November 28, 2010

What's Up in December
Lunar eclipse and meteor shower add thrills to chilly nights

December always marks the beginning of winter for us in the northern hemisphere. We have enjoyed a fairly warm autumn, but now it is time to prepare for winter.

click image to enlarge

This chart represents the sky as it appears over Maine during December. The stars are shown as they appear at 10:30 p.m. early in the month, at 9:30 p.m. at midmonth and at 8:30 p.m. at month' s end. Jupiter is shown in its midmonth position. To use the map, hold it vertically and turn it so the direction you are facing is at the bottom.

Sky chart prepared by George Ayers

The sun will reach its lowest point in the sky at 6:38 p.m. Dec. 21. That is called the winter solstice. The word solstice means, "Sun stands still." That is what it appears to be doing for a day before it slowly begins its upward climb in our sky again, causing the days to get longer and longer until the summer solstice is reached half a year later.

Dec. 21 also marks the summer solstice in the southern hemisphere.

There will be two very exciting events in December that are well worth braving the cold for, as long as the skies are clear. One of them, the Geminid meteor shower, happens every December, but the other one, a total lunar eclipse, will not happen again for us in this location until April 15, 2014.

The Geminid shower will peak the night of Dec. 13 into the morning of Dec. 14.

The moon will only be first quarter that night, so it will set around midnight, which is perfect because meteor showers don't usually pick up in intensity until well after midnight. That is because our spaceship Earth starts spinning towards the source of the meteors after midnight, continuing until dawn. It is similar to driving your car in a snowstorm. Before midnight, it's like you're seeing the snowflakes out the back window, but after midnight it's as if you're looking out your front window, driving directly into the snowflakes.

The story of what causes this mysterious Geminids is very different from all the other meteor showers. Most meteor showers are very old; they have been active for hundreds and even thousands of years. The Geminids were not even recorded until just over a hundred years ago and the object that creates these meteors wasn't found until 1983.

Most meteor showers are caused by comets, but the Geminids seem to be caused by an asteroid named 3200 Phaethon, which has a highly elliptical 1.4-year orbit around the sun.

The comet dust from the other meteor showers the earth passes through each year is not very dense, only 0.3 grams per cubic centimeter. Material from an asteroid is 10 times denser. The material from 3200 Phaethon seems to be a hybrid, since it is about 1 to 2 grams per cubic centimeter. This asteroid never grows a tail when it is near the sun like the comets do, but enough material gets broken off of it to cause one of the best meteor showers each year.

The most likely explanation for these contradictions is that this asteroid is actually a long-extinct comet nucleus that has accumulated a thick crust of interplanetary dust grains, some of which are now being lost during each approach to the sun and creating these wonderful meteors as they burn up high in our atmosphere. In a sense, 3200 Phaethon is wearing a mask, looking like an asteroid on the outside, while underneath beats the heart of the nucleus of a comet.

You can expect nearly 100 meteors per hour from a dark sky site during the peak of this enigmatic shower. The individual meteors tend to be much brighter than usual, because the tiny particles are denser than the usual comet dust.


The next major highlight in December is a total eclipse of the moon. The last one visible for us was nearly three years ago and the next one will be nearly three and a half years in the future, so make sure you catch this one if it is clear.

This eclipse will be later than usual, starting at 1: 30 a.m. Dec. 21, just a few hours before the winter solstice.

The total part of the eclipse, when the moon is completely engulfed in our shadow, will start at 2:41 a.m. and end at 3:53 a.m. The entire eclipse will not be over until 5 a.m. This means the moon will be nearly directly overhead when it plunges into the earth's shadow.

Every lunar eclipse is always different in character and tells you a lot about the nature of our atmosphere at the time. They range from almost completely black, with the full moon melting into our sky and disappearing from sight, which happened in 1982 and 1991 after major volcanic eruptions, to being bright and coppery like a new penny when the atmosphere is clear.

They are usually somewhere in between, glowing with a wonderful deep orange or reddish light that makes the moon look alive and three-dimensional, as if you could just reach up and touch it. The moon is only 1 1/2 seconds away at the speed of light, but that is still nearly a quarter of a million miles out in space.

As you carefully watch this lunar eclipse, you will become aware of many dynamic yet subtle changes occurring. Picture yourself above the plane of our solar system and you would see that the earth and the moon always have a shadow extending behind them as each continually spins on its axis and revolves around the sun. The lunar shadow cone is about a quarter of a million miles long and the earth's is about 4 times that long.

We are physically standing in the moon's shadow during a total solar eclipse. For this lunar eclipse, you witness our shadow slowly crawling across the face of the moon and giving us proof not only that the earth is round but also that it is four times bigger than our moon.

As the moon slowly dims, nearby stars become visible as the rest of the sky becomes darker and reveals more of its treasures.

Try looking at the moon with binoculars or a telescope to appreciate the second-by-second motion of our shadow across the moon and take a close look at its alien terrain.

Our life-giving atmosphere acts like a lens and bends the sunlight so that it can illuminate the moon even when it is in the deepest part of our shadow.

Remember that the reddish light you will see on the moon is actually the combined effect of all the sunrises and sunsets on Earth simultaneously projected onto the moon. An astronaut on the moon would see the sun covered up by a dark Earth ringed all around with a brilliant band of reddish light, as if illuminated from within.

Jupiter remains as the only bright evening planet. It will fade a little in December, but is still quite bright in Pisces, below Pegasus the flying horse. Both Venus and Saturn are prominent in the morning sky one hour before sunrise.

Look for Mercury and Mars with binoculars very low in the western evening sky half an hour after sunset during the middle of the month.


Dec. 1-3: Watch the waning crescent moon glide past Saturn, Spica and then Venus in the morning sky one hour before sunrise on these three mornings.

Dec. 5: New moon is at 12:36 p.m. EST.

Dec. 13: First quarter moon is at 8:59 a.m. The Geminids peak this night into the next morning.

Dec. 18: The nearly full moon will pass very close to the Pleiades this evening.

Dec. 21: A total eclipse of the moon occurs this morning. Full moon is at 3:13 a.m. This is also called the long night moon or the cold moon. Winter starts at 6:38 p.m.

Dec. 25: Isaac Newton was born on this day in 1642.

Dec. 27: Johannes Kepler was born on this day in 1571. He gave us the three laws of planetary motion. Last quarter moon is at 11:18 p.m.

Dec. 30-31: The waning crescent moon is near Venus these last two mornings of the year.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.


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