THE SOUTHWORTH PLANETARIUM
70 Falmouth Street  Portland, Maine 04103
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43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded
January 1970
2021-2022: VI
"We don't stop playing because we grow old. We grow old because we stop
playing."
-George Bernard Shaw

THE DAILY ASTRONOMER
Thursday, September 9, 2021
Restless Sky

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Happy 23rd Birthday, Infernally Adorable Young Adult!
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The one lesson astronomy teaches us all: nothing remains the same.
Everything changes, sometimes rapidly, other times slowly. Examine the sky
on successive nights. Although the constellations and planets appear to be
unchanged and to occupy more or less the same positions relative to the
setting Sun, alterations have occurred during the intervening time period
so that each night sky is unique.
[image: 939a083138a8238e1fff3e8ed1cc5409.jpg]
The moon experiences the greatest shift, for it moves half a degree an
hour, or twelve degrees a day. As its location changes, so, too, will its
illumination percentage. For instance, the moon was new on September 6th
and is currently progressing through the waxing crescent phase. One can see
it low in the western early evening sky. Each night it appears to grow
larger as its illuminated face turns slowly toward back toward us. Every
29.5 days, the moon completes another phase cycle as it goes from western
evening sky re-birth to gradual demise in the pre-dawn east.
[image:
vintage_sun_face_classic_round_sticker-r1bc6fa44a1f04778931cf305b269eee3_0ugmm_8byvr_704.webp]
Other alterations are not as readily noticeable. During a 24-hour period,
the Sun moves along its ecliptic path by approximately one degree.
Meanwhile, Earth's orientation relative to the Sun nudges slightly either
up or down depending on the hemisphere. Presently, the Sun's diurnal
(daily) path through the sky is becoming both shorter and lower and shall
continue to do so until the winter solstice. The combined motions change
the sunrise/sunset times by just 1 - 2 minutes. These are the alterations
we tend to only notice over many weeks.

[image: Planets2013.svg.png]
Planets, follow their own path through the stars, hence the word "planet,"
which derives from 'planetes,' meaning "wanderer."     Planetary motions
are not created equal:  the inferior planets, Mercury and Venus, dart back
and forth between the western evening and eastern morning skies at a
frenetic pace by virtue of their close proximity to the Sun.   The outer
planets meander through the firmament more gradually.    Old man Saturn
requires nearly thirty years to complete one orbit and so trudges
lethargically among the stars.  Yet, even that "trudge" amounts to slightly
more than 21,000 miles per hour.

[image: 2940651780_0cd2e7edb1_o.jpg]
Some changes are so subtle as to pass unnoticed even over the course of a
human lifetime.  Presently, Polaris serves as our north star as it is
almost precisely aligned with Earth's north celestial pole.   However,
Earth's precessional wobble, induced primarily by the gravitational
influences of the Sun and moon, will draw the north celestial pole away
from Polaris and toward other stars.     In 2,000 years, Errai, a star in
Cepheus the King, will sit at the night sky's apex.   Thirteen thousand
years from now, the brilliant star Vega, the brightest star in the Summer
Triangle, shall become our north star.      Yet, during our lifetime and
the lifetimes of our closest descendants, Polaris shall remain the axis
about which the stars appear to turn.

[image: 272B773900000578-0-image-a-16_1427819178050.jpg]
 On far larger timescales, even the constellations will dissipate like
cloud forms owing to proper stellar motions:  the independent movements of
stars within the galaxy.   Stellar motion is rapid!   The Sun races through
the galaxy at 143 miles per second and the surrounding stars travel at
comparable speeds.      The stars are so remote, however, that they
scarcely seem to move a micrometer over many centuries.     Even the
Stonehenge builders gazed upon the same star patterns that adorn our modern
sky.  However, over tens of thousands of years, the constellations will
become distorted due to these proper motions.   Notice in the above graphic
that Orion's belt will more or less maintain its shape while the
constellation's other features become bent and contorted.   (This graphic
does not take into account the distinct possibility that Betelgeuse,
Orion's eastern shoulder star, could go supernova within the next 100,000
years....or even in the next few minutes.)

We derive great comfort from the constellations.  Some are circumpolar and
so always remain in our sky. Others dissolve into the dusk at the same
times each year only to return to the pre-dawn sky a couple of months
later.    Throughout our lives, we can expect these transitions to occur
and recur.   We should bear in mind, however, that change is the only
constant, even if that change is beyond our perception.    The philosopher
Heraclitus explained that we never step in the same river twice.   If only
he knew that the night sky we behold tonight will never again be precisely
the same.



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