JohnHuth

One of the perq's of being an NSPN member... Actually, both cases are possible in tides. In case 2, there has to be a significant torque on the object to cause it to rotate like that. Case 1 is closer to the earth's, I believe - there is a torque due to tidal forces and you can even quantify this to say that the earth's rotation slows down by a few milliseconds every number of years or so (I think it's a decade, but it may be more). Eventually, the earth will stop rotation of a 24 hour day, and will simply put the same face toward the moon all the time (as the moon currently does with us). So, the answer, I believe is "mostly case 1 with a tiny bit of case 2 mixed in". but, feel free to question this.

Congratulations! It's a great feeling, isn't it? And - that initial soreness - you find you have muscles you didn't know about.

This has been one of the trips I've been lusting after. I'd be interested, if you haven't swamped your quota. I was surprised how well behaved Petit Manan was. 1st day: Mt Desert to Dry Island 2nd day: Dry to Rogue or Stephens 3rd day: Rogue to Machias (assuming you catch the tides)

Progress. I guess there are two pieces - one is to make the convincing argument that the centrifugal term doesn't exist (you had me going for a few days there) and that it's due to the divergence of the moon's (and sun's) gravitational field. The way I think of the near-far side argument is that any net gravitational force will cause the center of the earth-moon rotational system to shift, so any residual force must be symmetric and a bulge on the near side must be the same as the bulge on the far side - an effect which is born out from experience. If you buy the "egg-shape" potential, then the only thing left is the elastic properties of water and land. I think of it this way - the elastic properties of water and land are such that they will stretch under tidal forces and start to exert a "spring-like" force that holds the water and land from flying apart from the tidal forces. This "stretching" is necessary to hold things together. If you put a book on a table, the reason the book doesn't continue to accelerate downwards is because the table flexes ever so slightly, providing a counter-force to gravity. Now, water, being less elastic than land, will distort more than land, and hence will cause tides. We still have a bit of clean-up to do, here, because tides imply flow of water and we have to describe clearly why a flow is created. I think we're converging on the answer, however. I started out not-believing David Burch,then "almost" believing it, and now, I think that explanation he gives doesn't cut it. Now, it's only a paragraph, and you can readily argue that it doesn't matter. "Just hand me the friggin tables and a copy of Eldridge" - but it's been bugging me for some time.

I believe that Hopkinton Resevoir is open all year round. When I'm warming up for the season, I used Hopkinton last year.

OK, I think I got it - that website I posted in the previous helped me out a lot. After all the talk about centrifugal forces, I began to wonder if I'd missed something altogether, and actually did the "wrong" calculation that is shown in the website listed. The issue is that the earth is always in "free-fall" about the center of the earth-moon system, and, as a result there is no force felt from the rotation. The only force available is gravity. As the guy on the website points out, if you assume that there is come centrifugal force, by using the wrong frame of reference, you end up with a force that's about 70 times the force due to the moon. It would also have some strange characteristics - it would be largest at the equator and also largest at the side opposite the moon, it would also diminish with latitude. None of these are characteristics of the tides. The main thing is that the centrifugal force is zero - not even small - it is the divergence of the moon (and sun's) gravitational field lines. So, I'm now convinced.

Try this website http://www.vialattea.net/maree/eng/index.htm

I don't know the precise answer. What I have done was to take my barometer and make a 2-week chart of the pressure and annotated it with weather observations. When I did this, I found that cold fronts first had a major drop and then a rise in pressure. Some of this freakishness might have to do with local humidity conditions- air condensing and freezing. I do carry a Brunton "weather station" - which is pretty good, but I still prefer to look at the clouds.

