The term Supermoon is not a real scientific definition, but a word invented by the media to describe a full moon that looks slightly larger than the average.
To understand the origin of this term we must first consider the orbit of the Moon around the Earth. Our satellite is in fact moving on an elliptical orbit, whose distance from our planet varies during a revolution between 355 thousand km and 405 thousand km. The point of minimum distance is called perigee, while the one in which the separation between the two bodies is maximum is known as apogee.
Since there is no correlation between the phases of the Moon and its position along the orbit, a full Moon can occur at any distance from Earth between the apogee and perigee. The term Supermoon has been coined to identify the full Moons that occur near perigee: since at this point the distance from Earth is minimal this full Moon appears slightly larger than a normal full Moon.
In addition, although less known, the term Micromoon is used for those full Moons that occur at the apogee.
It can easily be inferred that a Supermoon is not a very rare phenomenon. Of the full moons that occur each year, at least three are supermoons.
That said, it is virtually impossible to distinguish the difference in size between a Supermoon and a Micromoon with the naked eye. The only way to make a difference is to compare them in an image like this.
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