A supermoon occurs when a full moon coincides with the closest point to theEarth that the Moon travels to. They happen about once a year.
The moon's orbit is not circular but elliptical, meaning its distance from the earth can vary between 357,000 km and 406,000 km.
When the Moon is at its closest point, known as 'perigee', it appears up to 14% larger and 30% brighter than at its farthest point, known as the apogee.
An optical illusion means that the moon always looks bigger when it is on the horizon, so when it is rising, it will look particularly impressive.
The moon creates tides on earth, because its gravity pulls harder on the side of the earth which is closest to it. The resultant tides are seen both in the sea and in the rock which moves up and down by up to half a metre.
The fact that the moon is closer at the moment will have an effect on tides - all be it a small one. Tides may be an inch higher, at most, but the difference is likely not to be noticeable for us humans.
The next supermoon will be seen on August 10th, with its closest pass at 7pm GMT. This will be the closest the earth comes to the moon all year, so is worth looking outfor.