While a "star" plays a central role in the Christmas story, it was the moon that dominated the night sky this year. Rising on Christmas Eve, the near-full moon produced shadows on the snowy landscape and was still shining brightly in the western sky early this morning.
When the moon is full, we always see the same side of its sphere; however, with all due respect to Pink Floyd, there is no permanently dark side of the moon. And, contrary to popular belief, the phases of the moon are not produced by Earth's shadow across our lone satellite; rather, they represent our changing view of its lit surface. Like the Earth, half of the moon is lit by the sun at all times (except during a lunar eclipse), but, as it orbits our planet, we see an ever-changing image of its lighted surface. Since the moon's rotation rate is synchronous with its 28-day orbit, we always see the same side of the moon when it is full and, from Earth, never see about 40% of its surface.
Like the Star of Bethlehem, which may have been a comet or just a fictional image, all features of the night sky are interpreted by humans from our unique perspective on planet Earth. This often leads to false conclusions and, throughout human history, has produced many rituals and erroneous beliefs. Modern science, in its effort to understand and explain our Universe, must also confront these ingrained convictions, most of which are sustained by mythology and religion.
