The orbits of planets around the sun are not perfectly circular, but rather slightly elliptical.

Here's why:

* Ellipse: An ellipse is a closed curve where the sum of the distances from any point on the curve to two fixed points (called foci) is constant.

* Kepler's Laws: Johannes Kepler discovered that planets orbit the sun in elliptical paths, with the sun at one focus of the ellipse.

While the orbits are elliptical, they are often close to circular, especially for planets like Earth. However, the slightly elliptical nature of the orbits leads to variations in the distance between a planet and the sun throughout its orbit.