The Moon
A supermoon happens when a full moon coincides with the Moon's closest approach to Earth.
A supermoon is a full moon that occurs when the Moon is near perigee, its closest point to Earth, making it appear up to 14% larger and 30% brighter than average.
The Moon's orbit is elliptical, so its distance from Earth varies between about 356,500 km (perigee) and 406,700 km (apogee).
When a full moon happens within about 90% of perigee distance, astronomers call it a supermoon. The effect is most noticeable near the horizon due to the Moon illusion.
Typically three or four times per year, though the exact count depends on the definition used.