Supermoon
The Moon has an elliptical orbit around Earth, and the Earth is not exactly in the middle (like the yolk in the egg). Therefore each month the Moon has a close position to Earth (Perigee) and a far position to Earth (Apogee). When it is close, it appears on average 14% larger (in diameter) and 30% (surface area) brighter when it is full moon. When both Perigee moon and full moon occur together, we speak of a supermoon (in far position of a micromoon).
The Earth has a similar elliptical orbit around the sun and also has a close and far position to the sun, together with the Moon. Hence, sometimes the gravity effect of the Sun is stronger than other times, causing the Moon to vary in its far and close position. The close position varies roughly between
356.000 and 371.000 km while the far position varies roughly between 404.000 and 407.000 km.
Far and close distances for every moon rotation from 2001 to 2100. Data Source: Moon Perigee and Apogee Table Courtesy of Fred Espenak, www.Astropixels.com. Full moons are in yellow, data Source: Keith Cooley, http://home.hiwaay.net/~krcool/Astro/moon/fullmoon.htm.
When the Earth would not turn around the Sun, the close distance would always fall together with a full moon. But because of the rotation around the sun, the light angle changes, causing a small shift in the cycle of full moon. After 14 moon cycles, both cycles are in the same position again, with a full moon in the close position, hence almost once every year! The interesting thing is that it is mostly never the same position relative to the sun, and therefore somewhere between 356.000 - 371.000 km as explained above. And that makes the the 14.11.2016 special; this full moon in Perigee position is closer than after most other 14 moon cycles. A super close, superbig moon! However this difference is much smaller than between a Perigee and Apogee month.
A close-up of the previous graph (only the very low band with full moons!), with the 100 closest moons from 2001 to 2100 and full moon in yellow and the dates everyone is talking about in orange. Closer moons will follow several times this century, so it is not so rare. And remember from the first graph: the distance and hence appearance diameters of all full moons in this graph do not much differ to the eye and are actually all supermoons!
The moon illusion
People often think they see a supermoon. But in fact they are victim of the so called moon illusion! A very good explanation is on Wikipedia.
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