Here's why:

* Kepler's Laws of Planetary Motion: Johannes Kepler, through his observations and calculations, established that planets move in elliptical orbits, not circular ones. The Sun is at one focus of this ellipse.

* Gravitational Force: The Earth's motion is governed by the gravitational force between it and the Sun. This force is not constant as the Earth moves, leading to variations in the Earth's speed throughout its orbit.

* Varying Distance: The Earth's distance from the Sun varies throughout its orbit. It's closer to the Sun at perihelion (around January) and further away at aphelion (around July).

While the Earth's orbit is not perfectly circular, it is very close to being one. This is why we often simplify it as circular motion for educational purposes, but it's important to remember that the true nature of the Earth's orbit is elliptical.