Tide Lab: Two High Tides a Day, Spring and Neap
Understand tides as two bulges of ocean raised by the Moon (and smaller ones by the Sun) that a coast passes through as Earth turns, adding up at new and full moon and partly cancelling at the quarters.
Before you try
What if the Moon vanished? Would the sea still have tides?
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What happened?
Move a control to see what changes and why.
An ocean covering the whole planet with no coasts, so heights are relative; real ranges depend on the shape of the coastline.
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Try these ideas
- Press Full moon and run a day. Count the high tides.
- Press First quarter and compare the range with the full-moon range.
- Find the time of day when your town sits exactly between the two bulges.
Why it works
The Moon pulls the water on the near side of Earth a little more than it pulls Earth’s centre, and the centre more than the far-side water, so the ocean is stretched into two bulges on opposite sides of the planet. As Earth turns once a day, a coast passes through both bulges: two high tides and two low tides, each day about 50 minutes later than the last because the Moon has moved on.
The Sun raises its own pair of bulges, about half as big. At new moon and full moon the Sun and Moon line up and the bulges add, giving spring tides with the biggest range; at the quarter moons they are at right angles and partly cancel, giving neap tides. In this lab you turn the Earth, move the Moon and read the sea level at your town on a graph.
Check your understanding
Question 1 of 4
Why are there two high tides a day rather than one?
Spring tides happen at
Neap tides have
High tide comes about 50 minutes later each day because
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Look up today’s tide times for Mumbai or Chennai and check the gap between highs.
- Ask: why is there a bulge on the side away from the Moon?
- Discuss how fishermen and ports plan around tides.