THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

Altitude:  10 feet below sea level

Founded January 1970
Julian Date: 2458820.16

                "Heavens above!"

THE DAILY ASTRONOMER

Monday, December 2, 2019

December 2019 Night Sky Calendar   Part I

What's happening in December!

Well, what ISN'T happening.  This month, astronomical winter begins, the Geminid meteor shower peaks, we bid adieu temporarily to Jupiter, the moon swings by three planets, Venus appears to nudge close to Saturn, the Long Night Moon describes a wide arc over our crystallines snowscapes, the Sun will start to set later each night and one never knows if an aurora borealis will bedazzle our skies.   

THURSDAY, DECEMBER 4:  FIRST QUARTER MOON

"First Quarter" is a curious term applied to this lunar phase.  It implies that we see only one quarter of the moon's "facing area,"  the region of the moon directed toward Earth.   Instead, we see half of the "facing area" at first quarter.    The term refers to the first quarter of the moon's orbit.  After a new lunation cycle begins at new moon, or conjunction, the moon must complete one quarter of its orbit to attain the first quarter (or quadrature) position.    At this time, we see it as a "half moon."  

FRIDAY, DECEMBER 5:  MOON AT APOGEE

Yes, we know what Ptolemy said.  In his elaborately designed, brilliantly conceived, yet utterly incorrect model, the planets and the moon all revolve around Earth along perfectly circular orbits.  He derived his notion of circular orbits from the Greek idea that the sky was an exquisitely beautiful and timeless realm in which all the empyreal orbs described perfect orbits.  Even though Copernicus properly placed the Sun in the solar system center, he retained the circular orbits because a. they seemed to move that way and b. he was a political pragmatist who realized that kicking the hornet's nest twice was slightly more problematic than doing it once.  

Kepler, who actually coined the term "orbit," was the first to accurately describe the planetary and lunar paths as elliptical, meaning that their distance from the parent body wasn't constant, but changed constantly: veering from a minimum to a maximum.   The moon's minimum distance is called "perigee," while the maximum is known as "apogee."    Today, the moon reaches "apogee," and will be 404,446 kilometers from Earth at the precise apogee moment.   As the moon is at such a far distance around the time of first quarter, we can expect tides to be lower.  Apart from this effect, nothing unusual should occur as a consequence of this lunar apogee.

TUESDAY, DECEMBER 10:  MOON NEAR PLEIADES

One will barely see the Seven Sisters "close" to the nearly full moon (97% illuminated) tonight.   The Pleiades Star Cluster is the night sky's brightest (but not the closest) star cluster.*

As it appears so luminous, this cluster will remain visible despite the gibbous moon's close proximity.     Now that we're veering precariously close to astronomical winter, the Pleiades Star Cluster will remain visible all night long.   Those who experience difficulty in locating it, should look to the moon tonight as a guide to finding it.

THURSDAY, DECEMBER 11: VENUS 1.8 DEGREES SOUTH OF SATURN (BRONZE EVENT!)

One will find the brightest planet and the most distant naked eye planet apparently close together in the western evening sky tonight.    Distinguishing between these two worlds will prove quite easy, as Venus (magnitude -4) will be 70 times brighter than Saturn (magnitude 0.6)

Both planets will set just after 6 p.m.

FRIDAY, DECEMBER 12: FULL MOON

The long night moon, so named because the nights are longest this time of year.  Also, the full moon close to the winter solstice follows the longest path of any full moon during the year.  When the moon is full around the winter solstice, it moves along an arc the Sun describes on the summer solstice. Conversely, when the moon is full around the summer solstice, it moves along the sun's winter solstice arc.    As the Sun remains above the horizon for more than 15 hours on the summer solstice, the winter's first full moon will also be visible for the same amount of time.

*The closest is the Hyades Star Cluster at a mean distance of 153 light years.  The Pleiades Star Cluster's mean distance is 445 light years.

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