The Moon exits Stage East, Pegasus Perches on High, and the Geminid Meteors Peak at Mid-week!
The Geminid Meteor Peaks
The Geminid Meteor Shower, one of the most spectacular of the year, runs from December 4 to 16, and peaks before dawn this Thursday, December 14. Geminid meteors are frequently bright and intensely colored, and slower moving than average. This shower is prolific! Under optimal conditions, more than 120 Geminids per hour are possible. And this year, the moon will be a slim old crescent, leaving the sky nice and dark for meteor hunters!
I wrote about how meteor showers work last month. Throughout the shower period, the best time to see meteors is when the sky overhead is plowing straight into the debris field, like bugs splatting on a moving car’s windshield. That’s because, when the constellation containing the shower’s radiant is overhead, the entire sky is available for observing. When a radiant lies near the horizon, the Earth will hide up to half the meteors.
To see more meteors, try to find a dark viewing location that has as much open sky as possible. Light pollution washes out the night sky, dramatically reducing the number of fainter meteors you will see. So a drive to a park or rural site away from city lights is helpful. To preserve your eyes’ dark adaptation, remember to turn your phone’s screen brightness to minimum or, even better, cover it with a red film. Disabling app notifications will reduce the chances of unexpected bright light, too.
You can start watching as soon as it’s dark. Even if the peak number occurs before dawn, meteors will still be visible before midnight, too. Don’t worry about looking directly at the radiant. Bring a blanket and a chaise to avoid neck strain. And remember that binoculars and telescopes will not help: Their field of view is too narrow to see the long meteor trails. If you have friends or family along, don’t look at each other while chatting. Keep your eyes to the skies!
Andromeda’s Family — the Wings of Pegasus
Late fall and winter evening skies feature a group of easy-to-see constellation that are the characters in a grand story from Greek mythology — the tale of Princess Andromeda and the hero Perseus. Last week, I told their story here, and described the constellation of Cassiopeia. This week, we’ll look at Pegasus — the westernmost constellation in the set. But let’s flesh out his story a little first…
Before hunting the Gorgon Medusa, Perseus was given some magical gifts to aid him in his quest, including winged silver sandals called the Shoes of Swiftness. Immediately after slaying the snake-haired Medusa by cutting off her head, he flew into the air to avoid Medusa’s angry sisters. While hovering there, some of the gorgon’s blood dripped on the seashore below. Poseidon mixed the blood with the sea foam, and magical Pegasus, whose name is derived from the Greek word for wellspring “pegai”, sprang from the sea.
Pegasus has traditionally been associated with beauty, wisdom, and poetry. In some stories, the majestic wild white stallion was tamed by the warrior Bellerophon, the most successful hero of ancient Greece — until Hercules came along later. After many heroic deeds astride Pegasus, Bellerophon felt that he was worthy of living in Olympus with the gods, and attempted to ride Pegasus there. But they denied him — sending a bee to sting Pegasus, who bucked and tossed Bellerophon from his saddle. The hero died from the fall to Earth, but he has an interesting role among the stars — as we’ll cover below. Zeus then added Pegasus to his stables, using him to transport thunderbolts. As a reward for his service, at the end of the horse’s days, he was granted a place among the stars.
The constellation Pegasus (the Winged Horse) enters the eastern evening sky in September, becomes well placed for evening astronomy, high in the southern sky during November and December, and then gradually sinks into the western twilight in February. It is one of the largest (seventh by area) and oldest of the 88 modern constellations. The stars of Pegasus only depict the horse’s head, forelegs, and wings rising out of the ocean, while the rest of the animal is occupied by the constellation of Pisces (the Fishes). In Chinese astronomy, the stars of Pegasus make up the Black Tortoise of the North, one of the legendary Four Benevolent Animals in the stars.
Pegasus contains one of the most obvious asterisms in the sky. An asterism is a shape or pattern made up of prominent stars, and it can use some of the stars in a single constellation, or combine stars from adjacent constellations. (The Big Dipper is an example of the former case, since the dipper uses only part of the large constellation Ursa Major.) Pegasus’ asterism is a giant square of four equally bright stars called the Great Square of Pegasus. But it might remind you of a baseball diamond when you see it, because it’s usually tilted with one corner downwards. For the Lakota people, the square represented the great shell of Keya, the Turtle.
Pegasus has traditionally been depicted upside-down with the square representing the wings, the stars to the west (to the right) forming the head and neck, and two chains of stars at the upper right of the square representing the front legs. To the west (or right) of Pegasus’ head is a tiny and dim constellation of four stars called Equuleus (the Little Horse). Beyond Pegasus’ legs sits the prominent constellation of Cygnus (the Swan), with its bright Summer Triangle star Deneb. Adjoining Pegasus on the eastern (lefthand) side, and sharing a major star with him, is Andromeda (the Princess). The water constellations of Pisces and Aquarius (the Water-Bearer) are below Pegasus.
