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: 2457644.16
"My powers are ordinary. Only my application brings me
success."
-Isaac Newton (1642-1727)
*THE DAILY ASTRONOMER*
*Monday, September 12, 2016*
*Night Sky Calendar: Week 2 - Sept 12 - 18, 2016*
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*ASTRONOMY CAFÉ: "THE TAMING OF THE TELESCOPE"*Monday, September 12,
2016 7:00 p.m. at the Southworth Planetarium
This month's Astronomy Café is all about telescopes.
Their usage, their purpose and how to get the most out of the telescope
you have at home. This café will also help you decide which telescope
to buy for the holidays.
Admission by donation
Call 207-780-4249, consult our web-site www.usm.maine.edu/planet
or contact [log in to unmask] for more information.
Ron Thompson, a telescope expert from the Southern Maine Astronomers,
will be delivering tonight's lecture.
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Once, and not too long ago, we posted monthly sky calendars. Now that
we're attempting to change the DA format, we decided to post weekly night
sky calendars, so as to allow us more time to devote to each event.
This coming week, Mercury will pass between the Sun and Earth for the
first time since its May transit and we'll see the famous harvest moon!
Also, and ominously, the Sun inches into Virgo the maiden, marking the last
throes of this masterpiece summer.
*MONDAY, SEPTEMBER 12, 2016: MERCURY AT INFERIOR CONJUNCTION*
Simply, a planet is in "inferior conjunction" when it is between Earth and
the Sun. Mercury and Venus, being the only two inferior planets (those
closer to the Sun than Earth), are the only planets that can ever be in
inferior conjunction, at least from Earth's perspective. Now, in case
that last sentence with its surfeit of "inferiors" didn't make sense,
simply know that only Mercury and Venus can ever move between Earth and the
Sun. During most inferior conjunctions, a planet will either be south of
north of the ecliptic, the plane connecting the Sun and Earth. (During
this inferior conjunction, Mercury will be 3.39 degrees south of the
Sun.) All planets are inclined relative to this plane by a certain
angle. Mercury's inclination is about 7 degrees, Venus's is about 3.4
degrees. Were these planets not inclined relative to the ecliptic, they
would always be directly between the Sun and Earth at each inferior
conjunction and we would see a "transit," the passage of a planet directly
across the Sun. Such transits can only happen when an inferior planet is
at or near a node, the intersection of the two orbital planes. On May
9, 2016, Mercury reached a node just seven hours prior to arriving at
inferior conjunction. Consequently, we saw a transit of Mercury.
Today's inferior conjunction is the first Mercurian inferior conjunction
since that transit.
*FRIDAY, SEPTEMBER 16, 2016: SUN ENTERS VIRGO*
Earth moves in a void. It spins on its axis and revolves around a
star. As our planet revolves, the Sun appears to migrate through
thirteen different constellations comprising a band known as the ecliptic,
or "zodiac." The Sun doesn't actually move in this manner any more than
it moves through the sky each day. The movement through the
constellations results from the planet's revolution motion, while Earth's
rotation causes the Sun's apparent diurnal motion. The graph below
shows how the Sun appears to occupy certain constellations at different
times of year as a consequence of Earth's orbit.
*[image: cid:image007.jpg@01D1F0BC.FA0478D0]*
*The Zodiac:** The Sun appears to move through thirteen constellations as
Earth revolves around it. This month, the Sun appears to migrate from Leo
the Lion into Virgo the Maiden. We can see that in September, Earth is
positioned on the far side of the Sun relative to the stars comprising
these constellations. [Note: this graphic does not include the '13th
constellation' Ophiuchus, which is poised between Scorpius and
Sagittarius.] Image credit: Addison Wesley Longman*
The Zodiac consists of VIRGO THE MAIDEN, Libra the Scales, Scorpius the
Scorpion, Ophiuchus the Serpent Charmer, Sagittarius the Archer,
Capricornus the Seagoat, Aquarius the Water Bearer, Pisces the Fish, Aries
the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab and Leo the
Lion. The Sun "enters" Libra the Scales on October 30.
*FRIDAY, SEPTEMBER 16, 2016: FULL MOON [HARVEST MOON!]*
The other term is "opposition," as the moon is on the far side of Earth
relative to the Sun. The moon and Sun therefore appear to occupy opposite
parts of the sky. This month's full moon is the famous "Harvest
Moon," defined as the full moon closest to the autumnal equinox. Harvest
moons most often occur in September, but they can sometimes fall in early
October if the October full moon date is closer to the equinox than
September's. The term "Harvest Moon" dates back to at least the early 18th
century, though its original source is uncertain. It was so named because
the full moon provides ample light throughout the night, much to the
delight and convenience of farmers harvesting their crops. Of all the full
moon names, the "Harvest Moon" is the most well known. Second, perhaps, is
the "Hunter's Moon," which always immediately follows the "Harvest Moon,"
and therefore will occur in either October or November. This year, the
Hunter's Moon occurs on October 16th.
