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Tide/Moon Questions

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During the full and new moons, are the PM tidal differences always substantially more extreme than the AM tidal differences?


If so, why?


Tough one...extra credit even wink.gif



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Time of moon rise at night


That would make perfect sense...except during the new moon it generally rises between 4-7am smile.gif The full moon rise is generally between 7-10pm. Yet even during the new moon, the PM tides are more extreme than the am tides. If it were just moon rise, it should be exactly opposite during new and the full smile.gif




JP, I'm not sure there's any science backing up your observation...doesn't mean it ain't true though wink.gif



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Hmmmmmm......interesting question....


If you look at a tide chart, your observation is correct, for now....


For example, looking at a tide chart for SSP:


June New Moon 6/17


AM low 2:36 AM 0.2 feet

AM high 8:29 AM 4.4 feet

PM low 2:22 PM 0.5 feet

PM high 8:35 PM 5.4 feet


AM tidal difference 4.2 feet

PM tidal Difference 4.9 feet


July Full Moon 7/2


AM low 2:31 AM -0.6 feet

AM high 8:22 AM 5.0 feet

PM low 2:32 PM -0.2 feet

PM high 8:43 PM 6.4 feet


AM tidal difference 5.6 feet

PM tidal difference 6.6 feet


So far, your observations are correct, and this trend continues until the Full Moon in September....though thru July and August, the differences between the AM difference and the PM difference lessen with each cycle...




Full Moon in September 9/28


AM low 2:15 AM -0.4 feet

AM high 8:20 AM 5.7 feet

PM low 2:39 PM -0.2 feet

PM high 8:38 PM 5.7 feet


AM tidal difference 6.1 feet

PM tidal differenc 5.9 feet


New Moon in October 10/14


AM low 2:27 AM -0.2 feet

AM high 8:30 AM 5.8 feet

PM low 2:58 PM -0.2 feet

PM low 8:44 PM 5.3 feet


AM tidal difference 6.0 feet

PM tidal difference 5.5 feet


Full Moon in October 10/28


AM low 2:24 AM -0.2 feet

AM high 8:34 AM 5.7 feet

PM low 3:03 PM -0.1 feet

PM High 8:55 PM 5.0 feet


AM tidal difference 5.9 feet

PM tidal difference 5.1 feet


And as we go thru the fall and winter, this gets exaggerated until I'm guessing about January or February, and then recedes, and eventually flips the other way again sometime next late spring.


You will also notice that MR. Powers mentioned some time ago that the only "minus tide" this summer at the canal during a new moon was in September (though for the rest of the year after that, the minus tides will also be on the new moon).


It all has to do with the moons orbit......for the past 4-5 months the moon has been at perigee (closest to the Earth) near the time of the full moon. (Back in June, the Moon was at perigee on the same exact day that it was full, thus the extreme tides we saw back then).


That slowly changes such that it is at perigee later and later after the full moon, when finally in January 2005, it is at perigee at new moon.


The flip-flop from being closer to the full moon to being closer to the new moon happens in September and thus the change from the greater tidal differences from being in the PM to being in the AM, and also the "minus tides" at the canal from being near the full moon to being near the new moon.


After January, this cycle will go back the other way, and sometime late next spring, we'll be back where we started (though a few weeks later than this year....each passing year makes the cycle later and later, until sometime a decade or more from now, the summer/winter cycle will be reverse of what it is now).....





BTW Tim, so what do I win?..... wink.gif

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PM, because the moon enters the fullest/wanest part of it's phase during these hours. In other words, the moon's gravitational pull is at it's greatest/weakest during the night.(I don't trust my reasoning worth a damn btw)

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Tim, With respect to the differences between the heights of the 2 high tides of the day, I think its got to do with the tilt of the moon's plane of rotation around the earth. The greater the angle of this plane with respect to the equator, the greater the height difference between the 2 high or low tides of the day. When the moon is north of the equator, the tidal bulge points north to the moon and also to the south on the opposite side of the earth. You get the "high" high tide when we (in the northern hemisphere) pass under middle this northern tidal bulge. 12 hours later, we pass under the edge southern bulge, and therfore see a "low" high tide. The moon's plane of rotation goes from maximum north to south to north again every 30 days.


