When Does High Tide Occur

Attempt This!
The tides get after each day

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At virtually places on world, there are two loftier tides each day. With each passing twenty-four hour period, the high tides occur well-nigh an 60 minutes later on. The moon rises virtually an hr later each twenty-four hours, too (actually, 54 minutes subsequently). Since the moon pulls up the tides, these two delays are connected. Every bit the globe rotates through one day, the moon moves in its orbit. A point on the world must move a footling farther than one rotation to line up with the moon over again.

Materials

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

Assembly

• Cut the globe-moon system (below) from the printout you've made of this activity. Paste it onto heavy newspaper or tagboard, and cutting 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 exist 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 globe from higher up the n pole.

• Go a piece of corrugated cardboard about the size of a piece of typing paper. Put the globe-moon model on top of the paper-thin. Put the northward pole view of the earth on elevation of the earth-moon model. Push a thumbtack through the center of the earth image, then through the center of the globe on the earth-moon prototype, into the cardboard.

To Practise and Notice

• Rotate the planet counterclockwise while looking at the tides. Discover that in this idealized model, any spot on the surface of the earth rotates nether two loftier tides each 24-hour interval.

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

• Rotate the earth through i full solar mean solar day, then that the Greenwich superlative goes around in 1 full circle.

• During i twenty-four hour period, the moon moves almost 13 degrees in its orbit. Movement the moon 13 degrees counterclockwise without moving the earth, and notice that the location of the high tide is no longer direct over Greenwich.

• Rotate the globe through an additional thirteen degrees to place Greenwich at the location of the high tide. This takes 54 minutes. And so the high tides occur 54 minutes later on each successive day.