During a tough Massanutten Mountain South Training Run a couple of months ago I entertain good friend Caren Jew with my impromptu lectures on differential equations, on "gold farming" in online multiplayer games, on the rôle of the Nurse in Shakespeare's Romeo and Juliet, on wiki spam, and on the unreasonable effectiveness of mathematics in the natural sciences. Five-plus hours pass as if so many minutes — for me anyway! Yes, I act like Mr. Know-It-All sometimes (always?). But Caren is one tough trail grrrl and survives the ordeal.
My ability to come up with a theory for any phenomenon is momentarily tested, however, when Caren observes the Moon rising in front of us at 3:30pm during the drive home. I am literally stunned. We saw the gibbous Moon setting during our outbound trip at 5:30am, only ten hours earlier. How can it be back so soon? It has to rise 50+ minutes later every day to circle the Earth in one lunar month. Did it take a short cut?
To Caren's vast amusement it takes me several minutes to come up with a theory to explain her observation. We're in the midst of winter, so the Sun is far south. The Moon's orbit is in the plane of the ecliptic and since it's nearly full at the moment, it's almost opposite the Sun, and thus is quite far north. Just as nights (when the Sun is below the horizon) are short in summertime, so also "full-moon-nights" (when the Moon is below the horizon) are short in wintertime. Therefore it's normal to see the Moon rising less than 12 hours after it has set, considering our northly latitude and the time of year.
How obvious — after a bit of head-scratching! (^_^)