StarWatch: Moravian College Astronomy

DECEMBER 2012

DECEMBER STAR MAP | STARWATCH INDEX | MOON PHASE CALENDAR

Print Large Sky Charts For 9 p.m. EST: NORTH | EAST | SOUTH | WEST | ZENITH

Current Solar X-rays:
Current Geomagnetic Field:

Current Moon Phase

Perigee-Apogee Moons are Compared: The difference in the angular size of the full moon at the time when the moon is closest to Earth (perigee) and farthest from Earth (apogee) is very apparent. The exact same photographic setup was used to record both moons. A Canon 60D camera was mounted at prime focus to a 3.5-inch Questar telescope which was being equatorially driven. The exposures were 1/250 second at F/14.4, ASA 400. Gary A. Becker images...

850 DECEMBER 2, 2012: Dressing for the Geminids: It honestly could not get any better for the biggest meteor shower of the year, the Geminids. They reach maximum on the morning of Thursday, December 13, about the same time as the radiant from which the meteors are streaming nears its highest point in the sky. The moon is also new on the 13th offering no diminution to the bountiful number of bright shooting stars which this shower produces. When devotees think about meteor observing, they most commonly defer to the balmy nights of Perseid viewing in August. The Geminids produce double the activity of the Perseids, about 120 meteors per hour near dawn from a rural site. It’s the cold, wintry conditions that commonly keep people from enjoying this event. Here are some suggestions for staying comfy while observing meteors in winter. Keeping your head, hands, and feet toasty is the big challenge. Consider buying hand and toe warmers at a local hardware store to help fortify the digits of your body from succumbing to the cold. “SmartWool” socks are very comfortable, even if your body has an aversion to wool. Layer your clothing, two to three pairs of polypropylene long johns will help wick away moisture without causing you to chill. You really don’t have to worry about the traditional shirt and pants, but an outer baggy shell of thick polyester fleece, top and bottom, will help trap extra body heat in your clothing. Make sure you have at least one balaclava (head gear), warm gloves, and a scarf. A ground tarp, air mattress and pillow will help with comfort and keep your equipment clean. Remember, if you have overdone your clothing and become too warm, you can always peel back your sleeping bag or your balaclava to make adjustments, but if you are underdressed and the shivering begins, know that your quality time under Geminid skies will be very limited. More about the Geminids next week...

851 DECEMBER 9, 2012: Geminids: Fire in the Sky: Thursday morning, December 13, is when Geminid meteors should be flying at their maximum rates. Geminid activity has been on the rise since the shower’s discovery in 1862 because the orbit of the dross which produces its bright meteors has been shifting ever closer to Earth. Jupiter’s gravitational tugs take responsibility. In 1877 rates of 17 meteors per hour were being witnessed, 27 meteors each hour by the century’s end. During the twentieth century rates climbed to over 80 events per hour. When I was a teen growing up in the 60’s beginning my journey in astronomy, the Geminids were a major event, but they still paled in contrast to the August Perseids. Now they surpass the Perseids, but are still less observed because of the cold and often cloudy conditions which prevail in mid-December. Geminids are the best bet for observing meteor activity if the weather cooperates. They are also the only shower related to an asteroid. In 1983, 3200 Phaethon was discovered, and its orbit was quickly connected with the Geminids. This gave credence to the belief that many short period comets ended their active days masquerading as minor planets. For North America, meteor activity should increase throughout the night of December 12/13. The highest rates, perhaps as great as two meteors per minute, should be seen just before dawn. You can start observing as early as 10 p.m. Face east, but observe near the darker zenith. Geminids will be diverging upward from the east. By 2 a.m., face south but continue to look towards the zenith. Geminids will appear to radiate from an area of the sky near the bright star, Castor. Maps for both of these times are posted at www.astronomy.org. Observe a day earlier if weather conditions will be inclement on maximum night. After the morning of highest activity, rates decrease rapidly. Read about cold weather attire in last week’s StarWatch. Ad Astra!

