Whyscope Question 42 Chapter 4 · Ocean & Water

Why do tides go in and out?

The Moon’s pull is strongest on the side of Earth facing it, so water there bulges toward the Moon. On the far side of Earth, the water tries to keep going in a straight line, which makes a second bulge. Earth spins through both bulges, so many beaches get two high tides and two low tides a day. The Sun pulls on the oceans too, but its effect on tides is only about half as strong as the Moon’s.

whyscope.org/q/42

How do we know?

↻ One of the pages with a moving picture

A tide gauge in San Francisco has been measuring since 1854.

Simplified and not to scale. The numbers match the steps below.

The Moon’s pull is strongest on the side of Earth facing it. Water there bulges toward the Moon.
On the far side of Earth, the water tries to keep going in a straight line. That makes a second bulge.
Earth spins through both bulges. So many beaches get two high tides and two low tides a day.
The Sun pulls on the oceans too, but its effect on tides is only about half as strong as the Moon’s.

Since 30 June 1854, a tide gauge in San Francisco has measured the water level. It is the oldest one still working in the Western Hemisphere!

Where the sea sloshes in time with the tides, they grow huge. The water in Canada’s Bay of Fundy sloshes about every 12 hours, so these are the highest tides in the world.

Mont-Saint-Michel in France is an abbey on a rocky island in the middle of sandbanks, and it is known for its powerful tides.

Wow number

53 feet is how much the water can rise from low tide to high tide in Canada’s Bay of Fundy. These are the highest tides in the world!

Myth or fact?

Spring tides only happen in spring. Myth. They happen twice a month, all year long.

The Great Lakes have tides, but they’re tiny. Fact. Under 5 cm (about 2 inches)!

Think about it

Why do you think a boat might get stuck on the sand at low tide?

Where in the world?

Minas Basin, Bay of Fundy, Canada where the water sloshes in this bay about every 12 hours, in time with the tides, so they grow huge. Mont-Saint-Michel in France sits on a rocky island among sandbanks with powerful tides — 45.2833° N, 64.2° W.

Where these facts come from

Every number on page 42, and who measured it

This page explains the two tidal bulges.

Gravity, Inertia, and the Two Bulges — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html

We checked it on 31 October 2026

This page tells us that many places get two high tides a day.

Tides — NOAA, the U.S. weather and ocean agency
noaa.gov/jetstream/ocean/tides

We checked it on 31 October 2026

This page tells us how strong the Sun's pull on the tides is compared with the Moon's.

What Causes Tides — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/education/tutorial_tides/tides02_cause.html

We checked it on 31 October 2026

This page tells us about the 53-foot tides in Minas Basin and why the bay sloshes so much.

JetStream Max: Bay of Fundy: The Highest Tides in the World — NOAA, the U.S. weather and ocean agency
noaa.gov/ocean/fundy-max

We checked it on 31 October 2026

This page tells us where the highest tides in the world are.

Where is the highest tide? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/highesttide.html

We checked it on 31 October 2026

This page tells us about Mont-Saint-Michel and its powerful tides.

Mont-Saint-Michel and its Bay — UNESCO World Heritage, the United Nations' list of the world's most special places
whc.unesco.org/en/list/80

We checked it on 31 October 2026

This page tells the history of the San Francisco tide gauge, working since 1854.

NOAA's tides and currents scientists
tidesandcurrents.noaa.gov/publications/tides_of_history.pdf

We checked it on 31 October 2026

This page also tells us about the 1854 San Francisco tide gauge.

Pier & Tide Gauge House at the Greater Farallones National Marine Sanctuary — NOAA's Greater Farallones marine sanctuary, near San Francisco
farallones.noaa.gov/about/pier.html

We checked it on 31 October 2026

This page tells us that spring tides happen twice a month.

What are spring and neap tides? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/springtide.html

We checked it on 31 October 2026

This page tells us how tiny the tides in the Great Lakes are.

Do the Great Lakes have tides? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/gltides.html

We checked it on 31 October 2026

Has this changed?

Why the tides rise and fall is not going to change.

Tide times change every day, so they are looked up ahead of time for each beach.

Words from this page

tide: The slow rise and fall of the sea.
spring tide: A tide with an extra-high high point and an extra-low low point. They happen twice a month.
tide gauge: An instrument that measures the water level.

Try it!

Track the tides at your beach.

1. With a grown-up, look up tide times for a beach online. 2. Write down the high-tide times for three days. 3. Compare the times. Are they the same each day?

What happens: many beaches get two high tides a day. Look closely: do the times stay the same, or move a little each day?

Go further

Gravity, Inertia, and the Two Bulges Tides