THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland,Maine 04103
(207) 780-4249      usm.maine.edu/planet
43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded
January 1970
2021-2022: XV
"My life has a superb cast, but I cannot figure out the plot."
-Ashleigh Brilliant

THE DAILY ASTRONOMER
Wednesday, September 22, 2021
A Sun at Both Poles
and more equinox talk


Three hours and 21 minutes from the time of this posting, astronomical
autumn will begin in the northern hemisphere. We are already observing the
telltale signs of the impending fall, most notably the vibrant hues of
crimson and tangerine spreading among the once uniformly green forest
canopies. We derive solace from the evergreens which, being the dominion of
heartier dryads, obstinately refuse to surrender their verdant needles to
the approaching chill and remain green even through deepest winter. Sigh.
It all sounds so familiar. Astronomical summer is drawing to a close. We've
been through this melancholy coda before and will most assuredly experience
it again.

The entire seasonal transition results simply from a tilted planet's
revolutionary motion around its parent star. Although the star produces
prodigious amounts of energy without any let-up, its angle relative to
various points along the planet's surface varies considerably as the
planet's orientation continually shifts.

[image: equinoxes-and-solstice.png]
As we proceed through autumn, the Sun''s altitude will become progressively
lower until it reaches its nadir on the winter solstice (December 21st.)
The Sun's altitude then starts slowly increasing until it reaches the apex
on the summer solstice (June 21st.)

We all know of this transition. We've been through it before. The aim of
this article is to explain what is happening around the world on this very
day.

*The Sun is visible at both poles*
If Earth contained no atmosphere, the sky would become dark the moment the
Sun set. The atmospheric light scattering is responsible for the slowly
fading illumination that follows the Sun's departure. It also causes
another effect called atmospheric refraction. This refraction projects
above the horizon images of celestial objects that have just set. Along the
horizon, this refraction amounts to half a degree, equal to the Sun's
angular diameter. Presently, the Sun is riding along the horizon at both
the north and south poles. Within the next week, the Sun will set at the
north pole while its gains a bit of height at the south pole. During a few
days around each equinox, one can find the Sun circling the horizon at both
the top and bottom of the world.

*The Sun is passing overhead along the equator.*
Wonderful fact to remember. The Sun is ALWAYS overhead somewhere on Earth.
This zenith Sun position constantly changes because Earth is rotating and
the Sun's altitude gradually changes. Observers along the equator will see
the Sun pass directly overhead today and on the vernal equinox (first day
of Spring.) On the December solstice, the Sun passes directly overhead
along the Tropic of Capricorn. On the June solstice, observers along the
Tropic of Cancer will see the sun at the zenith.

*You can balance eggs on their ends.*
Of course, you can do that on any day of the year! The ability to make eggs
behave in this unnatural way has nothing to do with the seasons and
everything to do with the patience level of the person holding the eggs.

*The Sun rises due east and sets due west.*
The statement that the "Sun rises in the east and sets in the west" isn't
entirely accurate. Most of the year, the Sun rises either north of east (in
the spring and summer) or south of east (in the fall and winter.) The Sun
rises due east and sets due west only on the vernal and autumnal equinoxes.

*We will not experience twelve hours of daylight today.*
Yes, we know the word "equinox" literally means "equal night." If Earth
were a perfectly spherical body traveling around a perfectly circular
orbit, we would have 12 hours of daylight on each equinox. However, we're
on a slightly pear-shaped planet that moves along an elliptical orbit.
Consequently, we in the mid northern latitudes will experience 12 hours of
daylight on September 25.


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