There are two pieces to the puzzle. The first is the fact that some kind of "differential" forces cause the earth to deform. The question is - how much of this is the "centrifugal" contribution and how much comes directly from the moon's gravity. Once you get the differential forces that tend to distort the earth, then you can talk about the relative influence on water versus land. Since land is less elastic, it doesn't deform the same way under stress and strain as water. I think that with any stress/strain - water would deform more readily than land. At least that's how I'm trying to figure it. Besides the raw physics, I'm trying to think through the "clues" we know about tides - 1.) They're very symmetric on the near-side and far-side of the moon/sun (high and low tides are pretty much the same heights on the near and far sides). 2.) The sun has an influence of about 1/5th of the moon. I'm pretty sure that for the sun, the centrifugal contribution has to be small - but there's probably some contribution from the diverging gravity field lines. The symmetry makes it difficult for me to "buy" the centrifugal explanation. I'd expect the tides to be asymmetric (highest on the far side of the moon) if this were the whole story. These are all hunches, mind you. I tried to calculate the center of rotation of the earth/moon system, and got that it was about 4000 km from the center of the earth, so, it's inside the earth. Maybe you can recheck my math on this?

OK, so I broke down and did the calculation. I was surpised to find that the differential of the "centrifugal force" was about the same size as the difference in gravity from one side of the earth to the other. I thought the term was negligible. Give me some time to play with the calculation, and I'll try to respond in a bit. John

>OK, you're a world-class physicist... I take your word for >it. Actually - PLEASE DON'T. The great physicist Richard Feynman was very adamant that people should only be content when they believe in their heart the explanations. Physics does provide good tools for thought. I'm thinking through this myself, so I don't claim to have the definitive answer. >* centrifugal force is a fictional "force" used to explain >the radial stability of an object in a rotating system Yes - although the rotational motion of the moon and earth around a common center-of-mass is the thing that keeps the moon and earth from crashing into each other. > >* fictional or not, the centrifugal force explanation of >opposite side tidal bulge is simply wrong; there would be an >opposite side tidal bulge even if the earth and moon were >not rotating. (Is this because the centrifugal "force" acts >equally on all points on and within the earth, so that it >cannot cause one part to move relative to another?) Yes - in essence - the effect of the orbit of the moon and the force of gravity can be said to be acting as if the earth were concentrated at a point - its center-of-mass. If you add up every force on every bit of the earth, you can say that is is as if the force, added over every little chunk of earth were happening at the center of the earth. What's left over are the forces that differ from one piece of the earth to the next. Now, "centrifugal forces" could create some differences between the near-side and the far side of the earth. I'm trying to figure out its magnitude compared to the effect of the divergence of the moon's gravitational field. There is some effect associated with the rotation of the earth, which makes it oblate - so, it's plausible that some residual effect from the rotation of the earth-moon system could create some effect, once you take out the overall average effect on the earth. Two things - I suspect that the effect is small, and can be neglected (this is something that one can calculate easily), and secondly, it should be relatively symmetric - where only the "differences" would create any kind of bulges. In any case, I think it's misleading to say that any bulge on the far side of the earth (from the moon) is "caused by" centrifugal forces. > >* the real cause of tidal bulge one the side of earth toward >the moon is that the gravitational pull of the moon upward >on the top of the ocean on that side of earth is (ever so >slightly) stronger than the upward pull at the bottom of the >ocean. That's because the bottom of the ocean there is a tad >farther from the moon than the top of the ocean, and >gravitation falls off over distance (by a cubic factor). >All together, that creates a net upward force acting on the >water, pulling it toward the moon, hence a bulge toward the >moon (though ever so slight). Yes, what's left over is the force of deformation on the earth. The forces of deformaion has to add up to zero, but there will be different forces acting on different bits of the earth, which will "deform them" - as if the earth were a slightly elastic rubber ball. > >* likewise on the the side of earth opposite the moon, the >moon's gravitational pull down toward the moon on the ocean >is (ever so slightly) stronger at the ocean floor (because >it's nearer the moon) than on the surface, creating a slight >net force away from the moon, or upwards on the ocean and >hence a bulge away from the moon. Yup - although, the force acts on the entire earth. If it were a rubber ball, the entire earth would be uniformly deformed. This is where the difference between the properties of water and the mantle of the earth come into play. Water is more readily deformed by the tidal forces than the mantle, which is why the water seems to move relative to the shores. > >* The above effects create tidal bulges, but not, per se, >tidal flow, because the forces on the ocean are straight up >and down Yup. > >* in between the points on earth nearest and farthest from >the moon, there are sideways components of this net force >which essentially pull the water around toward the nearest >bulge, creating tidal flow (do I have this right?) Yes. > >* These "ever so slight" effects actually have an equally >slight effect on the total shape of the earth (including >water), generally just a few (1-50) feet up and down, which >is pretty small in terms of the 25,000 mile diameter of the >earth. But that still generates the tides we observe and, >in human terms, move a lot of water around. Yes. > >The presentational problem for me is that centrifugal force, >fictional and irrelevant though it may be, corresponds to a >vivid everyday experience that we know from childhood -- >hang onto the rim of a rotating platform and the spin seems >to draw your body outward, flinging you off the platform. I >can't come up with an equally vivid and convincing image for >the differential gravitational effects, especially the far >side one. That may be because they are indeed so slight. But >until we can, it's gonna be hard to convince people of this >analysis. > That's a helpful point - one to keep in mind if I (or you or an enlightened instructor) make a presentation on the tides. The idea of centrifugal force has been invoked many times in the description of tides, so it's difficult to abandon. If you want to think about "centrifugal force" - in what us physicists call a "fictitious frame of reference" - it seems to be a real force. In the "frame of reference" of a rotating system like the earth-moon system, it might appear that a "centrifugal force" is pointing in the direction opposite to the moon's gravity. This "fictional force" keeps the earth from falling toward the moon. It "balances" the force of gravity. This isn't what happens if you adopt a non-accelerating frame of reference - as if you were floating above the earth-moon system and independent of the earth-moon orbit,then, you see the moon and earth describing a circular orbit about their mutual centers-of-gravity. In any case, the effect of any "centrifugal" force would be to balance the earth-moon system, and wouldn't cause any significant deformation of the earth, in any case. The only place where you get a modest deformation is from the rotation of the earth about its axis, and this causes a slight oblateness of the earth - but not relevant to the tides. Again, this deformation has to do with the difference in forces that different chunks of the earth experience.

