Earth rotates around its axis, an imaginary line that goes through the North and South poles. This axis is tilted 23.5 degrees. The earth's axis is tilted toward the sun for part of the year, and tilted away from the sun for the other part of the year. Therefore, the sun's rays strikes different parts of the earth at different angles. Earth has four seasons because it gets varying amounts of sunlight as it orbits the sun. The seasons therefore are a product of Earth's revolution about the sun and of the 23.5 degree tilt in the earth's axis.
When the Northern Hemisphere is tilted towards the sun, it receives direct rays. Direct rays heat much more than slanted rays from the sun. During this time, the Northern Hemisphere will experience more hours of daylight. As a result of direct heat and more daylight hours, the earth receives more heat. This increase in temperature causes Summer. At the same time the Southern Hemisphere is tilted away from the sun. The sun's rays are slanted and cause the heat to spread over a greater area. There are also fewer daylight hours. The combination of less daylight hours and slanted rays cause the earth to receive less heat. This drop in temperature causes Winter.
About June 21, Earth's axis tilts most toward the sun. Summer officially begins above the Equator, on the northern part of the world. On the same day, sunlight hits the southern part of the world at a slant, delivering less heat where Winter officially begins. Six months later, Earth's axis tilts most away from the sun. About December 22, the seasons are reversed in the world. Winter starts in the Northern Hemisphere while Summer starts in the Southern Hemisphere.
Twice a year, around March 21 and September 23, the sun shines directly over the equator. All parts of the world have equal day and night. These days are called the equinoxes.