Alex McColgan

You likely also know that this happens on the side of the planet furthest away from the moon,

Rather than being caused by gravity, this second bulge is caused by centrifugal forces, as the Earth and the Moon's gravitational pull on each other causes them to behave like two dancers holding each other by the arms and spinning across the dance floor.

While it's mostly the moon moving, due to the Earth being much more massive, the Earth is also swung around a little.

The water behind it is thus trying to fling off into space through its raucous spinning, causing the second high tide.

The Sun also has a role to play in tide formation, albeit to a lesser degree.

It's a bigger mass, which would cause a greater pull if it were closer, but its further distance means that the Sun's effect is only one third as big as the Moon.

When the Moon and the Sun are aligned, we get extra large tides, called spring tides.

This happens six to eight times a year.

When not aligned, they partially cancel each other out, causing smaller tidal extremes known as neap tides.

So, now consider the influence of lunar nodes on this tidal tug of war.

During spring tides, the pull of the Sun and the Moon working in unison causes the highest tides and the largest risk of floods.

However, the Sun and the Moon are never more aligned than they are at a node.

During the rest of each 9.3 year phase, they are not quite tugging in the same direction, so tides are more temperate

At a node, that's where things get more serious, and risk of floods become highest.

The last time this alignment occurred in September 2015, the UK and the US both issued major flood warnings to its citizens.

In September itself, there were floods, albeit minor ones, but it was only when heavy rain combined with the strength of the lunar nodes a couple of months later that the real damage was inflicted.

In the US in October, South Carolina saw flash flooding that caused property damage and people having to be rescued by emergency services.

At the end of December 2015, the UK was hit by some of the worst floods it had seen in a century.

Combined with the power of Storm Desmond, flooding and storm damage caused an estimated £1.3 billion in damages.

These floods can be highly damaging.