Answer:
Since the Earth is on a tilt or "Earth's axis", one side of the hemisphere is closer to the sun. As the Earth rotates, one side is closer to the sun, which causes summer. There are 2 sides of hemisphere, a Northern and a Southern. This is split by the equator. The Summer Solstices starts on June 21, and Winter Solstices starts on December 21. In the Northern Hemishere, the sun completely covers over the Tropic of Cancer by June 21. In the Southern Hemisphere the sun completly covers the Tropic of Capricorn by December 21.