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Commonly known as the winter solstice, the hours of daylight are the shortest today in the northern hemisphere, their longest in the southern. The northern line around the earth is called the Tropic of Cancer, the southern called the Tropic of Capricorn. (An easy way to remember which is which is alphabetical. Looking at the globe as normally seen with the north at the top, the Tropic of Cancer is the upper latitude and the “n” in caNcer comes before “p” in caPricorn. )
The Tropic of Cancer was named because at the time of its naming, the sun was positioned in the Cancer constellation during the June solstice. Likewise, the Tropic of Capricorn was named because the sun was in the constellation Capricorn during the December solstice. There are correlations with astrology of course, not that I believe in astrology. After all, I’m a Capricorn and Capricorns are very skeptical :-).
The study of the stars and the skies are as old as mankind and provided a means of measuring time, explaining seasons, and plenty of supposition. Still are. When all the planets lined up a few years ago many saw that as indicative of something cataclysmic (it wasn’t).
Many of the most ancient stone structures made by human beings were designed to pinpoint the precise date of the solstice. The stone circles of Stonehenge were arranged to receive the first rays of midwinter sun.
Some ancient peoples believed that because daylight was waning, it might go away forever, so they lit huge bonfires to tempt the sun to come back. The tradition of decorating our houses and our trees with lights at this time of year is passed down from those ancient bonfires. In ancient Egypt and Syria, people celebrated the winter solstice as the sun’s birthday. In ancient Rome, the winter solstice was celebrated with the festival of Saturnalia, during which all business transactions and even wars were suspended, and slaves were waited upon by their masters.
Henry David Thoreau said: “In winter we lead a more inward life. Our hearts are warm and cheery, like cottages under drifts, whose windows and doors are half concealed, but from whose chimneys the smoke cheerfully ascends.”
But, I want to keep this is a mostly astronomical setting.
The Tropic of Cancer and The Tropic of Capricorn
The Tropic of Cancer and the Tropic of Capricorn each lie at 23.5 degrees latitude. The Tropic of Cancer is located at 23.5° North of the equator and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China. The Tropic of Capricorn lies at 23.5° South of the equator and runs through Australia, Chile, southern Brazil (Brazil is the only country that passes through both the equator and a tropic), and northern South Africa.
The tropics are the two lines where the sun is directly overhead at noon on the two solstices – near June and December 21. The sun is directly overhead at noon on the Tropic of Cancer on June 21 (the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere) and the sun is directly overhead at noon on the Tropic of Capricorn on December 21 (the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere).
The reason for the location of the Tropic of Cancer and the Tropic of Capricorn at 23.5° north and south respectively is due to the axial tilt of the Earth. The Earth is titled 23.5 degrees from the plane of the Earth’s revolution around the sun each year.
The area bounded by the Tropic of Cancer on the north and Tropic of Capricorn on the south is known as the “tropics.” This area does not experience seasons because the sun is always high in the sky. Only higher latitudes, north of the Tropic of Cancer and south of the Tropic of Capricorn, experience significant seasonal variation in climate.
Thanks to the Geography Channel, Wikipedia, and Worldatlas.com for substantive parts of this explanation.