Why does the eccentricity of the earths orbit make the contrast between summer and winter ?
It does not. The contrast between the summer and winter is the “obliquity”: the angle that the spin axis makes with the orbital plane. In the (northern) summer, the north end of the axis is tilted towards the Sun. The Northern hemisphere gets sunlight from a more direct angle (Sun higher int he sky) and for a longer time (days are longer). Therefore the ground gets to store more heat. In the (northern) winter, the Earth in its orbit is now on the other side of the Sun. Therefore the north end of the axis is pointing away from the Sun. The Sunlight comes at a much lower angle and the days are shorter. The ground lose heat faster than it can get it from the weak (and scarce) sunlight. — Eccentricity is the fact that our orbit is not perfectly circular. The orbit is an ellipse with one part a bit closer to the Sun and a part a bit further out from the Sun. We are closest to the Sun in January (when it is winter in the northern hemisphere) and we are furthest away in July. The difference
