2005-08-18 / Columnists

Drawing On Science

by Stephen Yaeger


My niece, Jesseca, has a summer assignment where she has to record high and low tides and relate her observations to the phases (her assignment says ‘shapes’) of the moon (I’m sure that all of her fellow classmates are as excited to do this assignment as she is).

So, I decided to write about tides and the moon’s affect on them. (The sun also influences the tides, but we will not include its influence in this column.) But before doing so I thought a review from one of my previous columns is in order.

Unlike Earth the moon does not rotate on its axis so that one side of the moon always faces Earth and the moon’s appearance changes daily as it orbits Earth. These changes are called phases and are the result of the moon’s reflected light and its position in space as it orbits Earth. The sun lights the half of the moon that faces it while the other half always remains in darkness. But the half that always faces Earth is not always fully lit by the sun.

You can demonstrate this with a flashlight, a large ball, and a small ball. Mark an E on the larger ball representing Earth. Mark an M on the smaller ball representing the half of the moon that always faces Earth. The flashlight, of course, is the sun. Place the ‘Earth’ and the ‘moon’ so that the M and E face each other and have a friend aim the flashlight on the M. Note that if you were at E and looking at the moon you would see a full moon.

Now move the ‘moon’ to the right of ‘Earth’ so that M is again facing the Earth and point E. Stand behind ‘Earth’ so that you are facing the M too. Your friend keeps the flashlight in the same position and shines it on the moon once more. Now you can see that if you were at point E only 1/2 of the moon facing Earth is lit or, more properly, you see a quarter moon.

Place the ‘moon’ at different positions around ‘Earth’ remembering always to keep the flashlight in the same spot and the M and E facing each other. You have now demonstrated the phases of the moon as it revolves around Earth.

Refer to Diagram 1 accompanying this article and look at the eight phases of the moon. The new moon (1) is dark because the sun is lighting the half not facing Earth; at the crescent phases (2 & 8) we see only a small portion of the lighted half; at the quarter phases (3 & 7) the face of the moon facing Earth is half-lighted and half in darkness; at the gibbous phases (4 & 6), almost all of the lighted half faces Earth; and when the moon is full (5) all of the half facing Earth is lighted.

When the moon is opposite the sun, it rises mostly in the night sky. When it is between Earth and the sun, we can see it mostly during the day. A lunar month is the time it takes from new moon to new moon. The lunar month is 29-1/2 days. The difference between the time it takes for the moon to revolve around Earth (27-1/2 days) and a lunar month (29-1/2 days) is due to the distance the Earth and the moon move in their orbits in one day.

OK, now for the tides. As the moon revolves around the earth not only does it change in appearance, its gravitational influence causes the waters to rise toward it. This gravitational pull is the cause of tides. Refer to Diagram 2. The oceans that directly face the moon are drawn to it by gravity—they tend to bulge upward. This is called direct high tides . The oceans on the opposite side will also experience high tides, but this is due to the moon’s influence on the Earth itself with little effect on the waters. Such tides are indirect high tides.

As the Earth rotates from west to east all parts of it pass under the moon every 24 hours and 50 minutes. About every 6 hours and 12 1/2 minutes high tides slowly change to low tides and low tides slowly change to high tides. Now, 6 hours and 12 1/2 minutes later the tides change once again: low tides to high tides and high tides to low tides.

The distance that the moon travels in its orbit in 24 hours combined with the distance that the Earth rotates in the same 24 hours results in the moon rising and setting about 50 minutes later each day. The tides, then, will occur about 50 minutes later each day. For example suppose low tides occur at 3:43 am and 4:05 pm and high tides occur at 9:43 am and 10:02 pm on one day. The next day low tides will occur 50 minutes later at 4:29 am and 4:55 pm. High tides will be at 10:35 am and 10:53 pm.

As the Earth and the moon do their thing the moon will have its phases and the Earth will have its tides—and kids will get assignments to do during the summer.

Questions/comments? Email

Steve: Drawingonscience@aol.com

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