Last month I wrote about the tides and why we get two high tides and low tides per day in NSW and why their timing changes each day. It was all about the way the earth, moon and sun interact and the gravitational pull that the moon exerts on the oceans. But there’s much more to tides than that.

I remember watching early seasons of the TV show Survivor and how the tribes would always build their shelter at the back of the beach, not noticing that the water crept closer every night until one night, to their amazement, they were finally swamped. Aside from the fact that they clearly didn’t pay attention to previous seasons of the show, they also didn’t understand tide range.

Tide range is the vertical elevation difference between high and low tide – think of it as the height of the tidal wave. Unlike many Survivor contestants, you may have noticed that the high tide creeps a little bit higher on the beach each day and then starts retreating on a daily basis. At the same time, the water seems to go further out at low tide and then not so far out. What’s going on?

Well, this is where the sun comes in. Both the earth and the moon orbit around the sun. When the moon and the sun line up, their gravitational pull on the oceans is combined and we get bigger tides. The high tide comes up higher and the low tide goes out further, creating a large tide range. These are called ‘spring tides’ and occur around a full and new moon, so about every two weeks. Conversely, when the moon and sun are lined up at right angles to the earth, their gravitational pull acts against each other and we get ‘neap tides’ where the high tide doesn’t come up very far and the low tide doesn’t go out very far – a small tide range. So every lunar month (about 29 days) we get two spring tides and two neap tides. ‘King tides’ are just a bigger spring tide that occurs when the moon and sun happen to be particularly close to the earth and that happens about twice a year (around Christmas and late May/early June).

While our tide range in NSW varies from about 1m at neaps to almost 2m at king tides, tide range varies around the world with some places, like Broome, getting 9m tides. It’s all about the shape of continents, offshore reefs and continental shelves squeezing and amplifying the tidal wave itself. If all this cosmic stuff has you bamboozled, just read the tide tables towards the back of every Flame issue – it’s all predictable years in advance!

Have a question? Email rbrander@unsw.edu.au