October’s Full Hunter’s Moon nearly coincides with the apogee of the moon’s orbit, or the point at which the moon is farthest from Earth. That makes October’s full moon appear smaller than usual, the opposite of the “supermoon” effect that occurred earlier last March when the moon was full during its closest approach to Earth.
The moon reached its peak fullness at 2:06 a.m. UT last Oct. 12. Shortly thereafter, the moon was at its farthest point from Earth, which it reaches once a month. The moon’s orbit is elliptical rather than perfectly circular, which is why the distance from Earth to the moon varies by tens of thousands of miles depending on the time of month and year. The moon’s orbit is also always slightly changing because of differing effects of the sun’s gravity.
Though we couldn’t notice with our own eyes, the Moon’s apparent size changes throughout the year and this is because of the phenomenon called Lunar libration, or the wobbling of the Moon.
Below is an animation which demonstrates this effect. It shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2011, at hourly intervals.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio