Coaching Centre
Tides
How the Moon's gravity makes two tidal bulges, why there are two high tides about every 24 hours 50 minutes, the Sun's smaller effect, spring and neap tides, reading a tide table, and Australia's extreme tides in the Kimberley.
In Talbot Bay in the Kimberley, the tide can rise or fall by more than 10 metres. It pours through two narrow gaps in the cliffs so fast that the water on one side is metres higher than the other, making a waterfall that runs sideways. The "Horizontal Falls" switch direction every few hours, and they are driven by something nearly 400 000 km away: the Moon.
The Moon pulls on the oceans
Every object with mass attracts every other object with a force called gravity. The bigger the masses and the closer they are, the stronger the pull. The Moon is much smaller than the Sun, but it's so much closer that its pull on Earth's oceans has more effect.
The Moon pulls hardest on the side of the Earth facing it, so the ocean there is pulled into a bulge. On the opposite side, the Moon pulls on the solid Earth more than on the distant water, so the Earth is pulled away from that water, leaving a second bulge. There are always two bulges: one facing the Moon and one on the far side.
Two high tides a day
As the Earth spins, each coastline is carried through both bulges, so most places have two high tides and two low tides a day. Because the Moon is also moving along its orbit in the same direction, the Earth has to turn a little further each day to catch up with it. So the tides arrive about 50 minutes later each day, and high tides are about 12 hours 25 minutes apart.
Spring and neap tides
The Sun makes tidal bulges too, a bit less than half the size of the Moon's. How the two sets of bulges combine depends on the phase of the Moon:
- Spring tides happen at new moon and full moon, when the Sun, Earth and Moon are in a line. The bulges add together, giving extra-high high tides and extra-low low tides. (The name has nothing to do with the season: it comes from the water "springing" up.)
- Neap tides happen at first and last quarter, when the Sun and Moon pull at right angles. The bulges partly cancel, so the difference between high and low tide is smallest.
So there are two spring tides and two neap tides every month.
- Start at day 0 (new moon) and look at the graph. How many high tides does Sydney get in a day?
- Move to day 7 (first quarter). What happens to the gap between high and low tide?
- Press "Run time" and watch Sydney travel through the bulges. Where is it when the graph peaks?
Tide times at Bondi on one Sunday: 1:44 am low 0.4 m; 8:19 am high 1.9 m; 3:01 pm low 0.2 m; 8:57 pm high 1.4 m.
Time between the two high tides: from 8:19 am to 8:57 pm is 12 hours 38 minutes, close to the average of 12 hours 25 minutes.
Tidal range in the afternoon: high 1.4 m − low 0.2 m = 1.2 m.
Prediction: the morning high tide on Monday should be roughly 50 minutes later, around 9:10 am.
Big tides and small tides
The open ocean only rises and falls by about half a metre. The shape of the coast makes a huge difference. In wide, shallow, funnel-shaped bays the water piles up, and the tidal range can be enormous: more than 11 metres at Derby in King Sound, Western Australia, one of the largest tidal ranges in the world, and about 16 metres in the Bay of Fundy in Canada. Sydney's tidal range is usually only 1 to 2 metres.
Tides matter to a lot of people: ship captains entering ports, fishers, surfers, rock-pool explorers and anyone launching a boat. Plants and animals on rocky shores are sorted into zones by the tide. Barnacles and periwinkles high on the rocks must survive hours out of the water in the sun, while sea anemones and many seaweeds live lower down, where they're only uncovered at the lowest tides.
Test yourself
Tides are mainly caused by
The Moon's pull makes the tidal bulges.
Most beaches have high tides about every
Two high tides in each 24 hours 50 minutes.
Spring tides happen when
At new and full moon the pulls add together.
During which phase of the Moon would you expect a neap tide?
At the quarters the Sun and Moon pull at right angles.
High tide at a beach is at 6:00 am today. The next morning's high tide will be at about
Tides arrive about 50 minutes later each day.
Label the tide diagram.
Spring or neap?
A family wants to explore rock pools at the lowest possible tide. Explain which phase of the Moon they should choose, and why.
They should go at a new moon or full moon, at low tide. At these phases the Sun, Earth and Moon are in a straight line, so the Sun's gravity adds to the Moon's. This makes the tidal bulges bigger, giving spring tides with the largest range: the high tides are highest and the low tides are lowest, uncovering more of the rock platform.