What causes tides?

You'll remember the familiar story from when you were in School, Isaac Newton sat under an apple tree when an apple fell, and the theory of universal gravitation was born. That force created by the earth caused the apple to fall to it.

How the moon causes tides

It's that very same force of gravity that influences our tides, except it's the gravitational force of the moon (and the sun) on our seas and oceans that causes tides.

As we can see above, the moon has a gravitational pull on the sea. This pull causes the tides to rise on the side of the earth closest to the moon. This is called a tidal bulge. In addition, on the opposite side, we get a tidal bulge too, causing a high tide on the side of the earth that's away from the moon. This is caused by inertia force.

Why are there two high tides a day?

The moon orbits the earth every 27 days. Throughout that time, its gravity pulls the sea towards it, resulting in the tidal bulge. At the same time as the moon is orbiting, the earth rotates on its axis one full turn every 24 hours, meaning that bulge is present for the same location roughly twice a day, one for the tidal bulge closest to the moon and one for the bulge opposite to it.

The sun's impact on the tides

As I mentioned, it's not just the gravitational pull of the moon that influences our tides, the sun has an impact on it too. When the sun and moon align, their gravitational pulls combine which results in a spring tide. You can read more about spring and neap tides in another article I've written.

Quick facts about tides

• There's a high tide every 12 hours and 25 minutes, the same applies to low tides
• The highest tidal range in the world can be found at the Bay of Fundy in Newfoundland, Canada
• There's a spring tide every 14-17 days, or twice per lunar month
• Spring tides are when the tidal range is at its highest
• Spring tides are caused by the combined gravitational pull of the sun and moon when they're aligned
• Neap tides are when the tidal range is at its lowest
• Neap tides occur 7 days after a high tide

Now, you'll understand a little more about tides and how they occur. If you want to learn more about spring and neap tides, I've got an article for that. Maybe you want to learn more about the Bay of Fundy? I've got an article for that too!