The Shifting Shadows of the Solstice

What did you do on 21^st December?  Struggling to remember?  That’s not a surprise; the 21^st December often passes with little fanfare, in Britain at least.  Most of us are too pre-occupied with Christmas shopping, or planning the New Year’s Eve party, to consider its significance.  A shame, perhaps, because the events of the 21^st are far more important; in fact without them, we probably wouldn’t be celebrating New Year at all.

To understand why, we have to turn to the Earth’s annual journey around the Sun.  As we hurtle through space, our distance to our parent star changes little; just over one-and-a-half percent – a mere two million miles!  But a few million miles aren’t enough to create our seasons; to generate those familiar yearly transformations requires a twist, and that twist is literal:

The school-child’s illustration of the Solar System describes it best:

Rather than sit upright in its orbit, the Earth’s axis is tilted; inclined at an angle of 23.5 degrees.  That small twist in the solar jigsaw has profound effects back on Earth:  In winter, in the Northern Hemisphere, the North Pole points away from the Sun.  As the Earth spins on its axis (one whole each day), the polar region remains in shadow.  Here, night last for 24 hours a day, and at the pole itself, the night is six months long.  Things are little better further south, with short days, and long nights, that lengthen as one moves towards the Equator.

In so much shadow, little of the Sun’s heat reaches the Northern Hemisphere, and winter is in full swing.  Travel south of the Equator, though, and the sun brings summer, with the seasonal regularity we have come to expect.  Here, then, is the origin of our seasons; a constant see-saw of light and shadow as our planet ploughs along its orbit.  That see-saw governs the lengths of our days and nights, the growing seasons for our food, and ultimately, our weather.

And the 21^st December?  Well that date marks the Winter Solstice; the date when the North Pole is furthest from the Sun, and the Northern Hemisphere experiences its shortest day of the year.  From then onwards, the shadow begins to recede, slipping south in anticipation of the Northern Hemisphere’s summer, and shortening our nights by a few minutes every day.

So next year, those of you who enjoy a good New Year’s party, spend a few minutes contemplating the Earth’s shifting shadow; not only does it bring our winter snows, autumn storms, and summer monsoons, it’s also responsible for the growing season that gave you your glass of wine, or bottle of beer.  And for those who can’t stand the New Year’s parties, console yourself with this:  New Year is just a change of date; the real New Year was 11 days ago, on the Winter Solstice.

Text and image copyright: Christopher Lee, 2011 - First published 6^th January 2011.