Shortest Day to occur on 21st: the winter solstice

For six months now, the days have grown shorter and the nights have grown longer -- but relief is in sight.

The shortest day of the year is soon to arrive in the Northern Hemisphere. Finally, the countdown to spring will begin.

It's no surprise that many cultures and religions will celebrate a holiday -- whether it be Christmas, Hanukkah, Kwanzaa or pagan festivals -- that coincides with the return of the sun and longer days to come.

What is the winter solstice? The winter solstice marks the shortest day of the year in the Northern Hemisphere, when the sun appears at its most southerly position, directly overhead at the Tropic of Capricorn. It marks the longest day of the year in the Southern Hemisphere.

When exactly does the winter solstice occur? The solstice happens for everyone on Earth on December 21 at 11:48 p.m. ET. That's Monday night in New York and Atlanta, Tuesday morning in Paris and Nairobi, and Tuesday afternoon in Tokyo and Sydney.

Why does it occur? Because the Earth is tilted, we experience seasons here on Earth. As the Earth moves around the sun, each hemisphere experiences winter when it is tilted away from the sun and summer when it is tilted toward the sun.

Wait. Why is the Earth tilted? Scientists are not entirely sure how this occurred, but they think that billions of years ago, as the solar system was taking shape, the earth was subject to violent collisions that caused the axis to tilt.

What other seasonal markers can we celebrate? The equinoxes, both spring and fall, mark when the sun's rays are directly over the equator, where we have equal length of day and night. The summer solstice is when the sun's rays are farthest north over the Tropic of Cancer, giving us our longest day and summer.