THE SOUTHWORTH PLANETARIUM
207-780-4249      www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine 04103
43.6667° N                   70.2667° W
Founded January 1970
Julian date:  2457739.16



THE DAILY ASTRONOMER
Monday, December 19, 2016
Week 16  Night Sky Calendar
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Happy belated birthday to my favorite (only) brother,
Richard!    Even though you're six light years to the south
of us, we hope your winter still hurts!   Happy Birthday!!
_________________________________________

Despite the misgivings of our citizens and the angry protests staged around
the country, winter is about to commence.     We are resolved to make the
best of this situation, however, and will find in every bone-cracking,
blood-congealing, eye-fluid-freezing day of reason to rejoice.    Of
course, the winter's brilliant constellations offer warmth and joy amidst
the season's assailing tempests.

Recently, we decided to focus on a different constellation at the beginning
of each weekly night sky calendar. This week we're going to do something
slightly different.  We'll train our spotlight on the Winter Hexagon.  This
"hexagon" consists of seven -some say eight- stars taken from six different
constellations.     Throughout early winter, we will proceed clockwise
along the Winter Hexagon and discuss each constellation in turn:

These stars and their associated constellations are as follows:

Capella in Auriga the Charioteer
Aldebaran in Taurus the Bull
Rigel in Orion the Hunter
Sirius in Canis Major
Procyon in Canis Minor
Castor/Pollux in Gemini the Twins
and, Betelgeuse in Orion the Hunter.
(Although Betelgeuse is not on the actual hexagon perimeter, it is often
considered part of the Winter Hexagon because of its central location.

Our Winter Hexagon tour will commence on January 2nd with Auriga!

Winter Hexagon Rising!      The Winter Hexagon rises in the eastern sky
this evening.  By 9:00 p.m. the entire star pattern looms high over the
eastern horizon.    ​

Apart from the winter solstice, this week's calendar includes a
'stationary' Mercury. a quarter moon, the underestimated Ursid meteors and,
amazingly, the equation of time zeroes out again.

*MONDAY, DECEMBER 19:  MERCURY STATIONARY*
If you were an astronomer and charged with tracking the planets, you would
need to observe the planets every clear night and note their positions
relative to the stars.  You would notice that the planets tend to move
eastward relative to the stars.    At times, however, this eastward
migration would stop and then the planet would move westward (retrograde)
against the stars.  Eventually the planet would appear to stop again before
resuming prograde motion.    When the planet reaches one of these
"stopping" points, it is said to be 'stationary.'  A planet never actually
halts in its orbit.  Instead, it appears to do so because we are moving
relative to the planet.    Mercury is an inferior planet, meaning that it
is closer to the Sun than Earth.  We're watching this little world move
around its orbit from an external perspective.   When Mercury reaches
either "end point" on this orbit, it appears stationary.    Similarly, a
car driving around a track will either appear to move to our left or our
right depending on which side of the track it occupies.


*WEDNESDAY, DECEMBER 21:  LAST QUARTER MOON*


*WEDNESDAY. DECEMBER 21:  WINTER SOLSTICE*

*THURSDAY, DECEMBER 22: URSID METEORS*

*THURSDAY, DECEMBER 22: MOON 2.3 DEGREEES NNE OF JUPITER*

*SATURDAY, DECEMBER 24: EQUATION OF TIME IS 0*
The Equation of Time might seem too esoteric a topic for polite company,
but, to us, it is quite an interesting concept because it demonstrates that
one will find complexity even in the simplest places.     We know that the
Sun appears to rise somewhere over the eastern horizon and sets somewhere
along the western horizon.     This apparent motion is illusory, because
the only thing moving is our own planet.   Earth's rotation merely makes
the Sun seem as though it soars across the sky, an illusion so convincing
that even astronomy types still employ Ptolemaic words such as "sunrise"
and "sunset."    Once a day, the Sun will reach its highest point above the
southern horizon during a moment called "upper culmination."   The Sun is
at this culmination when it crosses the meridian, the imaginary arc
connecting due south and due north.     Although many believe the Sun
reaches the meridian at noon, most of the time it doesn't.   It either lags
behind or is slightly ahead of the meridian at noon.  The "equation of
time" indicates the time difference between civil noon and upper
culmination, otherwise known as "true noon."   Today, the Sun crosses the
meridian precisely at noon time.    The graph below shows the approximate
equation of time throughout the year.



