Q & A: Tides -- towards the moon or at right angles?

My physics teacher told us the tide diagram is wrong in our physics book. It shows the tides lined up with the moon so the high tide is on both the side of the earth toward the moon and the side away from the moon. He says the low tides are on the line with the moon. He says the high tides are at right angles to the line to the moon. He says it is as if the moon pulls the water from the back of the earth down flat so it bulges out to the sides. He says it is not true that the moon pulls the water up toward it. Who is right, the physicists who wrote our book or our teacher? I did some research. I looked at online tide charts for a day with a full moon. That way the sun and moon are pulling together. If my teacher is right, then the high tide should be at 6 pm and 6 am. If the book is right the high tide is at noon and midnight. The east coast tides show my teacher is almost right. But tide charts for the west coast are more like the book. Hawaii which is out in the middle of the ocean is either three hours too soon or three hours too late. So who is right? Thank you,
- Jack Graham (age 16)
Incirlik American High School, Turkey

Hi Jack,

Those are the two main ingredients of being a scientist! Skepticism and a willingness to confront models with observations.

Tides are more complicated than the book explanation, as you have just discovered with actual tide data. The reason for the difference is that water has to move around from one place on the earth to another in order for the water levels to rise and fall. If there weren't any land on the earth and the oceans were of uniform depth, then the tides would travel as a gigantic wave with a wavelength of half of the circumference of the earth. There are continents and the ocean floor changes depth, so the waves are impeded in their paths. The tides will come at different times even for places not so far apart, if there are big impediments to water flow. For example, water flowing into a bay through a narrow inlet will make the tides lag. Tides propagate some distance up rivers that open out into the ocean, and these may lag as well (or exhibit even more interesting behavior, such as single waves -- "tidal bores").

It may even be that a resonating water sloshing effect goes on in a closed or mostly closed body of water (like the Mediterranean) that causes the tides to be more complicated than a simple rise and fall with the sun and moon -- local harmonics can be important too.

Tom

p.s., concerning what the books and teacher say. It really is true that the moon pulls the water toward it. It does pull hardest on the part closest, and weakest on the farthest part, and in-between on the Earth. The complication, as Tom described, is that when you pull on something it doesn't immediately adjust its position. If you take a mass on a spring with some friction on it, then push and pull at a regular rate, you can get:
1, the mass stretched out toward you when you pull
2. the mass moving toward you when you pull, and closest to you in-between pull and push.
3. the mass accelerating toward you when you pull, and closest to you when you push,

or anything in between, depending on the mass, the springiness, and the friction. As Tom points out, for the tides in different places there are a range of those conditions.

Mike W.

(published on 10/22/2007)