Tides
Why does the sea at the beach come in and go out twice a day? Drag the Moon around Earth and watch the oceans bulge, then see how Mumbai's tides change through the day and the month.
Class 9 GravitationTry this
0 of 5 doneMake it high tide in Mumbai.Hint: drag the Moon so it lines up with Mumbai, or with the far side
Watch a whole day. How many high tides does Mumbai get?Hint: press Play and wait at least 25 hours
Make the biggest tides of the month: a spring tide.Hint: line the Moon up with the Sun
Make the smallest tides: a neap tide.Hint: put the Moon at a right angle to the Sun
Switch off the Moon's pull. What's left?Hint: untick Moon's pull
What's happening
The Moon tugs on the oceans
The Moon's gravity pulls a little harder on the side of Earth facing it and a little more weakly on the far side. That stretches the oceans into two bulges: one under the Moon and one on the opposite side. Wherever a bulge is, it's high tide.
Earth turns underneath
Earth spins once a day, carrying each coast through both bulges. But the Moon moves on a little along its orbit, so Earth needs about 24 hours 50 minutes to catch up with it. High tides come roughly every 12 hours 25 minutes, about 50 minutes later each day.
The Sun joins in
The Sun raises tides too, a bit under half as big as the Moon's. At New Moon and Full Moon (Amavasya and Purnima) the Sun and Moon line up and their bulges add: extra-high spring tides. At half Moon they pull at right angles and partly cancel: gentle neap tides.
Who figured this out?
Lothal dockyard
About 2400 BCE, Gujarat, IndiaThis Harappan port has what may be the world's oldest dock. Archaeologists believe it was built so boats could come in on the high tide.
Seleucus of Seleucia
About 150 BCE, MesopotamiaLinked the tides to the Moon and noticed they change in strength with the Moon's position.
Isaac Newton
1687, EnglandExplained in the Principia that tides come from the difference in the Moon's and Sun's pull across Earth, including the bulge on the far side.
Check yourself
1. What mainly causes tides on Earth?
2. About how many high tides does a coast get each day?
3. When do spring tides (the biggest tides) happen?
4. Why are neap tides small?
Common questions
Why do tides happen?
Tides are caused mainly by the Moon's gravity. It pulls more strongly on the ocean nearest to it and more weakly on the ocean on the far side, stretching the oceans into two bulges. As Earth rotates, each coast passes through these bulges (high tides) and the dips between them (low tides).
What is the difference between spring tide and neap tide?
Spring tides are the highest high tides and lowest low tides, happening at New Moon and Full Moon when the Sun, Moon and Earth are in a line. Neap tides have the smallest difference between high and low tide, happening at the first and third quarter Moon when the Sun and Moon pull at right angles.
How many high tides and low tides are there in a day?
Most coasts have two high tides and two low tides in about 24 hours 50 minutes. So high tide comes about 50 minutes later each day. Some places, depending on the shape of the coast and sea floor, get just one high tide a day.
Why is there a high tide on the side of Earth away from the Moon?
The Moon pulls Earth's centre more strongly than it pulls the ocean on the far side. So the far ocean is 'left behind' a little compared with the solid Earth, making a second bulge pointing away from the Moon.
Does the Sun cause tides?
Yes. The Sun is far more massive than the Moon, but much farther away, so its tide-raising effect is a little under half the Moon's. It makes tides bigger when it lines up with the Moon (spring tides) and smaller when it doesn't (neap tides).
Why are tides important?
Tides decide when fishing boats can leave and enter harbours, shape coastlines and mangroves like the Sundarbans, mix ocean water and nutrients, and can be used to generate electricity. Knowing the tide times is essential for safety on beaches.
A simple model of the tide-raising forces. Real tide times and heights also depend on the shape of each coast and the sea floor.
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