The main reason behind the seasons on earth is itâ€™s tilt on its axis. Axis is an imaginary line passing through the poles. 23.5 degree angle is formed to the perpendicular of the orbit of earth and axis always points in the same direction throughout the year.
Rays coming from sun hits at smaller angle as compared to the other parts of globe. When the northern hemisphere is away from the sun or we can say it is leaning away from the sun then the amount of light received by the northern hemisphere is distributed in large area and therefore receives less heat. Now at this time there will be winter in the northern hemisphere and summer in the southern hemisphere. Same happens when southern hemisphere is leaning away from sun. It will be experiencing winters and northern hemisphere will experience summer in another half of year. So from this we can say all parts of earth receives the different amount of sunlight which causes seasons.
SUMMER SOLSTICE is when the northern hemisphere is nearest to the sun. At this time sun is in its highest point in the sky so it takes more time to cross sky. That is why it is longest day in a year somewhere around 21 June.
So as North Pole starts moving away from the sun it starts rising lower in the sky. Days gradually becomes shorter and when it reaches to the midpoint it is called AUTUMN EQUINOX somewhere around 22 September.
Now when North Pole is leaning farthest from sun it rises in lower point and crosses the sky fast. This is the shortest day somewhere around 22 December and is called WINTER SOLSTICE.
When northern hemisphere again start moving near the sun the sun starts rising upward day by day and when it reaches midpoint this is called SPRING EQUINOX somewhere around 21st march.
If the axis was at zero degree then no seasons would have been experienced and the sun rays reach the earth in same way throughout the year.
The most common misconception about seasons is that when earth moves closer to the sun it is summer and when farther it is winter. Earth orbits is in an ellipse, not in a perfect circle but this ellipse is very near to circle and hardly have any affect on the seasons.