Let’s tour Pegasus. After it gets dark, face the southeastern sky. Using unaided eyes only, the Great Square appears empty. You might be able to pick out one or two dim stars inside it — more if you are away from light-polluted skies. The square’s edges are about 16° long (or 1.6 fist widths held at arm’s length), and about 20° from corner to corner. In mid December at 6 pm local time, the square’s centre is fairly high in the sky — about two-thirds of the way from horizon to zenith. It’s highest, in the southern sky, about 6:30 pm local time, then sets in the west about 2 am local time.
The star at the southeast corner (lower left) of the diamond is Algenib, Arabic for “the Side”. It’s a very hot blue-white star of moderate brightness that sits about 350 light-years away, and actually emits 4,000 times more light than our Sun! Moving counter-clockwise, the white star at the lower right-hand corner is Markab “the Saddle”. This star appears slightly brighter than Algenib — it emits less light, but it is only 140 light-years away.
From Markab, look west for the dimmer stars that trace the horse’s neck and head. A palm’s width from Markab is blue Homam “Man of High Spirit”, then another fist’s width further on sits Baham. Both are about equal in brightness. From Baham look a palm’s width higher to the right for bright star Enif “the Nose”. Enif is a cool orange supergiant star 670 light-years away and nearing the last stages of its life. You should be able to tell that its colour is tinted. Enif is huge! Were it to replace our Sun, it would span 40° of sky, eighty times wider than the Sun or Moon! Enif is just at the lower mass limit for dying in a supernova explosion. Use binoculars to scan the sky about four fingers widths to the west of Enif. You’ll find a dim, but pretty little fuzzy patch of stars designated Messier 15.
Returning to the square, the fairly bright magnitude 2.4 star at the northwest (upper right) corner is Scheat “the Foreleg”, the second brightest star in the constellation. It’s a cool red giant star 200 light-years away. Pegasus’ two legs start at Scheat and extend upwards to the right. Five degrees to the right of Scheat is the dim yellow star Matar “Lucky Star of Rain”, and the leg terminates a palm’s width farther in the same direction at a double star designated Pi Pegasi. Using binoculars you should be able to see that Pi consists of two close together yellow-white stars.
The second foreleg takes a jog to the lower right before bending back up parallel to the first one. A well-spaced pair of yellowish stars named Sadalbari “Luck Star of the Splendid One” and Lambda Pegasi marks the knee. The foreleg extends to a faint white star a fist’s width to the upper right, and ends at a dim white star an additional 5° away.
A dim yellow star that sits a thumb’s width just outside of the baseball diamond, midway between the lower and upper right corners, is the sunlike star 51 Pegasi. It hosts the first exoplanet ever discovered, in 1995! Dubbed Bellerophon, it is a Jupiter-sized planet that orbits the star every 4.23 days at a distance MUCH closer than Mercury does in our solar system. Planets like this are called hot Jupiters. Take a look at the star in your binoculars and let your imagination soar like Bellerophon!
The final star of the square, at the northeastern (upper left) corner, is called Alpheratz “The Horse’s Shoulder”. It’s another hot blue-white supergiant star, but located only 97 light-years away from us. The spectrum of this star’s light indicates that it is highly enriched in the metal Mercury. In actuality, Alpheratz does not belong to Pegasus. It’s actually the brightest star in Andromeda, and marks the princess’ head, but that’s for another day!
For Skylights readers with good-sized telescopes, Pegasus is loaded with galaxies! This is because the constellation is well away from the obscuring material of our Milky Way, allowing us to peer deeper into the Universe. One of my favorite sights is NGC7479, or Caldwell 44, an S-shaped spiral galaxy.
The Moon and Planets
The Last Quarter Phase, when the moon is half illuminated by the pre-dawn sun, occurred early this morning (Sunday). Last quarter moons rise about midnight and linger after dawn to shine in the southern morning sky. As the week unfolds, the moon will wane to a thin crescent, landing a few finger widths above Mars in the eastern pre-dawn sky on Wednesday morning, then jumping lower to take up a similar position above Jupiter on Thursday morning. Also on Thursday morning, the large asteroid Vesta will be sitting only 2.5 degrees to the moon’s lower left, but you’ll need at least binoculars to see it.
On Friday, the moon will be slim indeed — and sitting a fist’s width to Jupiter’s lower left. Our last glimpse of the old crescent moon comes Saturday morning, before it vanishes into the sunlight during Monday’s New Moon.
With Mercury and Saturn lost in the evening twilight, Uranus and Neptune are the only planets left in the evening sky, setting about 3 am and 11 pm local time respectively. Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes). 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).
Your best bet for seeing a bright planet this month is Jupiter, which rises in the eastern sky shortly about 4:30 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 3:30 am local time, is sitting 1.2 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 close to a fist diameter on the weekend.
Venus, too, is hidden from view while it’s embedded in the eastern dawn twilight. But Mercury will complete its passage between us and the Sun later this week and commence a good appearance in the pre-dawn sky starting around Sunday.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.
Astronomy Skylights for this week (from December 3rd, 2017) by Chris Vaughan.