On 2014-02-11 14:34, Dan Asimov wrote:
This explanation from the text Mike sugggests is the same explanation I've long heard from reliable sources:
----- • Water on the side of Earth facing the moon is pulled hardest by the moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.
• Earth itself is pulled harder by the moon’s gravity than is the ocean on the side of Earth opposite the moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
• With water bulging on two sides of Earth, there’s less water left in between. This creates low tides on the other two sides of the planet. -----
But I'm curious:
* Are the tides on the sides of the earth nearest and farthest from the moon symmetrical?
I assumed they were not, and asked this same question when we were camping at Steep Ravine, and thought I had circumstantial evidence that I was correct that they were asymmetrical (I thought higher tide closer to the moon). There were tide tables posted, and I notice that the variance between the two high tides per day varied by more than the corresponding tide varied from day to day. The high high-tide seems to vary from the low high-tide by more than the variation in high high-tides for 2 consecutive days. I guessed that the low high-tide was approximately the same as the antipodal tide at the same time as the high high-tide. But my friend (and my son) didn't accept my argument (that the low high-tide corresponded to/approximated the antipodal tide at the time of the high high-tide) at all, saying the tidal system was far more complicated than I was modelling it. In particular, they said that the sun's measurable influence on tides was enough to make my argument unconvincing. They were probably right. I had planned on finding tide tables from antipodal points on the earth; I never looked. I tried just now, and did not find anything useful, although I did come across a middle-school/high-school lesson (distinct from Mike Stay's) that seems far less bogus than anything I learned about tides when I was a kid: pumas.nasa.gov/files/01_25_11_1.pdf It implies that the 2 tides are asymmetric. I suspect there are more reasonable middle-school level explanations of tides than I expected (I'm pleasantly surprised). There's probably good coverage of other topics, too, so I should alter my expectations (and stop complaining). Oh well. My favorite aspect of this lesson, and the biggest distinction between this lesson and my own elementary school experience (long, long, ago) is exemplified by these two quotes: "By studying tide tables for various places, you can see that the behavior of lunar tides is not nearly as simple as the analogy and the physical/mathematical calculations in the DISCUSSION section demonstrate. Additional complications include: ..." "Technical note: The spring analogy should not be pushed too far ..." If I had teachers who could admit that (or be aware that their analogies had limitations), then I would have been much less frustrated as a kid, and forgiven them much more significant simplifications.
* If so, why (since the reasons given for those tides are different) ?
--Dan
On 2014-02-11, at 2:17 PM, Mike Stay wrote:
On Tue, Feb 11, 2014 at 2:41 PM, Bill Gosper <billgosper@gmail.com> wrote:
PS, has anyone ever found a correct explanation of tides in a middle or high school text?
http://www.ck12.org/earth-science/Tides/lesson/Tides-Basic/
-- Mike Stay - metaweta@gmail.com http://www.cs.auckland.ac.nz/~mike http://reperiendi.wordpress.com
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