What in Lunations!?
Posted December 26, 2006
The term I used in the title, "lunation," means a full lunar illumination cycle, during which the moon makes one complete orbit about the earth and goes through its full series of phases. As it does so, the moon follows and elliptical orbit that brings it a little closer and then a little farther away from the earth, and at the same time it rotates on its own axis in a way that keeps the same side of the moon facing earth at all times. However, its rotation axis is tilted slightly, and that, combined with the non-circular orbit, means that there is actually a bit of "wobble" to the face of the moon as it is presented to us.
We don't usually notice this, because the changes are so slow and we usually see the moon for just a short time each time outing, but this turns out to be one of those natural processes that can be dramatically illustrated through something akin to time-lapse photography, as seen in a nifty pair of movies that can be accessed by scrolling to the bottom of the page at http://aa.usno.navy.mil/faq/docs/moon_phases.html. The larger movie is more detailed, but seems to animate much faster. The smaller movie runs at a slower pace, and, maybe it's just me, but I find it vaguely mesmerizing - makes me think a little differently about the old moon that otherwise seems to hang so stagnant in the sky...