A rare full moon on Christmas
If you're connected at all to the Internet you've likely seen the reports of a full moon on Christmas this year, and that this is a relatively rare phenomenon. Well, it's true.
Because our moon orbits the Earth every 28 days or so, the full moon doesn't always occur on the same calendar date each month, and the last time a full moon fell on Dec. 25 was in 1977. I remember gazing at it while listening to the music of "Saturday Night Fever" and wearing a yellow leisure suit. (OK, not really!)
If Santa brings you a telescope, you'll certainly want to turn it toward the moon that night, but you might be a bit disappointed. You see, the full moon lies directly opposite the sun and, being lit directly from the "front," displays no shadows at all. And it is these shadows that show the moon in all its 3-D glory. In other words, the full moon is one of the worst times to view our celestial neighbor with a telescope. Nevertheless, you may see some interesting features.
The most obvious of these are the dark patches that cover much of its face. Some ancient sky watchers believed that these were lunar oceans, and named them "mare" or "seas," a name they still carry today — even though we know there are no oceans there. Others suggested that, perhaps, the moon was a great mirror, which merely reflected the light from terrestrial land and seas.
Today we know that these dark regions are relatively flat, solidified lava plains, caused by molten material seeping out from the moon's interior during its early history. Perhaps the most famous of these "seas" is the Sea of Tranquility, made famous by the Apollo XI mission which landed the first man on the moon. It lies on the western-most side of the moon and, at moonrise, appears near the moon's upper right side.
Since the full moon lies directly opposite the sun in our sky, it is the most contrary of all celestial bodies, and it does everything opposite the sun.
For example, the full moon always rises at sunset and sets at sunrise. If you ever need to know just where the full moon will rise over the eastern horizon, pay attention to your shadow at sundown; it'll point right toward the spot of moonrise.
The full moon's motions are also opposite those of the sun. In summertime, for example, the full moon rises in the northeast, passes high overhead, and sets in the northwest. And the summertime full moon — just to be contrary — rises in the southeastern sky, passes low to the south around midnight, and sets in the southwest at sunrise.
Six months later, in the wintertime, the sun rises in the southeast, passes low across our southern sky at midday, and sets in the southwest. The winter full moon does the opposite — rising in the northeast at sunset and setting in the northwest at sunrise.
When you step outside late on the night of Dec. 25 and gaze skyward, you'll surely be stunned at the elevation, brilliance and beauty of the 2015 Christmas full moon.
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