Sunday Brings a Pre-Christmas Supermoon, Andromeda loves Perseus among the Stars, and the Geminid Meteors Germinate!
A Geminid Meteor Shower Early Heads Up
The Geminid Meteor Shower, one of the most spectacular of the year, kicks off on December 4. It won’t peak until next Thursday, December 14, but you can keep an eye out for a few, especially later this week when the moon has waned. Geminid meteors are commonly bright and intensely colored, and slower moving than average.
Andromeda’s Family — a Story in the Stars
The late fall and winter evening sky features a group of easy-to-see constellation that are the characters in a grand story from Greek mythology — Princess Andromeda and the hero Perseus. Over the next few Skylights, I’ll tour the individual constellations — telling you how to find them, and highlight some the best sights in binoculars and telescopes. Once upon a time…
In Greek mythology, Cassiopeia was the queen of Ethiopia, and married to King Cepheus. When Cassiopeia boasted of her daughter Andromeda’s unrivalled beauty, the sea nymphs took issue with this and asked Poseidon to unleash the fury of Cetus the Sea-monster on Ethiopia’s coast. To appease Poseidon, Cepheus offered to chain Andromeda to the seashore, where Cetus the Sea-monster would take her and return to the sea. Just in the nick of time, the hero Perseus, fresh from defeating the Gorgon Medusa and still carrying her head in a bag, flew to Andromeda’s rescue on the winged horse Pegasus. The couple lived happily ever after, but the queen and her husband were banished to the stars, never resting as they circle the spinning north celestial pole. Later, Andromeda, Perseus and Pegasus, and Cetus were all given places in the sky.
This time of year is ideal for taking in the sights of the constellation Cassiopeia. Its distinctive crooked “W” of five bright stars is situated high the northeastern evening sky. Cassiopeia is circumpolar — so close to the North Celestial Pole (and therefore Polaris) that, for Canadians and other high latitude observers, it never sets.
This time of year, in mid-evening, you’ll find Cassiopeia oriented sideways, with the broken end “dangling” below the rest. Many cultures have interpreted this set of bright stars. The Inuit saw a blubber oil lamp and stand. The Navajo saw a female form too, albeit upside down. Biblical scholars replaced the queen of Ethiopia with Bathsheba, Mary Magdalene, and others.
Cassiopeia is depicted as sitting in a chair, her hand combing her hair. The brightest star, second from the top, is named Shedir, Arabic for “breast”. It’s a giant star, 229 light-years from Earth. In binoculars, you’ll see that it’s an orange-ish star with a small companion. The second brightest star, named Caph (“hand”), is at the top of the “W” and is white, 54 light-years away. The fourth star from the top is named Ruchbah (“knee”). It’s a variable brightness eclipsing binary star that dims when an orbiting dimmer companion star partially blocks its light every 25 months.
The star marking the queen’s waist, in the middle of the constellation doesn’t have an Arabic name. Instead it is designated Gamma Cas, using the third Greek letter for the third brightest star in Cassiopeia. It also has the unofficial nickname Navi, which is Apollo astronaut Gus Grissom’s middle name spelled backwards. This star is also variable due to a high rate of spin and periodic gas ejection. The last of the five stars marks Cassiopeia’s foot. Its nickname, Segin, is of unknown origin. This modest looking star is actually a blue-white giant star about 440 light-years away. It’s about six times the mass of our Sun, but 2,500 times more luminous!
The thinning outer edge of the Milky Way passes directly through Cassiopeia, meaning that the area is rich in interesting objects and star fields. One of my favorite objects can be seen in binoculars or a telescope. It’s a cluster of stars called the Owl Cluster (or ET Cluster or Dragonfly Cluster). It consists of two prominent yellow stars that form the eyes. A sprinkling of dimmer stars forms the owl’s body and feet, and two curving chains of stars define upswept wings. Be aware that the critter is positioned with its head to the right. This will be the same in binoculars, but your telescope will flip things around depending on the type. It is located by taking Navi and Ruchbah and making them the two vertices of a right angle triangle. The cluster sits at the third vertex, where the 90 degree corner is. It’s about four finger widths to the lower right of Navi — as if the queen is bouncing a baby owl on her knee!
By about 10 pm local time, the critter will rotate upright. The cluster was discovered by William Herschel in 1787 and is more than 7,900 light-years away! Other astronomical names for the object include NGC 457 (from the New General Catalogue) and Caldwell 13.
