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: 2459186.16
2020-2021:  LII


THE DAILY ASTRONOMER
Wednesday, December 2, 2020
December 2020 Night Sky Calendar  Part III

Although winter's arrival might cause us some distress, we must admit that
few of nature's vistas are as enchantingly beautiful as the winter night
sky.   First,  one can easily see Orion and the array of constellations
surrounding him throughout most of the night.   The brightest stars within
these constellations -Aldebaran in Taurus; Capella in Auriga; Procyon in
Canis Minor; Sirius in Canis Major; and Rigel in Orion- form the brilliant
and immense star pattern called the *Winter Hexagon.    *

[image: f531fa1b599a031aee8879df70b17a64.jpg]

Secondly, the stars are generally easier to observe because there is less
radiational cooling during the night due to the colder daytime
temperatures.  When the weather is warm, the residual heat rises off the
ground into the air.  Such heat transference induces a great deal of
turbulence into the atmosphere that can obscure viewing.

So, while some people might gripe and carp about winter's return, we assert
that the coldest season has much to recommend it...at least at night.

*THURSDAY, DECEMBER 24:  MOON AT APOGEE*
The moon does not travel along a perfectly circular orbit.    Instead -as
is true with all the planets- the moon's revolutionary path is elliptical.
  Were its orbit circular, the moon's distance from Earth would remain
constant.   However, this distance constantly changes as the moon moves
along this slightly elongated orbit.      Today, the moon reaches the most
distant orbital point, called "apogee."    At this time, the moon will be
251,106 miles (405,010 km) from Earth.     This value isn't much greater
than its distance at perigee on December 12th:  224,301 miles (361,777
kilometers).    In fact, the moon's orbit is nearly circular.  Were its
elliptical orbit much more elongated, the difference between its perigee
and apogee distance would be far greater.

[image: lunarorbit_graph2.gif]
The above image shows a realistic depiction of the moon's orbit.   The
dotted line represents a perfect circle.  We can see that the moon's path
only slightly deviates from circularity.

One won't notice much of a difference in the moon's size tonight.  Consider
this event to be a purely academic one.

*FRIDAY, DECEMBER 25:    NORTHERN CROSS ON THE WESTERN HORIZON*

[image: northern-cross-november-bob-king.jpg]
Although it is one of the Summer Triangle constellations, Cygnus the Swan
(the Northern Cross) remains visible early this evening.   This cross
consists of a long, straight axis intersected by a slightly crooked shorter
line.    The longer line represents the swan's body.   Deneb, the
constellation's brightest star, marks the swan's tail.   Albireo, a
magnitude three star at the longer axis' opposite side, forms the head.
   The shorter axis represents its wings and contains no prominent star
apart from Gamma (Sadr), the "breast star" that marks the intersection
point between the two star lines.

Mythologically, Cygnus is the swan into which Zeus transformed himself
before he seduced Queen Leda, wife of Tyndareus.     Within one night Leda
had relations with both her husband and Zeus (in swan form).
 Consequently, she gave birth to four children. Two of the children -Castor
and Clytemnestra- were sired by Tyndareus.  The other two -Helen and
Pollux- were Zeus' children.     Castor and Pollux, the Gemini twins -or
Dioscuri- matured to become powerful and renown warriors.   Pollux enjoyed
the advantage of immortality by virtue of his lineage.  When Castor was
slain in battle, Pollux offered to relinquish his immortality and give it
to his fallen brother.   Although he declined this request, Zeus was so
moved by his selfless gesture that he placed the twins in the night sky as
the constellation Gemini.    Helen's abduction by Trojan prince Paris
precipitated the Trojan War.    Clytemnestra married Agamemnon, the King of
Mycenae. and brother of Menelaus, Helen's husband.    Prevailing winds
prevented Agamemnon from launching his ships to Troy to participate in the
war.   Having learned that moon goddess Artemis cast these winds to punish
him for slaying a hind sacred to her, Agamemnon was told that he had to
sacrifice his daughter Iphegenia in order to calm the winds.     Most
reluctantly, he performed the sacrifice and allowed the ships to sail for
Troy.  When he returned from the war ten years later, his Clytemnestra and
her lover Aegithus.

Astronomically, the Northern Cross' longer axis runs along the Milky Way, a
luminescent band extending between both horizons.  This luminous arc is
formed by the collective light of myriad stars aligned along the galactic
plane.  As the above image is a time exposure photograph, the Milky Way's
brightness is exaggerated.

