You’re spinning. We all are. Even as you read this we’re whipping around with the Earth as it rotates on its axis and revolves around the sun. You may not feel it, but that spin is putting some real force on the Earth’s water—mainly its oceans.
How do we know this? Because of tides. The water on Earth is pulled toward the moon creating a bulge of water. This bulge is high tide. As the Earth rotates, the position of the bulge on the earth moves. But here’s the thing, there isn’t only ONE high tide a day. In most places there are TWO high tides a day, which are about 12 hours and 25 minutes apart. This means there are two bulges.
So how the heck does that happen? It has to do with the spin. When the Earth spins it throws the water out away from the center of the spin. So there are two bulges -- one toward the moon and one away from the moon.
You can recreate this spin and fling water about, just like the Earth does, right in your own home.
What You Need
• A paper or plastic cup
• Hole punch
What You Do
• Punch two holes about ½ inch from the rim of your cup on opposite sides.
• Thread your string through the two holes and make a knot about 4 inches above the mouth of the cup. It should form a triangle.
• Fill your cup about ¾ of the way with water.
• You may want to do this part outside so you’ll have enough room: Start to swing the cup back and forth until you build up enough momentum to swing the whole thing around in a circle like a Ferris wheel. Swing hard and fast and eventually you can even swing the water above your head as if you’re doing a lasso trick.
• To stop, slow down and let the cup swing to a slower circle before stopping so you don’t get a shower.
What’s Going On?
Gravity and inertia. When the cup of water is just hanging, it is pulled toward the center of our planet by a force called gravity. This is the same force that pulls the water toward the moon. The moon‘s gravity pulls on the water closest to it and makes a bulge. That bulge is high tide.
There’s another force in motion called inertia. You might expect the water to fall out of your cup when the mouth of the cup is pointing down. But when you swing the cup in a circle, you are essentially pushing the water—and the water will want to keep going in the direction you have pushed it because of inertia. The water keeps moving in the direction you've pushed it until it encounters the bottom of the cup which stops the water from flying away.
When you swing the cup, you exert a force on the water which is stronger than gravity. This is like what is happening on the side of the Earth away from the moon. On the side closest to the moon, the gravitational pull of the moon is stronger than inertia and you get a bulge. But on the side opposite the moon, inertia is greater and you get a bulge there.
Ever been to the beach one day and the water level is way high up on the sand? Then come back the next day to see that the water is way down on the beach? Where did all that water go? Here’s an experiment you can do on the kitchen counter to show you.
What You Need
• Glass rectangular casserole dish
• Blue food coloring (optional)
What You Do
1. Fill the dish about 1/3 of the way with water. Add food coloring if you like.
2. Place the pencil under one end of the dish so the water sloshes to the other side.
3. Which side is low tide? Which side is high tide?
4. Tilt the pan the other way. Now which side is high tide? Low tide?
What’s Going On?
When the moon pulls on the water closest to it, the water bulges out toward the moon. That bulge is a high tide. The part of the Earth that is under the pull of the moon experiences a high tide while the parts of the planet where the water has been pulled away experience low tide.
The pull of the moon and the force of inertia make two bulges of water. Where there is a bulge there is high tide. Where there is no bulge there is low tide. Because the earth is spinning, a different part of the planet is under the bulges at any given time. The water never goes away; it just moves around the planet.
Tide Pool Exploration
If you’re lucky enough to be by the beach for awhile, you can explore how tides affect not only the landscape but the plants and animals that live there. Tide pools are amazing worlds to observe.
Bring along a notebook and some pencils and make drawings or write down descriptions of what you see. How does the tide pool change when the tide is high? How does it look when it’s low tide? How do the plants and animals look at each tide?