This past Sunday began the last lunar cycle of the year, which, for the first time in almost 20 years, includes a full moon on the winter solstice itself. I'm going to spend some of the time between now and then using the moon as a guide for a little tour of the night sky.
Tonight, if your clouds part enough in the evening to reveal the just-set sun - very nearly as far south as it will ever get - you should also get to see a tiny sliver of a new moon hanging like an ornament above it in the not-yet-dark skies. This sliver moon is, to my mind, one of the most impressive sights to periodically grace our skies. To me, the ethereal part is not the sliver itself, looking like a razor sharp sickle glowing in the sky, but the ghostly outline of the rest of the moon that can be faintly seen.
What is that ghostly outline? If you pay close attention you might even noticed that it disappears after a few days. By the time the moon is up to first quarter all you see is that bright sunlit half of the orb. It's hard to tell precisely what is happening, because as the moon waxes towards full it gets brighter and brighter and you might just think that you're having a harder time seeing that ghostly outline in the presence of that brighter moon. But, no, the outline is indeed getting fainter.
What's going on? With a little thinking about what is illuminating the moon we can figure it out pretty easily and even make sense of the little details of when it is brighter and when fainter.