Winter Solstice is What You're Celebrating

Tomorrow - Thursday, Dec. 22 - is officially the "First Day of Winter," which is just a lazy way of saying that Earth's winter solstice arrives in the Northern Hemisphere at 12:30 a.m. Eastern.

Find the history and cultural background of Winter Solstice here. Did you know that Julius Caesar figured the date of the Winter Solstice to be December 25, and that he did so about 50 years before Herod called a census in Judea? What a coincidence! Also, just about every culture on the planet has a mid-winter festival to celebrate the end of shortening days and the return of lengthening days. Bringing evergreens indoors, decorating our homes with extra lights, feasting, singing and exchanging gifts are millennia-old traditions that long predate Herod.

But here's the really cool stuff: Everything You Need to Know About the Solstice from astronomy website earthsky.org.

The winter solstice is this week for us in the northern hemisphere. After the winter solstice, the days will get longer. Celebration time!

Late dawn. Early sunset. Short day. Long night. For us in the northern hemisphere, the December solstice marks the longest night and shortest day of the year.

If you live in this hemisphere, it’s your signal to celebrate. The shortest day is here! After the winter solstice, the days will get longer, and the nights shorter. It’s a seasonal shift that nearly everyone notices.

The solstice happens at the same instant for all of us, everywhere on Earth. But our clocks say different times.

In 2011, the December solstice takes place on Wednesday, December 21 at 11:30 p.m. CST (Thursday, December 22 at 5:30 UTC).

SNIP

The earliest humans knew that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year. They built monuments, such as Stonehenge, to follow the sun’s yearly progress.

But we today see the solstice differently. We can picture it from the vantage point of space. Today, we know that the solstice is an astronomical event, caused by Earth’s tilt on its axis, and its motion in orbit around the sun.

Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s northern and southern hemispheres trade places in receiving the sun’s light and warmth most directly. That’s what causes winter and summer.

At the December solstice, Earth is positioned in its orbit so that the sun stays 23-and-a-half degrees below the north pole horizon. As seen from 23-and-a-half degrees south of the equator, at the imaginary line encircling the globe known as the Tropic of Capricorn, the sun shines directly overhead at noon. This is as far south as the sun ever gets. All locations south of the equator have day lengths greater than 12 hours at the December solstice. Meanwhile, all locations north of the equator have day lengths less than 12 hours.

Read the whole thing, including an explanation of why neither the earliest sunset nor the latest sunrise fall on the day of Winter Solstice.