A Guide To Winter Solstice (Summary)

December 20, 1999

1) Why do we have solstices?

The winter and summer solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: Earth spins on a tilted axis.

The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s why we have seasons.

2) Is the solstice really the first day of winter?

Well, depends: Are you asking a meteorologist or an astronomer?

Meteorologically speaking,summer is defined as the hottest three months of the year, winter is the coldest three months, and the inbetween months are spring and fall.

Here’s how NOAA breaks it down:

Meteorological spring includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.

Astronomically speaking, yes, winter begins on the winter solstice.

 Is the winter solstice the earliest sunset of the year?
Not usually. Just because December 21 is the shortest day of the year for the Northern Hemisphere, doesn’tmean every location has its earliest sunset or latest sunrise on that day.”

“No one knows for sure why Stonehenge was built some 5,000 years ago (at least we don’t, sorry). But one strong possibility is that it was used to mark solstices and equinoxes. That’s because the structure is directly aligned toward the sunset during the winter solstice. (The sun also rises directly over the Heel Stone during the summer solstice.)”

“While the summer solstice draws a larger crowd, the winter solstice may have been more important to the ancient builders. At this time, cattle were slaughtered so the animals did not need to be fed through the winter, and wine and beer made previously had finally fermented.””

“Ever since the Earth has had liquid oceans and a moon, its rotation has been gradually slowing over time due to tidal friction. That means that over very, very long periods of time, the days have been getting steadily longer. About 4.5 billion years ago, it took the Earth just six hours to complete one rotation. About 350 million years ago, it took 23 hours. Today, of course, it takes about 24 hours. And the days will gradually get longer still.”

“All the planets in our solar system rotate on a tilted axis and therefore have seasons, solstices, and equinoxes.”

“Sunsets are, in fact, better during the colder months of the year.”

“[N]ot only is the air clearer due to low humidity, it’s also a bit clearer to begin with.”

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