I've thought about a good presentation for this - I think I can do with with some simple visual diagrams. First, centripetal acceleration does have effect on the planet. The earth is slightly oblate - bulges out at the equator because of it's rotation. Second - the earth-moon system also has a centripetal effect associated with it. The earth actually orbits a common center-of-mass in the earth moon system. What distinguises the tides from these effect is that the tides are created by the divergence of the moon's gravitational field. You can look at it this way: the "first order" effect is having a constant field from the moon, acting on the earth. This, combined with the centripetal effects of the mutual orbits, acts to position the center of the earth, due to an "average" gravitational field, which could be represented by a set of parallel lines all pointing toward the moon. The tides are caused by any residual variation of the moon's gravitational field over the earth - because the field lines are both diverging from the moon, and the field is getting weaker as you go farther away from the moon. The residual variations must add up to zero, if you add them up over the surface of the earth. If they didn't, then there would be some left-over constant field, that would recenter the earth's center. What's left will sum to zero. The easiest way to see the bulge both nearest and farthest from the moon at the equator, and compare it to a "reference" or average field at the center of the earth. At the side of the earth closest to the moon - the gravitational field is pointing toward the moon, and is stronger than the value at the center of the earth, so it tends to make the earth bulge toward the moon. This is the easiest to buy and is intuitive. If you look at the opposite side of the earth from the moon, the force of the moon is still pointing toward the moon (down toward the center of the earth). BUT, the value of the field is smaller because it's farther from the moon than the center of the earth. As a result the *difference* between the average field and the field at the center of the earth points *away* from the center of the earth, and will also produce a bulge that sticks away from the center of the earth. At ninety degrees from a line connecting the moon and the earth's center, you have field lines that are pointing toward the moon. If you subtract out the average component (parallel lines, if you want to picture that), then the difference between the average and the actual fields tends to point inward toward the earth's center. This is a relatively straightforward way to see that you get this "egg-shape" force that causes bulges on the side pointing toward and away from the moon, and indentations on the sized at 90 degrees with respect to the bulge. Centripetal forces only come into play when you talk about the displacement of the earth's center from the center of rotation of the earth-moon system and also the oblateness of the earth. Another "centrifugal force" effect that does come into play is so-called "tidal drag" - the fact that water flows better than the earth's mantle, causes effects that slow down the earth's rotation, ever so slightly - I think it's a millisecond per year. Because the earth's field on the moon is so much stronger than the moon's on the earth, the tidal drag forces are much larger and tidal drag has basically frozen the moons rotation so that it always shows the same face toward the earth. When I get back to the States, if anyone is interested, I can draw up some diagrams that I think will make it more clear.

