A summer on Earth in the Northern Hemisphere is 93 days long. Summer begins on June 21 and ends on September 23. Since the Earth has four seasons and it takes 12 months to make a full revolution around the sun, each season lasts about three months.
Earth has different seasons because it rotates on a titled axis, which increases and decreases the amount and density of sunlight each hemisphere receives throughout the year. Earth orbits the sun in an almost perfect circular pattern, so the distance from the sun has very little to no impact on the seasons. When it is summer in the Northern Hemisphere, the Southern Hemisphere experiences winter.