THE SOUTHWORTH PLANETARIUM
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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.


[image: 3506.jpg]

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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