Tides and the Moon

A tide is the regular, daily rise and fall of the level of the sea. At high tide the water creeps up the beach and covers the sand. A few hours later, at low tide, the water has pulled back and leaves wet sand, rock pools and seaweed behind. This rise and fall happens over and over, day after day, all around the coast of Australia.

The most important thing to notice is that tides are predictable and observable. They follow a steady pattern that scientists can calculate years ahead. This is exactly the idea from your NSW syllabus dot point: predictable and observable events on Earth, such as the tides, are caused by the relative positions of the Sun, the Earth and the Moon (SC4-OTU-01).

That is why the Bureau of Meteorology can publish tide charts, why surfers check the tide before a session, and why boat skippers plan their trips around high and low water. None of this would be possible if tides were random. They are caused by gravity, and gravity behaves in a regular, reliable way.

• High tide: the sea is at its highest level and the water is furthest up the beach.
• Low tide: the sea is at its lowest level and the water has pulled furthest back.
• Tidal range: the difference in height between high tide and low tide. At Broome in Western Australia this range can be more than 9 metres.

Gravity is the invisible pull that every object has on every other object. The Moon is large and fairly close to Earth, so the Moon's gravity pulls on everything here, including the ocean water. Because water can move and flow easily, the Moon's pull tugs the ocean into a tidal bulge, a place where the sea is pushed higher, on the side of Earth facing the Moon.

There is also a second bulge on the opposite side of Earth, the side facing away from the Moon. So there are two bulges of high water, one near the Moon and one on the far side, with lower water in between.

Now here is the clever part. The Earth spins right round once a day. As it turns, each coast is carried through both bulges and both of the low spots. So most places, including most of the Australian coast, pass through about two high tides and two low tides every day. A high tide rolls in roughly every 12 hours or so.

The Moon is the main cause of tides, but the Sun pulls on the ocean too. The Sun is much further away, so its pull on the tides is weaker than the Moon's, but it still matters. Whether the tides are very big or quite small depends on how the Sun and Moon line up with Earth.

Spring tides, the biggest tides: When the Sun, the Earth and the Moon all line up in a row, the Sun's pull and the Moon's pull act in the same direction and add together. This makes the bulges extra large, so high tides are very high and low tides are very low. This lining-up happens at new Moon and at full Moon. Tides like this are called spring tides. (Warning: the name spring tide has nothing to do with the season spring. It means the tide seems to "spring up" high, and it happens all year round.)

Neap tides, the smallest tides: When the Sun and the Moon are at right angles to each other, as seen from Earth, their pulls partly cancel out. This makes the bulges smaller, so high tides are not very high and low tides are not very low. The difference between high and low water is small. This happens at the first quarter and last quarter Moon. Tides like this are called neap tides.

Because spring and neap tides are tied to the positions of the Sun, Earth and Moon, the shape of the Moon in the sky gives you a clue about the tides. The phase of the Moon (how much of it is lit up) tells you where the Moon is in its path around Earth.

• At new Moon (the Moon is dark) and at full Moon (the Moon is fully lit), the Sun, Earth and Moon are lined up, so you get the big spring tides.
• At the first quarter and last quarter Moon (you see a half-lit Moon), the Sun and Moon are at right angles, so you get the small neap tides.

This is why the tides repeat in a steady cycle that follows the Moon. Over hundreds of years, people living on the coast learned to read the Moon and the tides together. Today the Bureau of Meteorology uses the known positions of the Sun, Earth and Moon to calculate tide times for every Australian port, often years in advance. Surfers, fishers, sailors and people gathering shellfish all rely on these predictions.

Australia has some of the most dramatic tides in the world. The shape of the coast can funnel the water and make the tidal range even larger.

• Broome and King Sound, Western Australia: the tidal range here is huge, often more than 9 metres and reaching around 11 metres near Derby. The sea pours in and out so fast it can look like a moving wall of water.
• Surfers and the beach: the best waves at many surf beaches depend on the tide, so surfers check tide charts before they paddle out.
• Boating and fishing: boat skippers plan to enter shallow harbours at high tide so they do not run aground. Fishers know that fish often feed as the tide turns.
• Staying safe: people exploring rock platforms or mud flats must watch the tide, because a fast incoming tide can cut off the way back.

Every one of these uses depends on tides being predictable. Because we understand that tides come from the steady positions of the Sun, Earth and Moon, we can plan around them with confidence.