Why does the earth have a season

 Season

A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.
Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.
During May, June and July, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in November, December and January. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July and August are the hottest months in the northern hemisphere and December, January and February are the hottest months in the southern hemisphere.
In temperate and subpolar regions generally four calendar-based seasons are recognized: spring (adj. vernal), summer (adj. estival), autumn (adj. autumnal), and winter (adj. hibernal). However, ecologists are increasingly using a six-season model for temperate climate regions that includes pre-spring (adj. prevernal) and late summer (adj. seritonal) as distinct seasons along with the traditional four (See Ecological Seasons below).
In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, because the amount of precipitation may vary more dramatically than the average temperature. For example, in Nicaragua, the dry season (October to May) is called 'summer' and the rainy season (April to November) is called 'winter', even though it is located in the northern hemisphere.
In other tropical areas a three-way division into hot, rainy and cool season is used.
In some parts of the world, special "seasons" are loosely defined based upon important events such as a hurricane season, tornado season or a wildfire season.
Chinese seasons are traditionally based on 24 periods known as solar terms, and begin at the midpoint of solstices and equinoxes.

Astronomical

The following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.