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:  2457651.16
               "Jeanius at werk."

*THE DAILY ASTRONOMER*
*Monday, September 19, 2016*
*Night Sky Calendar: Week 3*
*September 19 - 25, 2016*

We do hope these weekly sky calendars have proven quite helpful.   We
switched from the monthly to weekly format so as to provide more detail for
each entry.  Please let us know your thoughts.    A subscriber did ask us
to bring back the "Planet Watch" section, which returns today   The Planet
Watch will still be monthly, but we'll include it at the end of each weekly
sky calendar.  Also, at the subscriber's behest, we'll continue to select a
pick planet.


WEDNESDAY, SEPTEMBER 21, 2016:    MOON 9.2 DEGREES SOUTH OF THE PLEIADES
The waning gibbous moon, being only 70% illuminated tonight, will not
wholly obscure the Pleiades Star Cluster, the open star cluster just to the
west of Taurus the Bull.    This event afford us the opportunity to explore
our next Messier object.  (Recall that this school year we're going to
"visit" every Messier object.  This is our way to pay tribute to Charles
Messier on the 200th anniversary of his death - April 12, 1817)


*Pleaides Star Cluster.  ** "The Seven Sisters." An open star*
*cluster within Taurus the Bull.    Designated M45, this cluster*
*is one of the few naked eye Messier objects.  To the unaided eye,*
*it appears like a faint cloud.  Viewing it through a telescope*
*or even binoculars reveals its richness.    IMAGE:  Hubble Space*
*Telescope*

The Pleiades is also known as the "Seven Sisters," because an observer can
observe about seven individual stars.     Through a telescope, one can
count a few hundred member stars.  Approximately one thousand stars have
been shown to be members of a cluster, the core of which extends over eight
light years.     This cluster formed about 120 million years ago and,
according to computer simulations, will slowly dissipate over 250 million
more years.  At that time, most of the component stars will have
established their own trajectories through the galaxy.      In the above
photo, we can observe rarefied gases enveloping the stars.      These gases
comprise the "Maia Nebula," so named as it was thought to have been the
remnant of the nebula from which these stars took  form.     Astronomers
have since determined that this nebula is not associated with the cluster's
birth.  Instead, the Pleiades Star Cluster is currently passing through the
cloud.   The starlight illuminates the cloud, producing a glow reminiscent
of the luster fireflies impart onto a fog bank.

As we proceed through autumn and then winter, the Pleiades will rise
earlier each night and will become ever more prominent.

WEDNESDAY, SEPTEMBER 21:  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.

THURSDAY, SEPTEMBER 22, 2016: AUTUMNAL EQUINOX!
We're passing from one astronomical season to the next.  We have seasons
because Earth is tilted on its axis by 23.5 degrees, a value known as the
"obliquity."    Our summer solstice occurs when Earth's northern hemisphere
is aligned toward the Sun as much as possible.    When Earth's southern
hemisphere is poised toward the Sun, our winter begins.    At either
equinox, neither pole is aligned toward the Sun more than the other.    The
autumnal equinox occurs after summer, but before winter. Interesting note:
 Earth reaches aphelion -its greatest distance from the Sun- in early July,
so our planet is farthest from its parent body in our summer. Consequently,
it's traveling at its slowest orbital speed between the summer solstice and
autumnal equinox.   Therefore, in the northern hemisphere, summer is the
longest season.  Of course, it still has to come crashing to an end.
 Autumn begins with a whimper at 10:21 a.m. EDT!

FRIDAY, SEPTEMBER  23, 2016:   LAST QUARTER MOON


PLANET WATCH

MERCURY:  becomes visible in the pre-dawn sky by the third week of
September.   It is elusive, as always, but by the end of September grows
brighter than Saturn and Mars.   VERDICT:   Wait until the very end of
September to see Mercury in the early morning eastern sky.

VENUS (PICK PLANET!)  Venus maintains a constant brightness throughout
September, but, of course, remains brighter than all the other planets and
all the night sky stars.    Even though Venus sets during the very last
twilight phase this month, it is still a brilliant western sky object and
easy to find if you have a low western horizon.
VERDICT: If you're out at dusk, seek out Venus all month!

MARS:   Mars had its time in the limelight in May when it reached
opposition.  Although it remains in the western evening sky for the rest of
the year, it will diminish sharply in brightness through autumn.
 VERDICT:     See Mars in the southwestern early evening sky each night
this month.       As autumn ages, Mars will grow dimmer..

JUPITER:  Not visible this month!    This bloated gas giant returns to the
pre-dawn eastern sky by mid October.    VERDICT:  Well.....please wait
until late next month.

SATURN:   Is dimmer than Venus, Mars and, by late September, will also be
dimmer than Mercury.  This planet will vanish into the dusk by November.
Now, it rises about four minutes earlier each night.   VERDICT:   Find
Saturn in the early evening this month.   It will become much harder to
find in October as it approaches its hiatus.