Solstice
The December Solstice happens when the Southern Hemisphere of the Earth is tilted toward the Sun and the Northern Hemisphere of the Earth is tilted away from the Sun. At the South Pole, the Sun reaches its highest point in the sky, and then begins moving lower. At the moment of the December solstice, the Sun is directly overhead some point on the Tropic of Capricorn; this is the furthest south that the subsolar point ever reaches. In the Southern Hemisphere the December solstice is called the Summer Solstice (and marks the longest day of the year), while in the Northern Hemisphere it is called the Winter Solstice (and marks the shortest day of the year). The December Solstice is usually on December 21.