As the earth rotates on its axis and revolves around the sun, the earth goes through seasonal changes. why does this happen?

The earth's axis points to the same spot in the sky as the earth orbits the sun. This relationship can be divided into four points of interest. 
1) When the earth's north pole is pointing at its maximum amount away from the sun. That is the winter solstice in the northern hemisphere. 
2) Six months later, the north pole is now tilted most *toward* the sun. This is the NH summer solstice. 
3,4) The points halfway between those are where the polar axis is "sideways" to the sun. The sun is on a direct line from the center of the earth through the equator. These are the two equinoxes. 

To illustrate this, take a globe and move it in a circle around some central point, with the angle of the polar tilt always pointed in the same direction. 

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