The moon is the most recognizable object in the night sky, yet for most people its movements remain a mystery. Take the full moon that will occur on June 21, for example. It will ride strangely low in the sky—so low that it will be lost among the background of buildings and treetops for viewers in the northern United States. In Fairbanks, Alaska, the moon will not even clear the horizon. For everyone there, this will be a June without a full moon.
There is a method to this lunacy, of course. The moon’s place in the sky is determined by its orbit, which in turn is rigorously controlled by gravity acting primarily among the moon, Earth, and the sun. These seemingly simple elements combine to produce an oddly elaborate piece of celestial choreography.
The first surprise is that the moon does not orbit above Earth’s equator. Unlike every other ...
