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Dennis Mammana: Winter Solstice and a Change of Seasons

The winter solstice occurs this week. Click to view larger
The winter solstice occurs this week. ( illustration)

Nights are growing longer. Snow is blanketing much of the land. And furnaces are working overtime. There’s definitely a change of seasons coming our way.

It’s surprising how many folks don’t really understand why seasons even exist. Many believe that wintertime occurs because our planet lies farthest from the sun at that time and our temperatures are lower as a result.

While it’s true that our distance from the sun changes during the year, we’re actually about 6 million miles closer to the sun during January compared to July!

The lower temperatures in wintertime occur mostly because our planet’s axis is tipped 23.4 degrees relative to the plane of our orbit around the sun, and the sun doesn’t appear in our daytime sky very long.

During the months of December, January and February, the Earth’s Northern Hemisphere is tilted away from the sun, causing sunlight to hit us at a much shallower angle. Six months and half an orbit later, our planet’s tilt aims the Northern Hemisphere toward the sun, and solar rays beat directly down onto the Northern Hemisphere.

The lower daily path of the sun at this time of year means that it is only in our sky for about nine or so hours every day, which is not enough time to heat up the air very much.

So why, then, isn’t the year’s coldest day the first day of winter, when the sun appears to travel lowest in our sky? That’s because our atmosphere takes time to respond to temperature variations.

When you remove a pot of hot water from the stove you know that it doesn’t cool down instantly, and you make allowances for that when cooking. The same is true with our atmosphere; the coldest time of the year in the Northern Hemisphere typically occurs during late January or February.

The exact moment of the Northern Hemisphere’s maximum tilt away from the sun, which astronomers know as the winter solstice, occurs this year at 2:44 a.m. PST Wednesday, Dec. 21. To an observer on Earth, this day marks the sun’s lowest midday position in the sky and the beginning of its midday climb once again.

This marvelous moment has been celebrated by cultures throughout the ages as the rebirth of sunlight, warmth and life on Earth. It’s not a coincidence that some of the world’s major holidays are celebrated around this time of year. In fact, many having been scheduled around this one important astronomical event.

Even the term “solstice” originates in antiquity, coming from the two Latin words “sol,” meaning “sun,” and “sister,” meaning “to stand still.” It is at the winter solstice that the sun’s southerly midday drop seems to end, the sun stands still, and the star that gives life to planet Earth begins its ascendance once again.

From this moment on the days become longer, the sun appears gradually higher, and the greens of life slowly return to our planet’s Northern Hemisphere.

Springtime is not that far away. And while the astronomer in me loves the long nights of winter, I secretly await the increasing warmth ahead!

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. The opinions expressed are his own.

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