The Earth doesn't orbit upright, but rather at a tilt on its axis by 23 1/2 degrees. And it's the tilt of the earth that causes winter and summer. On the December solstice the Northern Hemisphere is most tilted away from the sun causing the sun's rays to shine on the Tropic of Capricorn. This is the farthest south the sun's rays ever get. South of the equator locations experience days longer than 12 hours, while north of the equator locations experience days less than 12 hours. Following the Winter Solstice days get longer and nights get shorter.