Image by Aymen Ibrahem (Night Sky over Egypt) of the Bibliotheca Alexandrina, Egypt. ©Aymen Ibrahem, shown with permission. The 4500 year old bulk of the pyramid of Pharaoh Khafra (Chephren) shields the sun to reveal a sun dog, a 22 degree halo and a hinted upper tangent arc. Ancient Egyptians recognised several manifestations of the sun god Ra, Ra-Herakhty of the rising sun, Ra-Atum of the setting sun and Aten the solar disk. Strangely, there is no acknowledgement of the sundogs or parhelia that must at times have accompanied Ra on his daily journey across the sky.

December begins with the full moon within one degree of the Pleiades, the Seven Sisters, and ends with a blue moon, the term for a second full moon in the month. New Year's Eve will feature a partial lunar eclipse, but you will have to be in Africa or Asia to see it. Look for Mercury, an evening star, on Dec. 24 at 5:55 p.m. Mercury will form the bottom point of an equilateral triangle with Jupiter (above and to the south) and Altair, the eye of the Eagle (above and to the west). By the end of December, Mercury will appear as a slim crescent in a telescope.

Venus rises 20 minutes before sunrise on the first of the month, but by midmonth, it becomes hidden in the sun's glare. Mars appears to describe an arc on the border of Leo and Cancer in the night sky as it moves in retrograde, ''backwards,'' this month. Mars rises at 10 p.m. on the first of the month; only Jupiter and Sirius are brighter in the night sky. Jupiter stands right in the south at 5:15 p.m. on Tuesday, 15 minutes after sunset. At 7 p.m., Uranus is right in the south, about 45 degrees above the horizon at magnitude 5.8, visible to the unaided eye under dark, clear skies.

The ringed giant Saturn rises in the east at 1:51 a.m. Tuesday. The ring system will be tipped about 5 degrees by month's end, closing down to 2 degrees in May, and at the end of 2010, will be open about 10 degrees. Not until 2014 will the rings be fully open at 20.2 degrees inclination. On Dec. 20 at 8 p.m., note Jupiter and a waxing crescent moon in the southwest. In a pair of 7x50 binoculars, you may spot Neptune only half a degree distant from Jupiter, at about the 2 o'clock position.

On the evening of Dec. 13 into the morning of the 14th, the Geminid meteor shower peaks. This shower is fairly reliable, producing up to 100 meteors an hour. The moon will not interfere with viewing this multi-colored display — more than half usually white, a quarter yellow, and the remaining meteors blue, red and green. Only about 25 years ago did NASA discover an object in the same orbit as the Geminid meteoroid stream, a rocky asteroid or extinct comet named 3200 Phaethon. This is most probably the parent body of the Geminid meteor shower. On Dec. 22, the Ursid shower peaks. Only about 10 faint meteors an hour may be seen, and fortunately the moon will not be visible this evening, either. The shortest day of the year, the winter solstice, will be Monday, Dec. 21.

The Ursids meteor activity begins annually around December 17th and runs for a week plus, until the 25th or 26th. This meteor shower is named for its radiant point which is located near the star Beta Ursae Minoris (Kochab) in the constellation Ursa Minor.

Flurries of questions about mysterious triangle-shaped snowflakes may soon subside, thanks to new research on snowflake formation. Most snowflakes are hexagons because of the arrangement of hydrogen bonds in the water molecule. But the new study, appearing online at arxiv.org (http://arxiv.org/abs/0911.4267) and in an upcoming issue of The Microscope, suggests that after hexagonal flakes, oddball triangular flakes are the most prevalent. Study coauthors Kenneth Libbrecht and Hannah Arnold of Caltech in Pasadena propose an aeronautical reason for the triangular geometry. The results help solve the very old puzzle of how the unexpected flakes form, Libbrecht says.

Snowflake enthusiasts — such as Libbrecht, who photographs snowflakes — have spotted triangular snowflakes in the wild. The snowflake scientific literature, which goes back almost two centuries, is thick with such sightings, Libbrecht adds, but no one has explained why. “People have noticed them for hundreds of years.”

To address the mystery, the researchers created snowflakes in the laboratory and recorded the shapes. In conditions that simulate natural snowfall, the vast majority of flakes were the standard hexagons, but more of them were triangular than a statistical model had predicted, the team found. Some of these flakes still have six sides but an overall triangular shape, created by three short edges and three long ones. The abundance of triangle-shaped flakes suggests that they may be more common in nature than chance alone would allow.

Tiny impurities, such as dust particles, can cause one edge of the falling snowflake to tilt up as it falls, Libbrecht says. The snowflake sides that are pointed down grow faster as the wind blows by, leading to a stable triangular pattern. Once a triangle shape gets started, the snowflake remains triangular despite any later bumps as it falls, the researchers propose.