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.

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