During the second half of December, look for Mercury low in the southeast about an hour before sunrise. Only a very small percentage of people throughout the world have ever seen Mercury. It's the smallest major planet in the solar system, smaller than Jupiter's moon Ganymede and Saturn's moon Titan.
Venus appears as the ''Evening Star'' this month. It will shine as a bright beacon low in the southwestern sky during evening twilight. On Dec. 26, Venus and the waxing crescent moon will be quite close in the sky.
Mars rises in the east around midnight and is high in the southern sky at dawn.
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Jupiter appears in the southern evening sky. The moon passes just above Jupiter on the evenings of Dec. 5 and 6.
Saturn rises in the eastern predawn sky. This beautiful ringed planet is nearly twice as far away as Jupiter.
The Geminid Meteor Shower peaks on the night of Dec. 13-14. This year, light from the waning gibbous moon will drown out most of the faint meteors.
The sun arrives at the Winter Solstice on Dec. 22 at 12:30 a.m. EST. This marks the beginning of winter in the Northern Hemisphere and summer south of the equator.
HOW DO YOU SOLVE A PROBLEM LIKE MARIA?
''The Moon is the first milestone on the road to the stars.'' - Arthur C. Clarke
When people look up at the surface of the moon, they always see the same light and dark patterns that never change. This is because the moon is in synchronous rotation, always keeping the same side facing the Earth. Early Greek and Roman astronomers thought the smooth, dark regions were seas and named them 'maria', while the lighter, more cratered areas were thought to be land. We now know that the dark, smooth expanses are made of ancient, frozen lava. For many centuries, depending on their culture, people have imagined seeing optical illusions on the lunar surface, such as the man on the moon, the woman on the moon, the rabbit on the moon, etc. Since the surface markings never changed, everyone thought that the far side of the moon, the side we never see, probably looked much the same.
Then in 1959, the Soviet spacecraft Luna-3 returned the first images ever taken of the moon's far side, and astronomers were amazed. The strange and more heavily cratered far side looked very different from the familiar near side with its enormous smooth, dark maria. With many more craters, the far side appears as though it has been battered much more severely over many millions of years. Scientists were puzzled. Why would the lunar near side be covered with large, multiple maria while the far side displayed only a few small maria but was much more heavily cratered?
Scientists developed several hypotheses to explain the different appearance of the near side vs the far side. Some of these include tidal forces, uneven heating from below the lunar surface, Earth acting as a halfback by shielding the moon from impacts, and even possibly a side effect from the huge collision on the far side that created the more than 1,500-mile-wide crater known as the South Pole-Aitken impact basin. All of these theories have problems that prevent them from being generally accepted by the scientific community.
The most recent hypothesis proposes that the thicker crust and rugged mountains of the lunar far side resulted from a relatively slow collision between the moon and a smaller moonlet that shared the moon's orbit around the Earth. The collision would have taken place at a relatively slow speed due to the fact that the two objects shared a common orbit. Low-speed collisions would not create a crater or cause extensive melting. Instead, nearly all of the colliding material would pile up on the impacted side, resulting in a thicker crust and mountainous terrain on that side. Also, an enormous, low-speed collision would have shifted much of the molten subsurface magma to the near side, enhancing the formation of maria. That is exactly what we see today in the observations carried out by spacecraft in lunar orbit.
In just a few months, NASA's Gravity Recovery and Interior Laboratory spacecraft is due for a lunar orbit insertion. Its mission is to develop a detailed map of the moon's crust, revealing how its thickness varies in various locations on both the near and far sides. This will provide further evidence to support or disprove the different hypotheses.
The best evidence would be provided by studying rocks from the far side of the moon. However, no country has lunar far side sample-recovery missions planned, although China has a near side recovery trip planned for 2017. With all the setbacks in the world's economies, it may be a long, long time before scientists can examine rocks from the lunar far side.
Editor's note: This monthly guide to the stars is from the Marshall Martz Memorial Astronomical Association, the Southern Tier Astronomy Recreation Society, and The Post-Journal. For further information, contact the M.M.M.A.A. at www.martzobservatory.org or S.T.A.R.S. at www.UpStateAstro.org/stars/stars.html.