I've heard that the transponders don't work so well because you're too low in the water.

Evasive action is the only thing you can count on. Last year, I played with all sorts of improvised radar reflectors, and they just don't hack it. Also, forget VHF - although if you can tune into the frequencies that the lobstermen use, you can sometimes figure out where they're going. I mainly rely on my sense of hearing and I can usually tell the bearing and speed of lobster boats by the direction of the sound and the RPM of the engine. In parts of Maine, like around Mt. Desert Island, in the summer, it can be a real hazard. The lobsters move into the shallow ground in the summer and this really cramps up the space where the lobsterboats work. So, my ears are what I use when it's all foggy.

>If I understand this correctly, the field lines radiate >outward in all directions from the object so that the angle >between a moon field line which hits the top of the earth >and the line which hits the bottom of the earth is far >greater at the moon and far less at the earth than similar >lines from the sun and thus the force from the moon's lines >has a great impact in bulging the earth? Yup, that's precisely correct!! > >Is it also true that the tide in the Gulf of Maine is not >true ocean tide or perhaps I should say not part of the >Atlantic Ocean tide, but rather it is similar to the tidal >flow into a bay? Not that I understand the distinction. I don't think there's really a distinction - it's that different bodies of water have different natural periods. For example, harbors may have periods of something like a few minutes. If you "tweak" a body of water, it will oscillate. In the case of harbors, these are sometimes called seiches. As the body of water gets larger, the period gets longer. If you take a system and periodically tweak it, it will respond. If you happen to tweak it at its natural frequency, it will show a huge response. The best analogy to this is when you pump a swing. The swing, being a pendulum, has a natural period to it, and if you pump faster or slower than the natural period, it won't go so high, but if you pump it just right, it will respond dramatically. The size of the Gulf of Maine is such that it has a natural response which is close to the periodicity of the tides that tweak it. Nantucket Sound, on the other hand, is much smaller, so it has a shorter natural period and doesn't see as large an extreme. The middle of the ocean sees very little tidal variation. > >I read somewhere that the tide generally floods north and >east when north of Cape Ann and south and west when south of >Cape Ann. Why does that happen? >> > I don't know so much about Cape Ann, but I do know that there are some odd places - for example, at the tip of Monomoy, the flood current goes east and ebb goes west. That's because the predominant flood somes into both Long Island Sound and Nantucket Sound with flows that sort of originate around Newport RI. A lot of this has to do with the nature of the semi-trapped body of water that's being flooded, its natural period and how most efficient flow of water develops. I'd have to think about the topography of Cape Ann a bit, in order to give you a thoughtful answer. >I think your post demonstrates there are nerds and then >there are Nerds. > I hesitate to ask if I'm a candidate for the capital "N".

I had to fly to NYC yesterday and grabbed David Burch's book on kayak navigation and had a "grrr....." moment in reading his chapter on tides. So, in a bit of a rant, with nowhere to go - I thought I'd mention this. Tides are complicated. The explanation I saw in Burch's book is not consistent with my understanding of tides. They're not due to a bulge because of the moon on one side and "centrifugal force" on the opposite side of the earth. I even had the experience where a few high-end kayak instructors repeated the "Burch" explanation as gospel (although it's not really Burch's - and I think he has a doctorate in physics even!). Tidal forces arise because the gravitational field lines from the moon are spreading out. To be a bit technical - the sum of the earth's plus the moon's gravitational fields creates a kind of "egg-shape" that causes the tides. Water has different elastic properties than land, and this difference, coupled with the "egg-shape" bulges is responsible for tides. An easy way to convince yourself that this is true is the following: the effect of the moon on tides is about 5 times that of the sun, yet the gravitational pull of the sun is far, far larger than the moon. So, you would naively expect the sun to dominate the tide based on the erroneous explanations. The proximity of the moon and the fact that its gravitational field lines are spreading out far more rapidly than the sun's (which are nearly parallel at the distance of the orbit of the earth) is responsible for the fact that the moon's effect on tides is so much larger than the sun's. Here's a great URL that you can check out that will explain this. http://www.lhup.edu/~dsimanek/scenario/tides.htm So, no more use of the "centrifugal force" explanation, OK? (now that I know I'm safely among nerds!)