[image: cid:image002.jpg@01D1EFDE.8C95F0F0]

*Equation of time graph:**     The red curve indicates the 'equation of
time' throughout the year.   When the red line is below the mid-line
(0.00), the Sun has not yet reached the meridian at noon.  When the red
line is above the mid-line (0.00), the Sun reached the meridian before
noon.  When the red line is on the mid-line, the equation of time is 0.
For instance, on Feb 11, the equation of time is at a local minimum of
approximately 14 minutes.  At noon on Feb 11, the Sun was east of
the meridian.    Upper culmination actually occurred on that date at 12:14
p.m.      On December 24, the equation of time is zero. The Sun will cross
the meridian at noon. *



*SUNDAY, DECEMBER 25: MERCURY AT PERIHELION*
One must wonder why we bother to include this event in a calendar
specifically designed for observational astronomy.    We're not sure,
ourselves, although this event affords us an opportunity to discuss that
most scintillating of all astronomical topics.  "Elliptical planetary
orbits."   Planets move along elliptical orbits, which one can envision as
being like ovals.     Were a planet's orbit circular, its distance from the
Sun would remain constant.   As the planets in our solar system  travel
along ellipses, their distances from the Sun are constantly changing.
 They veer from a maximum distance (aphelion) to a minimum distance (
perihelion.)    Today, Mercury reaches perihelion and will be 28.6 million
miles (46 million km) from the Sun.




*PLANET WATCH:*

*MERCURY (Host constellations:   Sagittarius-Ophiuchus)*
The first world peeks up in the western evening sky.  This apparition
favors southern observers, but Mercury remains visible low in the southwest
for northern sky watchers.  Mercury will be at greatest eastern elongation
on December 11th, but at inferior conjunction on December 28th.     Mercury
will vanish around the third week of December.   VERDICT:   We would advise
you to venture out during the first half of December to find Mercury.
The elusive world vanishes late this month.

*VENUS (Host constellation:  Sagittarius)   PICK PLANET!*
People wonder why Venus is such a prima donna when she receives the coveted
"pick planet' diadem almost every month.    Venus enjoys the advantage of
being the brightest planet, so whenever it appears it will invariably
outshine it planetary cohorts.   This month Venus shines brilliantly as the
western evening sky star.   VERDICT:   Easy to find if you're outside in
the early evening.  Look west and just behold the spectacle.

*MARS  (Host constellation:   Capricornus - Aquarius)*
Here's an interesting observational exercise:   observe Mars in the western
evening sky throughout December.  One will notice that Mars is setting
around the same time each night.   Mars has been setting around 9:30 p.m.
each night this autumn because Earth and the red planet are moving in such
a way as to make their positions relative to each other nearly constant.
VERDICT:    Not only can you see the red planet, you can almost set your
watch by it.  What's not to love?

*JUPITER  (Host constellation:   Virgo)*
Rising earlier each morning, Jupiter is the only pre-dawn planet visible to
the naked eye.   The fifth sphere serves as a the brilliant eastern beacon
one can easily see provided one ventures outside a couple of hours before
sunrise.    VERDICT:   The perfect planet for joggers, dog walkers, and
those accursed few who always arise with alacrity before the alarm blares,

*SATURN  (Host constellation :   Ophiuchus)*
We're sorry to say that we have to bid Saturn adieu, at least for awhile.
The ringed world vanishes into the dusk, destined to return to the pre-dawn
sky by late December.   Saturn is in solar conjunction on December
11th. VERDICT:
 Don't bother to seek out Saturn until just before the New Year's
celebrations.

© 2016  Edward Gleason