Have you ever wondered what caused the seasons? It is because of the Earth's tilt, therefore the sun hits it at a different angle. In winter solstice, the Earth is tilted so the sun hits the Tropic of Capricorn directly. Therefore, people up north have less sunlight, which means less heat. The same thing happens with summer solstice, only the sun hits the Tropic of Cancer. That means northern people have more heat.  During seasons in between summer and winter, the sun is shining on the equator.  The southern hemisphere is opposite of us (We are closer to the Tropic of Cancer. They are closer to the Tropic of Capricorn.), so when it is our winter solstice, it is their summer solstice!