You're welcome! A great crowd. I had a few lingering questions to attend to from the weather/waves. First - no - light houses are not 100 ft tall - but if you don't know the height of a random lighthouse, it's a decent guess. A lot of them are in the 80-120 ft range. Second- I wanted to double check the etymology of "rule of thumb" - so when I got home, I googled it and was, at first, horrified to find out that it had something to do with a law about being able to beat your wife with a stick as long as the stick was not thicker than your thumb. Then, as I read on, I found out that this was an urban myth, and "rule of thumb" really comes from the use of parts of the body as measuring devices (e.g. feet). Yikes!

CRCK has done this trip a few times as part of their end-of-year trip for the instructors. The place to watch out for is the "middle ground" when the current and the wind are up. It's a bar halfway between - I've heard of a few hairy moments there under the wrong conditions. You can go around it, but then there's the west-chop - but staying a bit north and then cutting down, you can avoid some of the hairier bits. John H.

I know...y'all must be getting tired of this. Here's the deal - some of you probably know this already - I was out in the water at the same time and same rough location as the two girls who died on Columbus Day 03. Mainly because I was distraught, I wrote up a piece on it, which was a bit prosaic (if any of you want to see that, let me know, I can mail it to you). Anyway, a friend saw it, and liked it, and one thing led to another, and Sea Kayaker is interested in publishing it. They wanted something like 300-400 words on the status and opinions about the Aranoff/Jagoda bill in the legislation. I was hoping someone could give me the most up-to-date information on the legistlation - any additions, subtractions - the provisions of the bill in plain english. If any of you don't mind being quoted, please speak up - you may see yourself quoted in Sea Kayaker! John H.

No problem for me, if it's then.

Kevin has helped schedule this course for Saturday, March 3rd. Here's a tentative syllabus of topics. Feel free to comment or ask - this isn't set in stone. I'll make this one run for about 2 hours, with a break in the middle. Weather and waves 1.) Warm fronts and cold fronts 2.) Cyclonic and anti-cyclonic wind patterns with highs/lows 3.) Approach of warm fronts – cloud sequence 4.) Approach of cold fronts 5.) How thunderstorms develop 6.) Wind patterns with squall lines and thunderstorms 7.) Estimating distances of thunderstorms and squalls 8.) Other weather predictors 9.) Basics of water waves – amplitude, wavelength, period 10.) Swells 11.) Wind driven waves – breaking and building 12.) Waves and land a. Shoals and breaking b. Steep topography and dumping c. Cliffs and reflections d. Clapotis 13.) Currents – points, constrictions 14.) Eddies

I had a 10 and a 6 for a 4 day trip out of Jonesport. I think I would've done better with two 10's. I was going through about 6 liters per day, but the guy I was with was drinking 4 per day. Part of it was that I was doing more cooking with water (morning and evening), and part of it was that like to err on the side of more hydration. So, I'm going to get another 10. Oh yeah, and a 2 liter platypus and 2 1-liter nalgenes. My system was to use the nalgene's to monitor my water usage, since they're calibrated, and have the platypus for on-the-water

Thanks, Ed. I'll try to include that. I'll scan some charts and discuss conditions you might expect. Also on cloud formations - I was already planning on that. I'll include info on winds, highs and lows as well.

I was thinking about something like the following: 1.) Basics of weather - pressure and temperature. 2.) Water in the atmosphere 3.) What happens when a warm front comes through 4.) What happens when a cold front comes through 5.) Judging thunderstorms as they build 6.) Other seat-of-the-pants weather forecasting then for waves 1.) Basic characteristics of wave motion 2.) How and why waves break 3.) The effect of wind-speed and fetch on building waves 4.) Relation between underwater topography and waves This would be must simpler than the other 2x2 hour sessions and would be focused on "what you need to know" if you're kayaking. Let me know if this would be helpful.

There used to be a campground at Oracoke - which is just before the ferry to the mainland.