THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W 
Altitude:  10 feet below sea level
Founded January 1970
Julian Date: 2458870.16
2019-2020:  LXXXIX
           "Heavens Above!"


THE DAILY ASTRONOMER
Wednesday, January 22, 2020
Five O'Clock Shadows

Did you notice?
Yes, perhaps you did.
Even though we're now languishing in winter's deepest hollows and the biting cold has just sprouted a few more jagged bicuspids, the sky isn't stone black at 5 p.m. anymore!   While the sun is still setting before five, the civil twilight won't end until after five: a subtle reminder of spring's inevitable return.       Faint shadows on a greying landscape have replaced the midnight dark.

We wanted to focus on this lighter matter to both offer solace to those sad souls who believe that winter is interminable and to explain its astronomical causes.  A reminder that all terrestrial concerns ultimately have celestial causes.

First, Earth moves rapidly around the Sun.  Its current cruising speed is nearly 67,000 miles per hour.*  As it is tilted by 23.5 degrees relative to its vertical axis, the orientation of the poles relative to the Sun varies throughout the year.  On the summer (June) solstice,  the north pole is aligned as closely toward the pole as possible.   On the winter (December) solstice, the north pole is aligned as far away from the Sun as possible. Throughout the intervening six months between both solstice points, Earth's north polar alignment changes gradually.      This alignment shift alters the Sun's daily path.  In the summer, the Sun appears to describe a wide arc from its northeastern rising position to its northwestern setting position.  On the vernal (March) and autumnal (September) equinoxes, the Sun rises due east and sets due west.  In the winter, the Sun rises south of east and sets south of west.   See the graphic below.


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The Earth's rotation rate is constant for any given location, so if the Sun's arc is larger in our sky, it will remain above the horizon for a longer period than it does when following a shorter arc.     The duration of daylight is far shorter in the winter than the summer for precisely this reason. 

The next issue pertains to twilights.   We experience three of them: civil, nautical and astronomical.  Civil twilight, what we call dusk, occurs when the Sun is between 0 - 6 degrees below the horizon.   Nautical twilight refers to the time period when the Sun is more than six, but less than twelve degrees below the horizon.   Astronomical twilight is the final twilight phase. It occurs when the Sun is between 12 - 18 degrees below the horizon.   After astronomical twilight ends, the atmospheric light scattering is negligible.  

The civil twilight period is now ending just after five p.m.  As we proceed through winter and spring, the civil twilight will end progressively later until around the summer solstice when it ends just after nine p.m!

Winter shall continue for a while and the bitter cold's iron grip will remain firm for a few weeks to come. Yet, be comforted by the hints of civil twilight that one can now see after five p.m.    We're moving inexorably toward spring.

*Earth was at perihelion (its closest point to the Sun) on January 5th and is now moving toward aphelion (the most distant point).  Consequently, its revolutionary motion will slow down, but not by much.



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