Answer:
There are three basic factors affecting long term climatic changes of earth including change in the shape of orbit (eccentricity), the tilt of the axis changes (obliquity), and the unsteady movement from side to side of earth (precession).
The shape of the orbit (eccentricity) changes from oval to circle, when shape of earth become circle the sun comes at the center and affects very less to seasons while when orbit become oval, it makes earth more warmer and increase the length of the seasons.
When the tilt of earth (obliquity) becomes larger, it results into severe summer and winter and if the tilt is smaller there will be mild seasons.
The unsteady movement from side to side of earth ( precession) depends on distance of earth from sun, if the earth is nearer to the sun there will be extreme summer in one hemisphere and extreme cold in other hemisphere.