Today Earth has reached the farthest point in the orbit around the sun, which is also called aphelion. The opposite happens in early January when Earth is at the nearest point in the orbit which is called perihelion. The difference between the two is around 5000000 Km.
The reason for this is the elliptical orbit of Earth around the sun. The orbit is not completely circular but elliptical like an oval. The elliptical orbit has two foci and sun is present at one of the foci. The sun and the Earth both revolve around their center of mass. Since the mass of the sun is very large as compared to the mass of Earth, the motion of the sun is negligible.
Although, there is a difference in radius of the orbit at two positions but this is not the measure cause of seasons. Some people thought that the closest point corresponds to winter and farthest to summer which is not true.
The seasons are mainly caused by a tilt of the axis of rotation of the earth around the sun. Due to this tilt at one time only one pole faces the sun while the other remains in the dark. During the half year, the north pole faces the sun which corresponds to summer in the northern hemisphere while winter in the southern hemisphere. The opposite happens in the other half of the year when south pole faces the sun to cause summer in southern hemisphere and winter in the northern hemisphere.