I want to wish everyone happiness on this day that has been celebrated by a variety of cultures throughout time. In many ways, it is the start of a new year.
If you have ever wondered what makes a solstice a solstice, allow me to explain. Most people understand that Earth rotates on it's axis once every "24" hours and revolves around the sun once in "365" days. But many are not aware of a third relative motion: The Earth's axis tilts forward about 23 degrees for the Northern Hemisphere's summer and 23 degrees back for its winter. It is this tilt that causes the seasons. When the Northern Hemisphere is tilting forward, it gets the most direct sunlight for the longest time and produces summer. As we move toward Winter Solstice we get less direct sunlight and less time in the sun.
The line at which the Northern Hemisphere tilts its furthest distance forward is called the Tropic of Cancer. The line at which the Northern Hemisphere tilts furthest back is called the Tropic of Capricorn. The Equator is directly between these two lines. When the sun sits directly over the equator, we are having either our Spring or Autumnal Equinox.
Happy Winter Solstice!