The Shortening Autumn Days

Winter is not only the coldest time of the year but its early weeks also bring the shortest spans of daylight.  The hours of daylight recently dipped below ten hours here in central Pennsylvania and we're still a month from the winter solstice.  The length of our days and the trend of declining temperatures are directly related to the height of the sun in the sky.  The sun will continue to drift closer to the horizon until the shortest day of the year just before Christmas. 

On our winter solstice, the sun is directly overhead the Tropic of Capricorn (23.5 degrees south of the equator), the southern-most point that overhead rays reach.  That means that the solar radiation that reaches our Northern Hemisphere is at its lowest.  Above the Arctic Circle (23.5 degrees from the North Pole) the sun is not seen at all on the Winter Solstice.  The magic 23.5 degrees keeps coming up in our conversation because that is the tilt of the Earth on its axis.  That tilt means that the height of the sun in the sky changes from season to season.  Altoona, about 40.5 degrees north latitude, never experiences direct overhead sunlight, being located 17 degrees above the northern border of the tropics, the Tropic of Cancer.  That tropical border line is just below the southern end of the Florida Keys.

We all recognize that average temperatures generally drop as we move away from the equator and toward the poles (where the sun is much lower in the sky).  Shallotte, NC (34 degrees North Latitude and directly south of Altoona) is, on average, 18 degrees warmer than Altoona at this time of the year.  Riviere-Heva in northern Quebec, Canada (48 degrees North Latitude and due north of us) averages high temperatures 17 degrees colder than ours in late autumn.  We sometimes forget that the length of day and night are also influenced by our distance from the equator.  Shallotte, NC gets 35 minutes more daylight than we do in mid-November.  Riviere-Heva has a 38 minute shorter day than what we have here.  The sun will go down at 4:27 p.m. this afternoon in the remote Canadian town.

There is an upside to these short days in the wintertime; there are correspondingly long days in the summer.  An 8:20 day in Riviere-Heva near the first day of winter is replaced by a 15:40 long day on the Summer Solstice six months later.  The longer day and higher sun help bring an average high of 70 degrees in late June.  I met Alaska's state champion, Bill Tetraut, at the national bicycling championships some years ago and our conversation inevitably turned to our training challenges.  He lamented that winter and spring training could be very difficult that far north. 

Summers were another matter.  Besides the warmer weather, nights were only a little more than six hours long where he lived and he could safely ride until 11 p.m. for several weeks in early summer.  This interesting tilting of our planet, then, brings the seasonal temperature variations and changes in the length of our days and nights.  After living through a few decades of these changes, we often forget how these little things can change our lives in very big ways.