852 DECEMBER 16, 2012: Low Sun, High Spirits: As we head full speed towards the Holidays, the sun is putting on the brakes as it reaches its lowest point in the sky, the winter solstice on December 21 at 6:12 a.m., EST. It’s the tilt of the Earth’s axis, 23.5 degrees from the perpendicular to its orbital plane that creates this effect and all of the other consequences of the changing seasons that we experience. During each quarter the sun’s noontime position either moves up or down by the same amount as Earth’s tilt. Why am I in such high spirits? I can’t deny the Holiday festivities, which are also related to the time of the solstice, play a part; but the downward trend of the sun is nearly over and Sol’s motions will soon be reversed and headed in an upward direction. On the day of solstice, which literary means “sun standstill,” Sol will rise at its most southerly position from the east and set at its most southerly position to the west. At noon when the sun is highest in the sky for the day, it will be at its lowest noontime position for the year. If the daily spin of the Earth could be stopped at local noon, when the sun was due south, and if we could take a hike over land and sea, what would we witness? Traveling southward over the curvature of the Earth towards the sun would find Sol getting one degree higher in the sky for each degree of latitude we traversed. We would have to stride some 4400 miles south or 64 degrees in latitude to move the sun into a position where it would be directly overhead. We’d find ourselves on the Tropic of Capricorn, assuming that we lived where I live at 40.5 degrees north latitude and celebrating the first day of summer in the Southern Hemisphere. Going northward would eventually cause the sun to dip below the horizon. So even though shadows are at their longest this week, the lowly sun starts coming back, slowly at first, but with the promise of another spring and the warmth of summer on the horizon. Happy winter to all!

853 DECEMBER 23, 2012: Comet to Grace Spring Sky: “Oh no, Mr. Bill, it’s not another comet!” Yes, 2013 seems to be shaping up as a banner year for these hairy interlopers. First, it was Comet ISON, C/2012 S1, named after the International Scientific Optical Network, its mission to track high altitude geocentric space debris. Comet ISON is predicted to whip past the sun in late November 2013 at only a scant million miles from the solar photosphere, and this is where things might get tricky. Will the comet hold together under the intense heat of a close solar passage or will Comet ISON simply break apart and go puff, creating some beautiful space images, but little to see from the Earth? ISON’s orbit has some similarities to the Great Comet of 1680 which survived an even closer passage around the sun, and then became spectacular. Now, in addition to ISON, there is Comet PANSTARRS, C/2011 L4, named for the Panoramic Survey Telescope and Rapid Response System which monitors and detects objects which could strike the Earth. PANSTARRS reaches perihelion, its closest distance to the sun on March 3, and will become visible about one week later, low in the west as a fuzzy speck, 45 minutes after sunset. Because of strong twilight and its very low altitude, binoculars will most likely be needed to see the tail and maybe even the comet. PANSTARRS will be less than a half degree from the planet Uranus and 4.5 degrees from a razor thin crescent moon on the evening of March 12, but again an exceptionally good western horizon will be needed as well as binoculars to enhance the show. By spring, March 20, Comet PANSTARRS has gained a little altitude, but it has faded to the brightness of the stars of the Big Dipper, while the moon has blossomed beyond first quarter. With all of this hype, keep in mind that “comets are like cats. They both have tails and they do exactly what they want.” I’m still excited!

854 DECEMBER 30, 2012: In the Goldilocks Zone: Do you remember Goldilocks and the Three Bears? Goldilocks tastes three bowls of porridge; one is too hot, the second too cool, but the third bowl is just right. The “Goldilocks” tale has been applied to the desire of astronomers trying to discover exoplanets orbiting stars in their habitable (just right) zone. With over 850 exoplanets identified and thousands more waiting to be confirmed, all but seven of these new worlds orbit outside of their Goldilocks zone. That number jumped in December 2012 by two with the discovery of five new planets orbiting a sun-like star named Tau Ceti, only 12 light years distant. All of the new planets are fairly similar to the Earth in mass, and one of them lies squarely within the Goldilocks zone, where conditions would be just right for life. Another planet lies just within the outer boundary of habitability of Tau Ceti. The favored world, Tau Ceti e, is about four times the mass of the Earth and completes one orbit around Tau Ceti in 168 days. Planets tug and pull on their parent star, creating minute oscillations in the motion of the star towards or away from the Earth. Currently with new data-crunching techniques, astronomers can measure changes in this radial motion as small as the speed of a crawling baby. The time period of the oscillatory motion yields the orbital period of the planet, as well as the planet’s distance from the star. The amount of induced wiggling of Tau Ceti, a star of known mass, yields the mass of the orbiting planet. One problem of the Tau Ceti system is that it is debris laden, with meteorite impacts occurring more frequently, including many more probable extinction events like Earth’s dinosaur-killer of 65 million years ago. Still many astronomers believe that Tau Ceti e has the potential for primitive life forms even with its higher bombardment rate. ET, we are almost home.