The Earth rotates about its own axis and orbits around the Sun due to the combination of two fundamental principles: conservation of angular momentum and the gravitational force between celestial bodies.
- Conservation of Angular Momentum: Angular momentum is a property of rotating objects and is defined as the product of an object's moment of inertia and its angular velocity. The conservation of angular momentum states that in the absence of external torques, the total angular momentum of a system remains constant.
When the Earth formed from the primordial solar nebula, it had some initial angular momentum. As the cloud of gas and dust collapsed under its own gravity, conservation of angular momentum caused the system to rotate faster due to the decreasing moment of inertia. This rotational motion has been preserved since then.
- Gravitational Force: The Earth orbits around the Sun due to the gravitational force between the two bodies. Gravity is a fundamental force of attraction that depends on the masses of the objects and the distance between them. The Sun's massive gravitational pull on the Earth keeps it in orbit.
The gravitational force between the Sun and the Earth acts as a centripetal force, continually pulling the Earth towards the Sun. This force creates an inward acceleration, but since the Earth also has an initial velocity tangential to its orbit, it results in a curved path—the Earth follows an elliptical orbit around the Sun.
The combination of the Earth's rotation on its axis and its orbit around the Sun leads to the cycle of day and night as well as the changing seasons. As the Earth rotates, different parts of the planet receive sunlight, resulting in the alternation between day and night. The tilt of the Earth's axis relative to its orbital plane causes the variation in the amount of sunlight different regions receive, leading to the changing seasons throughout the year.