THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.
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."
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.
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.
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.
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)
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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