Highlights of our December Sky Transcript


Hello, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences; welcome to The Night Sky.

The holiday season is upon us and before we all succumb to the rigors of gift buying, family get-togethers and too much eggnog let’s take a little bit of time to step outside and see what’s happening in our night sky.

But be sure and bundle up when you do because the nights are much chillier now, especially as we get closer to the winter solstice on December 21st.    As you may know, the reason for the seasons is the Earth’s 23 and a half-degree tilt upon its axis as it orbits around the Sun.   This axial tilt means that the two hemispheres of our planet are either tilted towards or away from the Sun at various times throughout the year and this brings about a corresponding change in the amount of solar radiation and heating a given hemisphere will receive.  On June 20th or 21st our Northern Hemisphere is at its maximum amount of tilt towards the Sun and this marks the summer solstice.  Six months later, on either December 21st or 22nd the northern hemisphere is at its maximum amount of tilt away from the Sun.  On the day of the winter solstice the Sun at noon will be at its lowest position all year and it will also spend the least amount of time above the horizon.  The winter solstice is the shortest day and longest night of the entire year.  Be sure and mark the Sun’s noon position on or around this date and watch what happens over the coming days.  Slowly, but surely, the Sun’s noon time position will get higher and higher over time and the day length will gradually increase until finally, during June’s summer solstice, we see the Sun reach its highest point of the year only to start getting lower and lower in the days that lead up to winter and the whole process starts all over again.

OK, moving on from our own planet, let’s take a look at one that hardly has any axial tilt at all: it’s the King of the Planets, Jupiter, and it returns to our evening skies this month.  Look for Jupiter high up in our eastern sky by midnight within the constellation of Leo the Lion. In fact, Jupiter will be within just a few degrees of Leo’s brightest star, Regulus (the Heart of the Lion).  By the end of the month Jupiter will have risen two hours earlier than it did at the beginning of the month, making it available for viewing much earlier in the evening.   By the middle of the month Jupiter will be the brightest object in our evening sky outside of the moon.

With a pair of binoculars you should be able to see four of its brightest moons: Io, Europa, Ganymede, and Callisto.   If you watch closely you can witness these Jovian satellites waltzing around their lord and master.   Some nights you may only see one or two while the others are hiding behind Jupiter’s other side, on other nights you might be treated to seeing all four strung out like Christmas lights around an ornament.    The Jovian moons all orbit within the same plane as the planet’s equator and so we always see them just to the left or right of Jupiter’s equator.  Once every six years we get to see the orbital plane of these satellites tilted edge on to our perspective and we are currently within one of these alignments.  What this means for owners of small telescopes is that you can occasionally see the moons passing in front of one another or passing through another’s shadow.   There are more than a dozen of these events occurring throughout the month of December and if you pick up a copy of this month’s Astronomy or Sky & Telescope magazine you can find a schedule with the times for when they occur.

Even a small telescope will reveal Jupiter’s most striking feature: its stripes.   The bright stripes are known as zones and represent upcurrents of cooler gas rising to the upper cloud decks while the darker bands are referred to as belts, which represent warmer material descending back into the interior of the planet.    Then there is that other distinctive mark of Jupiter’s, the Great Red Spot, a gigantic storm that has been raging in the Jovian atmosphere for at least 350 years.   We don’t know exactly why the storm is red, perhaps it is due to material being dredged up from deep inside Jupiter or maybe, as recent studies suggest, it could be due to chemical compounds undergoing changes after having been exposed to ultraviolet radiation from the Sun.  But there is something very weird going on with the Red Spot.  Over the past few years it’s color has changed from that of a brick red to a pale, salmon pink.  Even more disturbing is the fact that the Spot is shrinking in size.  Over the past 40 years the Spot has lost 30 percent of its original width and the rate of shrinkage has even accelerated within recent years.  It may be that within our lifetime the famous Red Spot will disappear altogether.   You’ll need a telescope to see it as well as a schedule for it’s visible transit times, which can be obtained by visiting the Sky & Telescope website.

No celebration of Christmas is complete without a festive light display and this month nature provides us with a truly spectacular one, the Geminid Meteor Shower, one of the year’s very best meteor shower events.   While you can see Geminid meteors anywhere from December 4th to the 17th, the peak nights will be on December 13th-14th and the 14th-15th.   As the name suggests, the radiant (or place upon the sky from which all the meteors appear to originate) is the constellation of Gemini; which can be seen just above the eastern horizon at around 8PM.   The nominal peak night is on the 14th and under good viewing conditions it’s possible to see anywhere from 90 to 100 or more meteor streaks per hour.   Fortunately the waning moon does not rise until much later in the evening so it won’t spoil the show.

The Geminids have been lighting up our mid-December skies since at least 1862 and has steadily increased in intensity since the 1930’s.  But for many years there was a mystery associated with this shower.  Most of our meteor showers are the result of comet dust burning up in our atmosphere but, oddly, there is no known comet to account for the Geminid meteor stream.   It wasn’t until October of 1983 that astronomers finally found the source for this spectacular meteor shower: it wasn’t a comet but an asteroid named Phaethon 3200.   Phaethon orbits around the Sun in just 1.4 years and at its closest approach to the Sun it’s less than half the Mercury-Sun distance.   This close encounter with the Sun heats Phaethon up to around 1400 degrees Fahrenheit, enough to cause thermal fracturing of the asteroid and to release a trail of gravel and dust in the wake of its orbit.  It’s this debris that we see burning up in our atmosphere when Earth plows through the meteor stream.

Here are a few of my tips for enjoying this wonderful celestial display:

First off you must get to the best dark sky site you can find.   That may mean driving into the country for you city dwellers and away from all the light pollution.

Let your eyes become adapted to seeing in the dark.  This usually takes about 20 to 30 minutes of being in the dark without having any bright light hitting your eye.  Avoid house lights, car lights, or light from cell phones and tablet devices.  It only takes a few seconds of exposure to bright light to ruin your night vision.  Use a flashlight with red plastic film over it in order to see with, red light will not harm your dark adaptation.

Stay both warm and hydrated.  Wear clothes suitable for the weather, I often suggest wearing layers that you take off or put back on as needed.  Keep water and warm beverages on hand.  Avoid alcoholic beverages as they can impair your night vision.  Also, keep lots of snacks high in calories on hand.  If the night is cold your body is going to expend lots of calories to stay warm and you’ll need the snacks as fuel to keep you going.

While the shower’s radiant is in the east during the early evening you can expect to see meteors in just about any part of the sky.  Spread some blankets out on the ground or use reclining lawn chairs.  Mainly, just get comfortable and enjoy the show!

That’s all for now but be sure and check out our Night Sky web page for more astronomy news and viewing tips.

Happy Holiday’s from all of us here at The Night Sky and UALR Television.  Until next time, be sure to get outside, look up, and wonder.

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