There are two high tides and two low tides at most coasts each day, roughly 12 hours 25 minutes apart.
Step 1: Something to Notice
Watch first. The explanation comes later.
A tide table for any coast shows high water roughly every 12 hours and 25 minutes — two high tides and two low tides each day, arriving about fifty minutes later each day.
If the Moon is only ever on one side of Earth at a time, why are there two high tides a day and not one?
Step 2: Find Out
Watch the red town go round once and read its tide off the curve underneath.
Step 1 — Predict
In one rotation of Earth, how many high tides will the town pass through?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: Find Out
The Sun raises tides too, at about 46% of the strength. Move it around and watch the range.
Step 1 — Predict
When the Sun and Moon are lined up, what happens to the difference between high and low tide?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 4: So Here Is Why
Now the explanation, after you have seen it happen.
It is the difference that matters
Gravity gets weaker with distance, so the Moon pulls the near water harder than it pulls the solid Earth, and the solid Earth harder than the far water. It is that difference across Earth's width — not the pull itself — that raises tides.
Tidal force The difference in gravitational pull across an object's width, which stretches it.
Stretched, not pulled
The result is that Earth is stretched along the line to the Moon, with a bulge of water at each end. The far bulge is water being left behind, not water being pushed.
Two bulges, one rotation
The bulges stay lined up with the Moon while Earth turns underneath them. Any coast passes through both in one day, which is the two high tides.
The Sun helps
The Sun raises tides too, at about 46% of the Moon's strength. Lined up with the Moon they add, giving the large spring tides at new and full moon. At right angles they partly cancel, giving the small neap tides at the quarters.
Spring tide The largest tides, when the Sun and Moon pull along the same line.
What this model leaves out
Real tides are also shaped by the ocean basin the water sloshes around in. That is why the Bay of Fundy has a range of fifty feet and parts of the Gulf of Mexico get only one tide a day. The two-bulge model explains the cause; the coastline explains the local details.
Two a day, and one on the wrong side
There is a tidal bulge facing the Moon and another on the opposite side. The near side is pulled hardest, the far side least, so the Earth is stretched — which produces two high tides daily.
The Sun matters too
When Sun and Moon line up, their effects add and tides are largest. When they are at right angles the tides are smallest. The pattern of spring and neap tides follows the Moon's phases.
Step 5: A Common Mistake
Lots of people think
“The Moon pulls the ocean toward it, so there is one high tide — on the side of Earth facing the Moon.”
Step 6: Why the Tide Is Later Each Day
The same idea somewhere new.
Earth takes 24 hours to spin once, but the Moon has moved about 13° further round its orbit in that time, so the town needs an extra 50 minutes to catch up with it. That is why high tide is roughly fifty minutes later each day, and why the tide table on a harbor wall is a list rather than a single time. The same arithmetic explains why the tidal day is 24 hours 50 minutes rather than 24.
Step 7: Think It Through
Practice makes it stick.
The Tide Table
Problem 1 of 2
High tide today is at 6:00 in the morning. Tides run about 12 hours 25 minutes apart. To the nearest hour, what time is the next high tide? Give a 24-hour clock hour.
hundred hours
Catching Up
Problem 2 of 2
Earth spins once in 24 hours, but the Moon has moved on, so the town needs about 50 more minutes to line up with it again. How many minutes is a tidal day?
minutes
Bulges and Ranges
1 of 4
How many high tides does a coast get in one rotation of Earth?
2 of 4
What causes the bulge on the side facing away from the Moon?
3 of 4
At which two phases of the Moon do spring tides happen?
4 of 4
Match each arrangement to the tide it produces.
Tap a card on the left to start.
Step 8: Quick Check
Show what you know.
Question 1 of 1
Why are there two high tides a day rather than one?
Step 9: Explain It in Writing
Claim, evidence, then reasoning.
The question
A tide table shows two high tides a day. A classmate says this is impossible if the Moon pulls the water toward itself, because the Moon is only on one side. Using the model, explain what causes the second high tide.
Words to usetidal bulgerotationstretchfar sidepulldifference
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Tides come from the difference in the Moon's pull across Earth, not from the pull itself.
- Earth is stretched along the Moon line, so there is a bulge at each end.
- One rotation past two bulges is two high tides a day.
- Sun and Moon aligned gives spring tides; at right angles gives neap tides.