Today marks the official date of the December’s Full Cold Moon, named so for obvious reasons. Last night’s moon was a little more than a half-day before full; tonight’s will be closer — only about 6 hours past full (Central Time). The moon will rise a little past sunset in Gemini the Twins and shine the entire night. Would that we could the constellation better, but moonlight will blank most of Gemini’s stars except the bright pair of Castor and Pollux located 2 fists to the left of the rising moon.
Pollux and Castor are the mythical Greek twin brothers who joined Jason and the Argonauts in search of the golden fleece. Tonight, the moon will tickle their toes tonight. Mostly Castor’s. From mid-northern latitudes, the moon will stand highest around 12:30 a.m. tomorrow morning. But if you live at 21° north — the latitude of Honolulu and cities in southern Mexico and Central America — you’ll have to look straight up to see the moon because it will shine down directly from overhead.
If you’re in a holiday mood, this year’s stocking stuffer will be the small asteroid 2003 SD220. The approximately 1-mile-long (1.6 km) asteroid will safely fly by Earth today at a distance of 1.8 million miles (2.9 million km), its closest approach in more than 400 years and the closest until 2070.
Fresh radar images reveal an elongated bowling-pin-like shape that someone at NASA described whimsically as “similar to that of the exposed portion of a hippopotamus wading in a river.”
Now that we’re past the winter solstice, the days are already getting longer. It takes at least a month before most of us will notice the change in the time of sunset. Did you know that the sun sets more slowly at the solstices and fastest at the equinoxes? Around the time of winter solstice, the angle the setting sun makes to the horizon is very shallow, closer to horizontal, so it s-l-i-d-e-s along the horizon until it sets. At the equinoxes, the sun sets due west at its steepest angle and goes down almost vertically in a hurry.
At 40° north, the latitude of Pittsburgh and Beijing, it takes about 3¼ minutes to set compared to about 2¾ minutes at the equinoxes. What’s more, Earth is closer to the sun at the winter solstice compared to the summer; its shiny disk appears slightly larger and therefore takes a bit longer to travel through the horizon and set. The sun appears to travel east each day (to the left as you face south) because of Earth’s orbital motion. Since we’re closer to the sun in December it appears to travel east a little faster compared to summertime, delaying the sunset further. From Winnipeg, Manitoba at 50° north latitude, these factors combine to slow the setting sun to 4 minutes and 18 seconds. That’s 8 seconds longer than the summer solstice sunset.