*TUESDAY, DECEMBER 29:  FULL MOON*
Every full moon has a special name.  Actually, any full moon will have a
variety of sobriquets. December's full moon is known as the "Cold Moon,"
"Hoar Frost Moon." "Snow Moon," "Winter Maker Moon," and the "Long Night
Moon."
All but the final name refers to the season.   Winter, as we all know, will
begin this month.      The "Long Night Moon" refers to the moon's angle.
 The full moon that occurs closest to the winter solstice will follow a
higher path than any of the year's other full moons.  For this reason, it
remains in the sky longer than any other full moon.

For instance:  on December 29th, the moon will rise at 3:46 p.m. and will
set at 6:53 a.m. on December 30th.  The moon will remain above the horizon
for 15 hours and 7 minutes
The June 5th full moon rose at 8:24 p.m. and set at 4:53 a.m. the following
morning.  That moon spent 8 hours and 19 minutes above the horizon.

A high-flying full moon is yet another reason to experience the gorgeous
winter night sky.


*PLANET WATCH*

*MERCURY*
*Host constellations: Scorpius; Libra; Ophiuchus; Sagittarius*
Planet One is barely visible early in the predawn eastern sky during the
first three days of December.    After this apparition, Mercury will appear
too close to the Sun to be visible or the rest of the month.   VERDICT:
Venture outside on 12/3  around 6:20 - 6:40 a.m. and  you MIGHT see Mercury
quite low above the eastern horizon.    As the Sun rises at 6:57 a.m. on
that date, Mercury will quickly vanish into the dawn light.     Try again,
perhaps, later this week, but by the weekend, Mercury will be out of sight
until next year.

*VENUS      PICK PLANET!! *
*Host constellations:   Libra; Scorpius; Ophiuchus*
Except for Mercury during the earliest part of December, Venus will be the
only planet visible in the eastern morning sky.  As it is always brighter
than any other world (save the Moon), Venus is often our pick planet.
One will find it easily after 5:00 a.m.    Venus resembles a dazzlingly
bright star that will slowly fade out of sight as the dawn light
intensifies.    VERDICT:  If you're an early riser, you will want to brave
the late autumn/early winter chill and seek out Venus.  Our sister world
will remain the 'morning star' throughout most of the winter, as it is due
to move into inferior conjunction on March 26, 2021.
*HIGHLIGHT:   Venus will appear extremely close to the moon on December
`12th. (See star chart below.)  *

[image: skychart.png]


*MARSHost constellation:  Pisces*
Though not nearly as bright this month as it was when it reached opposition
in mid-October, Mars remains a conspicuously bright planet through the
month.  At month's beginning, Mars (magnitude -1.0) will outshine all the
night sky stars except Sirius.   At month's end Mars will shine at
magnitude -0.2, making it slightly brighter than Arcturus, the night sky's
fifth brightest star.   Mars begins the night high in the eastern sky.   At
the start of December, the fourth world will set around 2:20 a.m.   At
month's end, it will set by 1:30 a.m.    VERDICT:  Perfectly placed for
skywatchers who prefer early evening to early morning.  Even though its
brightness will diminish throughout December, Mars is still easy to find
provided one looks into the eastern sky soon after darkness descends.
*HIGHLIGHT:  Mars will be six degrees north of the moon on December 23rd.*

*JUPITER*
*Host constellations:   Sagittarius (first half of December); Capricornus
(second half) *
Jupiter is brighter than any planet, save Venus.     One will find it well
over in the western sky after dusk.      Jupiter will set before 7:30 p.m.
at the beginning of December and by 6:00 p.m at month's end.  VERDICT:
Venture out early to find Jupiter.    Look over into the western sky during
the early evening anytime this month.    As Jupiter will move into superior
conjunction on January 28, 2021, it will draw ever closer to the Sun and by
early January will vanish.   Catch it while you can!
*HIGHLIGHT # 1:   See the moon, Jupiter and Saturn close together on
December 17th   (This will be about the time Jupiter shifts from
Sagittarius into Capricornus.) *

[image: skychart (1).png]

*HIGHLIGHT # 2:   Jupiter will be in conjunction with Saturn. At closest
approach, the two planets will be within 6' of each other.  The closest
Jupiter-Saturn conjunction since 1623!*

*SATURN*
*Host constellations:  *
*Sagittarius (first half of December); Capricornus (second half)  *
Poor Saturn will always play second fiddle to Jupiter.  This secondary role
is particularly evident now that the two planets are so close together in
the western early evening sky.    At magnitude 0.6, Saturn will be about
nine times dimmer than Jupiter throughout the month.    VERDICT:   If
you're out to find Jupiter, anyway, might as well seek out Saturn.   Saturn
will appear to draw ever closer to Jupiter during the first three weeks of
December.
*HIGHLIGHT:  Saturn will be in conjunction with Jupiter on December 21st.
At closest approach, the two worlds will be separated by 6'.   The closest
conjunction since 1623!  (Yes, we know you read about that already.)  *


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