This animation shows at hourly intervals, the geocentric phase, libration, position angle of the Moon’s axis, and apparent diameter of the Moon throughout the year 2013.
The Moon always keeps the same side to us, but not always the same face. Because of the tilt and shape of its orbit, Earth observers see the Moon from slightly different angles over the course of a month. When a month is compressed into 24 seconds, as it is in this animation, the changing view of the Moon makes it look like it is wobbling. This wobble is called libration.
The most noticeable monthly variation in the Moon’s appearance is the cycle of phases, caused by the changing angle of the Sun as the Moon orbits the Earth. The cycle begins with the waxing (growing) crescent Moon visible in the west just after sunset. By first quarter, the Moon is high in the sky at sunset and sets around midnight. The full Moon rises at sunset and is high in the sky at midnight. The third quarter Moon is often surprisingly conspicuous in the western sky long after sunrise.
CREDIT: Ernie Wright, Scientific Visualization Studio (SVS), NASA Goddard Space Flight Center, Greenbelt, Md.