There are a lot of misconceptions surrounding the reasons why we have seasons.
Some people improperly believe that the Earth is closer to the sun in summer and farther from the sun during winter. Because of this, there is often the common misconception that winter is from December to March and summer is from June to September.
This misconception is often held in the Northern Hemisphere, the area of the world that is North of the Equator — including North America, Europe and parts of Asia. In the Northern hemisphere winter is from December to March, but in the Southern Hemisphere, the area of the world that is South of the Equator — including South America, Africa, parts of Asia and Oceania, winter is from June to September. Some people also believe that the Earth's orbit around the sun is a perfect circle.
Technically, the sun is closer to parts of the Earth during summer and farther away from parts of the Earth during winter. This is because the Earth’s axis, the imaginary pole that goes through the centre of Earth from top to bottom, is tilted at an angle of 23 1/2 degrees.
As Earth orbits the sun on an elliptical (an elongated oval orbit), the axis remains pointed in the same direction, so different parts of Earth get direct rays from the sun at different times. In the northern hemisphere, the axis points most toward the sun at approx. June 21st and the axis is furthest away at approx. December 21st. In the Southern Hemisphere, this is reversed.
At approximately March 21st and September 21st, both hemispheres are approximately 90 degrees away from the sun. This is called an equinox, which is derived from the Latin term for equal night. During the days leading up to and directly after the equinox there are approximately equal day and night (12 hours each).