth.