About once every year, this river in China will exhibit Sea-like waves that attract professional surfers from all over the world.
The wave occurs when the Sun and the Moon are both in just the right place such that the gravitational pull of not only the Moon, but also the Sun, influences the tide of the river as it travels downstream.
The reason this only happens about once every year, and not all the time, is because the placement of the Moon and the Sun in the right place only happens about once every year - other times of the year, the Moon and the Sun are in different locations in the sky and the gravitational force isn't strong enough in a single direction to create this effect.
While it's widely known that the Moon's gravitational forces influence the world's tides, it's often overlooked that the Sun, as far away as it is, can also have an effect.
