The orbit of the earth around the Sun is not circular. It is not even an ellipse. The shape of the Earth’s orbit changes from being elliptical (high eccentricity) to being nearly circular (low eccentricity) in a cycle that takes between 90,000 and 100,000 years. At the moment the Earth's orbit is nearly circular with a radius of about 150 million km. The perihelion - point of closest approach on 3rd January is about 5 million km less than aphelion - greatest distance, which occurs on or about July 4th. About 6% more solar radiation reaches the Earth at perihelion than at aphelion. The tilt of the Earth is also not constant. At the moment the Earth spin axis is tilted at about 23.5 degrees to the orbital axis of the Earth around the Sun, but it varies between 22.1 and 24.5 degrees in a cycle of about 40,000 years.. A bigger tilt means warmer summers and cooler winters in both North and South hemispheres.