Supermoon is a natural phenomenon that occurs when the moon gets super close to earth. This makes the moon appear larger and more brighter. But why exactly does this happen and why is it so rare?
The answer lies in the way the moon orbits around the earth. You may recall a diagram from your primary school science class that shows celestial bodies orbiting the sun in a perfect circle. However, the actual orbits are much more complex and are often slightly elliptical. Same goes for the orbit of moon around earth, in fact the oval shape of the lunar orbit causes the distance of the earth and moon to change by more than 50,000 kilometers.
A supermoon occurs when the timing of a full moon coincides with that of the closest approach the Moon makes to the Earth on its elliptical orbit. This makes the moon appear upto 30% brighter and 14% larger. And since this alignment has to match to make a supermoon, they are a rare sight.
So if there is a supermoon out there today, go out and enjoy the view, because you might not be able to see another one for years.