Tide Turner

Tide Turner: spring and neap tides for Cambridge IGCSE Marine Science

This free game is part of our Cambridge iGCSE Marine Science (0697) course for exams in EXAM YEARS 2027-2029.

High and low tides: the two tidal bulges

In Tide Turner you are the harbour master of Port Haven. You see the Earth from above the North Pole, wrapped in an ocean that is hugely exaggerated so the tides are easy to see. It covers the tides part of the Cambridge IGCSE Marine Science (0697) course: how high and low tides, and spring and neap tides, are produced by the gravitational effects of the Moon and Sun.

• The Moon's gravity pulls the ocean into two bulges. One bulge faces the Moon. The other is on the far side of the Earth.
• Why there is a far-side bulge. The Moon pulls hardest on the water nearest to it, a bit less on the Earth's centre and least on the water on the far side. The Earth is pulled away from that far water, so it is left behind as a second bulge.
• High tide and low tide. A place inside a bulge has a high tide. A place halfway between the bulges, where water has been drawn away, has a low tide.
• Two tides a day. The Earth spins once a day under the bulges, so most coasts get two high tides and two low tides a day. The Moon moves on a little each day, so high tides come about 12 hours 25 minutes apart.

Spring tides and neap tides

The Sun's gravity makes bulges too. The Sun is far bigger than the Moon but much further away, so its bulges are a bit less than half the size of the Moon's. In the game you drag the Moon round its orbit and watch the Sun's bulges add to or partly cancel the Moon's.

• Spring tides happen when the Sun, Moon and Earth are in a straight line, at new moon and full moon. The effects add together: the highest high tides, the lowest low tides and the biggest tidal range. The name has nothing to do with the season.
• Full moon is a spring tide too. The Sun and Moon are on opposite sides of the Earth, but each one bulges both sides of the Earth, so their bulges still line up.
• Neap tides happen when the Sun and Moon are at right angles, at first quarter and third quarter. The Sun's bulge fills in the Moon's low tide, so the high tides are lower, the low tides are higher and the tidal range is smallest.
• Spring and neap tides come about 7 days apart, so there are two of each in every 29.5-day lunar cycle.

How to measure tidal amplitude

In the Tide Pole mission you go down to the harbour and measure the tide yourself.

• Use a tide pole: a board marked in metres and 10 cm blocks, fixed firmly to a harbour wall or pier so it cannot move, and long enough to stay in the water at low tide.
• Read the water level at regular intervals, for example every hour, through a full tidal cycle so that both high tide and low tide are caught.
• Waves make the water bob up and down, so read the middle of the waves (the still-water level), not the tops.
• Plot the readings, find the highest (high tide) and the lowest (low tide), then tidal amplitude = height of high tide − height of low tide.

At Port Haven the spring tide amplitude is 4.6 m and the neap tide amplitude a week later is only 1.8 m, which shows that the amplitude is biggest at spring tides and smallest at neap tides.

The five missions

Two Bulges: turn the Earth to give ports a high or low tide and discover the far-side bulge. Busy Coast: requests pop up at ports all round the Earth against the clock. Sun and Moon: line up the Moon for spring tides and swing it to right angles for neap tides. Tide Pole: read the pole every hour, plot the graph and work out the tidal amplitude on a spring day and a neap day. Harbour Master: everything at once, ending with the biggest high tide of all. Every mistake shows a short explanation, and quick checks between missions test the key ideas.