THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W
Altitude:  10 feet below sea level
Founded January 1970
Julian Date: 2458827.16
2019-2020:  LXVII
              "We sent a quiz about time traveling on Friday, but we don't
know where it went."


THE DAILY ASTRONOMER
Monday, December 9, 2019
Earliest Sunset Today!

Have you noticed the Sun?     Correct.   Our gloriously beautiful,
blindingly bright, theoretically warm parent star beats a hasty retreat
below the western horizon soon after mid afternoon, currently.   Those who
slave away in a subterranean planetarium all day playing "Asteroids" on the
full dome projector see little more than fading twilight when venturing up
into the upper world.    Such is the nature of things on a tilted planet
this time of year for Northern Hemisphere dwellers.   Those who find this
situation a bit mood-dampening should take heart, however, for although
we're still more than two weeks shy of the winter solstice, the earliest
sunset will occur THIS coming weekend!   After that, the sunset time will
gradually increase until July!

As we can see from the graphic below, the date of earliest sunset is
latitude dependent.    Those spoiled sods in the south have already
experienced their earliest sunset and so the sunset times have already
started to increase.      We here along America's left eyebrow will
experience the earliest sunset today.  The Sun will set at 4:04 p.m.
 Now, the sun set time will still be 4:04 p.m. until December 15th, on
which date the Sun will set at 4:05 p.m!   Granted, the onset of night will
occur in the afternoon for quite awhile.  However, the sun will not set any
earlier than it sets today.

[image: earliest-sunset-stephen-aman.jpg]
*Hey!  When is YOUR earliest sunset?   The answer depends on your
latitude.  Those residents in the south have already experienced
their earliest sunset.  Those of us dwelling in the Great White North (or,
a hair's breadth south of it) will experience the earliestsunset.    Note
for the curious:    the duration of daylight at the Equator is always 12
hours every day of the year.  *

Why isn't the earliest sunset date on the actual solstice?   Well, because
Earth is not a uniformly dense, perfectly spherical planet following  a
precisely circular orbit.     Instead, Earth is an oblate spheroid of
unequal mass distribution so that its shape is not perfectly spherical.
Sunlight strikes it unevenly.  Moreover, Earth moves along an elliptical
orbit so that its distance from the Sun veers from perihelion (point of
least distance) in early January to aphelion (point of greatest distance)
in early July.      All the while, Earth rotates uniformly on its axis.
 Because of this complex motion, the Sun doesn't always cross the due south
point -the meridian-precisely at noon every day.  In fact, the Sun only
passes due south at noon on four days a year, when the "Equation of Time"
equals zero.  (Please refer to the second graphic.)
[image: time-equation.png]

Now, if Earth were a perfect ball traveling around a perfect circle, the
Sun would reach the meridian at noon every day.    The earliest sunset would
occur on the winter solstice.  Yet, our exasperatingly complicated Universe
doesn't allow for such convenience.     The passage of the Sun across the
meridian occurs before noon this time of year.   But, as one will notice
from the above graphic, the equation of time equals zero by late December.
  Although the daylight duration continues to decrease slightly, the
sunset time
starts to increase by the second week of December simply because the
meridian passage is happening slightly later each day.

Realize that at the same time, the sunrises will continue to happen later
and later until January!      For us, the earliest sunset date occurs
before the winter solstice which happens before the latest sunrise.
Conversely, the earliest sunrise occurs before the summer solstice which
occurs before the latest sunset.

Those who are despairing at the sight of the afternoon darkness can derive
solace from the realization that the sunsets will start happening later
very soon.


To subscribe or unsubscribe from the "Daily Astronomer"
http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER