The Earth is typically considered to be part of a non-inertial frame of reference because it undergoes both rotational motion and non-uniform motion around the Sun. An inertial frame of reference is one in which an object remains at rest or moves at a constant velocity in a straight line unless acted upon by external forces. In such a frame, the laws of physics, particularly Newton's laws of motion, hold true without the need for additional corrections.
However, due to Earth's rotation on its axis, objects on the surface experience a centrifugal force that appears to push them outward. This force, known as the centrifugal force, is a pseudo-force that arises due to the acceleration caused by the rotational motion of the Earth. Similarly, Earth's orbit around the Sun is not perfectly circular but rather elliptical, resulting in a changing velocity and acceleration.
Because of these accelerations and non-uniform motions, Earth's frame of reference is considered non-inertial. In a non-inertial frame, additional forces or pseudo-forces, such as the centrifugal force and the Coriolis force, need to be taken into account to accurately describe the motion of objects within that frame.
The consideration of Earth as a non-inertial frame of reference becomes particularly important when dealing with phenomena like the Coriolis effect, which affects the apparent motion of objects moving across Earth's surface. It also has implications in the field of general relativity, where the curvature of spacetime caused by mass and energy distributions affects the motion of objects.
It's worth noting that in many practical situations, the effects of Earth's non-inertial frame are negligible and can be safely ignored. However, in precise scientific measurements or when dealing with systems involving high velocities or long time scales, these considerations become significant.