Large telescope owners can look for the large nebulas that reside in Cassiopiea. Extending a line from Shedar to Caph by a distance equal to their separation brings you to the Bubble Nebula and Salt-and-Pepper Cluster. There’s a smaller nebula less than half a finger’s width below Navi. And near the bottom of the constellation’s official region are the spectacular Heart and Soul Nebulas. They sit only two degrees apart (two finger widths) and can be found only about a palm’s width to the lower right of the star Segin.
While you’re gazing at that part of the sky, use the top three stars of Cassiopeia as an arrowhead that points directly at the great Andromeda Galaxy. The galaxy is approximately 1.5 fist widths (at arm’s length) from the tip of the arrow. At 2.5 million light years away, its faint smudge is among the farthest objects visible to unaided human eyes. Under dark skies, you might be able to detect its glowing patch spanning more than two finger widths across, or two full moon diameters. In actuality, it’s six moon diameters across!
I’ll post a sky chart here. In a future Skylights, we’ll tour Perseus, Andromeda, and some other characters in our story.
The Moon and Planets
Tonight (Sunday) brings us the December full moon, traditionally known as the Oak Moon, Cold Moon, Long Nights Moon, and the Moon before Yule. It rises at sunset and sets at sunrise and, because the full phase occurs on Sunday morning, it will be a little past full. Binoculars will reveal that the moon’s eastern edge (the right-hand side) is already rimmed with shadowed craters while the opposite side is not. Sunday’s full moon occurs less than one day before perigee, the point in the moon’s orbit when it is closest to Earth, making Sunday’s full moon the largest (about 7% larger than average) and brightest supermoon for 2017.
Just like the sun, the moon travels through (or at least near to) the constellations of the Zodiac. On each subsequent evening this week, the moon will slide eastward — through the top of Orion (the Hunter), the legs of Gemini (the Twins), past dim Cancer (the Crab), and into Leo (the Lion). It will also wane and rise about half an hour later each night. Last Quarter Phase, when the moon is half illuminated by the pre-dawn sun, occurs next Sunday about 3 am EST. Last quarter moons rise about midnight and can still be spotted in the southern morning sky in daytime.
Over the next few nights (Sunday through Wednesday), you might be able to catch a glimpse of Mercury low in the southwestern sky just before 5:20 pm local time. The speedy planet will also be sitting only two finger widths to the lower left of Saturn. Both planets are the same brightness. (I’ll post a sky chart here.) As the week unfolds, Mercury will drop sunward faster than Saturn, moving below the ringed planet on Thursday, and then racing out of sight by the weekend. Saturn sets about 5:30 pm local time this week, but it’s truly immersed in the evening twilight on its way to solar conjunction on December 21. Use binoculars to hunt for these two planets, but be careful to avoid the sun!
Slow moving Uranus and Neptune are the only planets left in the evening sky, setting about 3:30 am and midnight local time respectively. Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes). A medium-bright star called Omicron Piscium sits about 2.5 finger widths to the lower left of Uranus and another star of comparable brightness called Mu Piscium is three fingers to the planet’s lower right. Later in the evening this triangle of two stars plus Uranus will be tipped towards the west. Tiny blue Neptune, only observable in a backyard telescope, is about half a finger’s width below the medium-bright star Hydor in Aquarius (the Water-Bearer).
The most eye-catching planet nowadays is Jupiter, which rises in the eastern sky shortly before 5 am local time. The extremely bright planet is well above the horizon by dawn. Much dimmer and red-tinted, Mars, which will be rising about 3:40 am local time, is sitting 1.5 fist diameters to Jupiter’s upper right, about a palm’s width to the lower left of the bright star Spica in Virgo (the Maiden). Over the course of this week, Mars’ eastward orbital motion will carry it lower, so the separation between Mars and Jupiter will diminish steadily, until they “kiss” in early January! Finally, very bright Venus, is mirroring Saturn. It’s embedded in the eastern dawn twilight for about 45 minutes before sunrise, and will all but disappear after this week. I’ll post sky charts for the planets here.
Iridium flares are glints of sunlight off of the flat reflecting sides of one of the satellites that comprise the Iridium pager and sat-phone network. The flares occur before dawn and after dusk, when the satellite passing overhead is still illuminated by the Sun, which is below the horizon for observers on the ground. The duration and brightness depend on the angles between the observer, the satellite, and the Sun.
The Iridium company has started replacing these satellites, so enjoy them while you can. Using an accurate clock, go outside a few minutes ahead and look in the direction indicated. You should first see the dim Iridium satellite moving quickly across the sky, and then it will rapidly brighten for 3 to 8 seconds, and fade out. Truly spectacular! The more negative the Magnitude number, the brighter. The larger the Alt. number, the higher up it is!
Astronomy Skylights for this week (from December 3rd, 2017) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.