The Winter Solstice

Today marks the Winter Solstice.  At 10:49 PM CST the sun will be directly over the Tropic of Capricorn which is approximately 23.5° south of the equator.  That means the sun angle in North Dakota will vary from from 20.5° above the horizon at solar noon today to just 17.5° along the Canadian border.  It is this low sun angle (less solar energy) in combination with our frequent snow cover (reflection of the Sun’s energy) and lack of a modifying influence (an ocean) that makes this region a catalyst for extremely cold weather from mid November through February.

Although it is technically the “shortest day of the year”, it is neither the day of the earliest sunset or the latest sunrise.  The earliest sunset came several days ago, around December 17, with sunsets having already increased by 2 or 3 minutes.  But the sunrise times will continue to get later until the first few days of January.  Why is that?  We think of each day being 24 hours and granted our clocks are set that way as an overall average, but the time from solar noon to solar noon is rarely 24 hours.  The Earth travels in an ellipse around the Sun, not a circle.  By coincidence, the Earth is the closest to the Sun this time of year (perihelion) and with the closest approach to the Sun, the earth is also moving at the highest speed of the yearly orbit.

As a reference, about two weeks before the Winter Solstice the solar noon time (not taking into account the time zones) occurs near 11:52 a.m. local standard time but by the winter solstice, the solar noon occurs much closer to true solar noon, so 7 or 8 minutes later.  The result is that this later “solar noon” also means a later clock time for the sunrise and sunset.  Meaning, earlier sunsets before the winter solstice and later sunrises for the days and weeks after the winter solstice.  By mid January, the sunset times will be a good 20 minutes later than the earliest sunset time, whereas, the sunrise times will have only increased a few minutes.

Daryl Ritchison

 

 

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