The other day there was a partial Lunar eclipse (shown in this photo from NASA’s Astronomy Picture of the Day website). A friend of mine, James, noted that the Moon was really big that night too as it rose with a chunk missing in the evening. The size is exaggerated in the above image by a clever use of a zoom lens but regardless the moon was a little larger than usual.

The Moon goes through a cycle during its orbit, being at one point closest and at another, farthest. If these points occur during the Full Moon the effect is very obvious to the eye as shown in the two comparison photos below.

The Moon’s orbit is not circular in shape but rather it is elliptical (as are all orbits) and thus it cycles between being nearer to us, at what is called perigee (~348,000 km), and then farthest from us at apogee (~398,000 km). In August, this year perigee occurred just one day before full moon.

The Moon takes 27 days and 7 hours to complete a rotation around the Earth. However during that time, the Earth has moved in its own orbit around the Sun and so it takes 29 and a half days to go from Full Moon to Full Moon since the lunar phases depend on the angle between the Sun, Moon and Earth.

Therefore if apogee or perigee occur at Full Moon in any given Month they will next occur slightly earlier in the next Month and so on back through the lunar cycle. This neat animation shows both effects quite nicely in one complete lunar phase cycle.

Therefore if apogee or perigee occur at Full Moon in any given Month they will next occur slightly earlier in the next Month and so on back through the lunar cycle.

So large moons occur once a month, but the effect is most pronounce at Full Moon. They occur at a different time each month and the cycle is not annual. It takes many decades for a large, full moon to land exactly on the same day in the year.

As a side note, down the left hand side of the answer is a scale picture of the Earth Moon system. The moon here is shown midway between apogee and perigee but it does give you an idea of the sizes and angles involved.