*[image: cid:image011.jpg@01D1F0BC.FA0478D0]*
*Harvest moon:** Despite popular beliefs to the contrary, the Harvest
Moon is not always the largest moon and is not so* *called because it is
"orange." The moon in the above image [courtesy of Wikipedia.com] appears
orange because it is low on the horizon, where the abundance of atmospheric
dust scatters the blue light away from our sight line, which lends the moon
its crimson coloring. *
*FRIDAY, SEPTEMBER 16, 2016: PENUMBRAL LUNAR ECLIPSE*
The full moon passes through the outer part of Earth's shadow today. The
event will not be visible from the United States, but, honestly, there
isn't much to see with a penumbral lunar eclipse. The outer shadow,
called the "penumbra" is quite faint and the obscuration of a penumbral
lunar eclipse is subtle. However, any eclipse is noteworthy and so we'll
include it. The graphic below shows the moon's path during this
event. The moon passes through the southern penumbral region. At maximum
eclipse, it is entirely within the penumbra and is quite close to the
umbra. The dark shadow is only visible, Barnabas, when any part of the
moon enters the umbra.
*[image: cid:image014.png@01D1F0BC.FA0478D0]*
*Penumbral Eclipse path: **During this penumbral eclipse, the moon passes
through the southern region of the penumbra. Penumbral eclipses are not
visible as this outer shadow region only faintly obscures the full moon.
Image credit: Wikipedia.com*
*SUNDAY, SEPTEMBER 18, 2016: MOON AT PERIGEE* *(DISTANCE 56.74 EARTH
RADII)*
On September 6, the moon reached apogee, its point of greatest distance
from Earth (for this particular orbit.) Today, the moon reaches perigee,
its point of least distance. The moon will be approximately 361,490 km
(224,633 miles) from Earth at the moment of perigee. As we discussed
on Sept 6, the perigee moon appears larger than the apogee moon, but one
would be hard pressed to notice the difference because one can never see
both the perigee and apogee moon together in the sky. The perigee moon
does influence the tides, which are very distance sensitive. The perigee
moon and the full moon are only separated by two days this month, so the
tides will currently be somewhat higher than usual.
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FROM THE CATACOMBS OF INFINITE KNOWLEDGE:
*The Great Globular Cluster in Hercules (M13)*
*[image: cid:image007.jpg@01D20B94.730EA290]*
*The Hercules Globular Cluster*
*A beautiful and star-rich globular cluster within the Hercules Keystone.
The Hercules Cluster contains more than 300,000 stars within a volume of*
*approximately 1.6 million cubic light years. This stellar density
averages*
*0.4 stars per cubic parsec:* much higher than the 0.14 stars per cubic
parsec density within*
*the solar system's region of the Milky Way Galaxy. This density is
greatest*
*around the globular cluster core and diminishes with increasing distance
from the core.*
*Image: Bareket Observatory, Israel*
We continue our Messier tour today with a swift fly by of the Great
Globular Cluster in Hercules. This magnificent object is designated M13,
meaning that it was the 13th entry in Charles Messier's famous
catalog. Our first stop on this year long Messier odyssey was at M1.
However, we are not going to visit the Messiers in numerical order. As
any adamant sky watcher will attest, certain Messiers are best seen at
specific times. As we proceed through the catalog this school year,
we'll focus on the Messiers that are best seen at the given time.
*M13: The cluster in the keystone**One can locate the Hercules Globular
Cluster within the Hercules'"Keystone."*
*a quadrilaterial asterism representing Hercules' torso. Even though
M13 is one of the*
*sky's brightest globular clusters, it is barely visible to the unaided in
the darkest sky.*
With a stellar population exceeding 300,000 the Hercules Globular Cluster
is one of the galaxy's richest globular clusters. Tucked inside the
Hercules' "Keystone" asterism, this cluster was the thirteenth entry in
Charles Messier's catalog, even though Edmond Halley (of comet fame)
discovered it fifty years earlier. Globular clusters are among the
Universe's oldest objects. With an estimated age of 11.65 billion
years, M13 formed relatively soon after the Big Bang. Like the other
globulars, the Hercules Cluster describes a wide orbit around the galactic
nucleus: an orbit it completes after millions of years. Despite the tidal
stresses the galactic plane induces on these clusters when they pass
through it, the clusters maintain their structures, hence their advanced
age. Galactic clusters (also called "open clusters") dissipate
relatively quickly, as they contain fewer stars than globulars and
consequently their gravitational adhesion is more tenuous.
Tonight, one can observe Hercules high in the evening sky. Though its
namesake is arguably mythology's most luminous character, the constellation
representing him is suprisingly faint. To find Hercules and its
keystone, look to the west of Vega, the Summer Triangle's brightest star.
M13 is hidden just to the south of the star eta Herculis. Some romantic
astronomers claim that M13 is just below Hercules' armpit.
While dozens of globulars are readily visible in telescopes, the Hercules
Cluster is the northern hemisphere's brightest example!
*Parsec: One parsec equals 3.26 light years. A cubic parsec is equal to
about 34 cubic light years.
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