They are caused by Earth’s orbit around the sun and the way that Earth tilts on its axis.
As Earth revolves around the sun, it stays tilted on its axis. The tilt when summer begins in the Northern Hemisphere is toward the sun. At the same time the Southern Hemisphere tilts away from the sun causing winter in this hemisphere. Whatever season the Northern Hemisphere is experiencing the Southern Hemisphere is the opposite.
On the first day of winter for the Northern Hemisphere the tilt is away from the sun, at the same time the Southern Hemisphere is experiencing summer because the tilt is toward the sun.
There are 2 points in Earth’s orbit when it is not tilted toward or away from the sun. These points mark the beginning of spring and fall. When spring comes in the Northern Hemisphere, fall comes in the Southern Hemisphere. Areas near the equator do not have much seasonal change because Earth’s equator never tilts very much toward or away from the sun.
Because Earth is tilted, the suns rays strike Earth at different angles at different times of year. In summer, the tilt makes the sun’s rays shine more directly on Tennessee.
In winter, the sun’s rays come in from a greater angle causing the rays to spread out. They are less direct so less heat energy reaches Tennessee.
The amount of daylight changes with the seasons. This has an effect on the temperatures of the seasons. The period of daylight gets longer every day from December 21 until June 21. Then the period of daylight gets shorter from June 21 to December 21.
Summer receives more direct sun rays for a longer period of time causing warmer temperatures. Winter receives less direct sun rays for a shorter period of time causing cooler temperatures.
Areas near the equator do not have much seasonal change because Earth’s equator never tilts very much toward or away from the sun. The north and south poles, even when they tilt toward the sun, get less direct sunlight than areas farther from the poles. So, even in the summer it is still very cold at the poles.