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The tides get later each day

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At most places on earth, there are two high tides each day. With each passing day, the high tides occur about an hour later. The moon rises about an hour later each day, too (actually, 54 minutes later). Since the moon pulls up the tides, these two delays are connected. As the earth rotates through one day, the moon moves in its orbit. A point on the earth must move a little farther than one rotation to line up with the moon again.

Materials

• Paper and pencil, or a photocopier
• Scissors
• Corrugated cardboard
• A thumbtack
• A printed copy of this activity

Assembly

• Cut the earth-moon system (below) from the printout you've made of this activity. Paste it onto heavy paper or tagboard, and cut around it. Leave the moon connected to the planet by cutting along the dotted lines. (Note: If this were a true scale drawing of the earth and moon, the moon would be an arm’s length, 75 centimeters, away. The heights of the tidal bulges are also greatly exaggerated.)

• Do the same thing with this view of the earth from above the north pole.

• Get a piece of corrugated cardboard about the size of a piece of typing paper. Put the earth-moon model on top of the cardboard. Put the north pole view of the earth on top of the earth-moon model. Push a thumbtack through the center of the earth image, then through the center of the earth on the earth-moon image, into the cardboard.

To Do and Notice

• Rotate the planet counterclockwise while looking at the tides. Notice that in this idealized model, any spot on the surface of the earth rotates under two high tides each day.

• Set the earth so that the Greenwich meridian, indicated by the arrow, points toward the moon. (The Greenwich meridian, or prime meridian, is a line of longitude that passes through Greenwich, England.) In our simple model, the tide at Greenwich (at the tip of the arrow) is high.

• Rotate the earth through one full solar day, so that the Greenwich meridian goes around in one full circle.

• During one day, the moon moves about 13 degrees in its orbit. Move the moon 13 degrees counterclockwise without moving the earth, and notice that the location of the high tide is no longer directly over Greenwich.

• Rotate the earth through an additional 13 degrees to place Greenwich at the location of the high tide. This takes 54 minutes. So the high tides occur 54 minutes later each successive day.