The orbit of the Earth around the Sun is elliptical, not circular. This discovery was made by Johannes Kepler in the early 17th century. Kepler's laws of planetary motion describe the motion of planets around the Sun, and his first law states that the planets move in elliptical orbits with the Sun at one of the foci of the ellipse.
An ellipse is a geometric shape that resembles a elongated circle. It has two foci, and the sum of the distances from any point on the ellipse to the two foci remains constant. In the case of the Earth's orbit, the Sun is located at one of the foci.
The eccentricity of an ellipse describes how elongated or flattened the ellipse is. For a perfectly circular orbit, the eccentricity is zero, and as the eccentricity increases, the orbit becomes more elongated. The Earth's orbit has an eccentricity of approximately 0.0167, which means it is slightly elongated but still very close to a circle.
This elliptical shape of the Earth's orbit means that the distance between the Earth and the Sun varies slightly over the course of a year. The point of closest approach is called perihelion, while the point of farthest distance is called aphelion. Perihelion occurs around early January, and aphelion occurs around early July.