From looking at the tide charts, the plane seems to coincide with the equator about 3 days past the new and full moons as the height difference between the highs is only a 1/10ft or so. It looks like the angle is at it maximum about 3 days before the new or full moon when the tide differences are their greatest.


I think Steve in Mass may have the answer to why the higher high is in the pm.


Phew, I need to rest my head now.

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The change in the apogee and perigee explains the flip flop from the extreme of extremes in tides going from Full Moon to New moon......the change in the orbit around the equator would explain the flip-flop from AM to PM extremes (so your theory on Tim's original question is better than my explanation.... smile.gif ).....and I think these two things may somewhat coincide, given the laws of the universe.....


In fact, upon a quick look, according to my Old Farmer's Almanac, on the day of the flip flop I cited above in September, the moon is listed as "On the Equator".......


Good call..... smile.gif

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Mass, water absorbs energy. Energy moves mass. The high point would be at the pm tide (here in the northern hemisphere) as the ocean absorbs (for lack of a better word) energy, it stores then releases said energy.


Hey, I'm an outta work Network Management Engineer - spare me!

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Paul, sorry.....that could possibly be a good hypothesis, but what about the fall flip-flop in the phenomena?


Sorry bud, that just don't cut it.... wink.gif

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Steve, you observation for the flip flop occurring in september got me thinking. There is a corresponding flip flop in march. Both flip flops are close to the vernal and autumnal equinoxes where there is no tilt of the earth's axis with respect to the sun.


According to the tide charts, in the winter (Jan and Feb 2005) the full/new moon high tides are greater in the am. In the summer they are greater in the PM. This,I think, is caused by the tilt of the earth's axis with respect to the sun. In the summer the northern hemisphere is tilted towards the sun, in the winter its tiled away from the sun. The maximum tilt occurs at the winter and summer solstices. During the year the moon does its rotation thing around the earth causing the daily tides but from summer to winter the earth's tilt goes from one extreme to another (north pole tipped towards to away from the sun) causinng the higher of the 2 tides to shift from pm to am.


Tim, are we close??

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I do know that the extreme of extreme tides, meaning that when the tidal difference is greatest, switches from times of the full moon to the times of the new moon....(there are ALWAYS extremes at both), but the extremes of the extremes switch from full moon to new moon dependant upon when the moon is at apogee and perigee....right now (annually speaking) the moon is at perigee near the full moon during late spring and summer, and then it is at perigee near a new moon in late fall and winter.....and that slowly progresses over the years, so that it will be opposite of what we have now in some "x" years.....and will be at perigee at new moon in the summer sometime 10-20 years from now.


On the other hand, I am not sure about the moon's orbit above and below the equator (which, by your very well thought out idea, would effect whether the tidal difference is greater in the PM or AM), and whether it changes just relative to the seasons and the tilt of the Earth to the sun, as you suggest, or if that also progresses as the years pass, so that it may, in fact, slowly change, and "x" amount of years from now, that cycle will be reversed as well.....


Since Donna is an astrologer (but hardly understands atronomy), I have a Ephemeris here through 2050, so if I can make heads or tails of it, I may be able to figure out the answer to that question.....


So now that we've probably confused everybody but ourselves........ wink.gif

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Not sure, but I think it has to do with the additional effect of the gravitational pull on the water by the sun- From "


Shifting tides


Gravity is said to be the weakest of all the fundamental forces. But one aspect of it is very consequential: Gravity never goes away. It weakens with distance, but it is always at work. This fact is the primary driver of tides. The side of Earth nearest the Moon always gets tugged more than the other side, by about 6 percent.


Hey, you might say, there are two high tides on this planet at any given moment. True. And another far more complex set of phenomena explains this.


The Moon does not just go around the Earth. In reality, the two objects orbit about a common gravitational midpoint, called a barycenter. The mass of each object and the distance between them dictates that this barycenter is inside Earth, about three-fourths of the way out from the center.


So picture this: The center of the Earth actually orbits around this barycenter, once a month. The effect of this is very important. Think, for a second, of a spacecraft orbiting Earth. Its astronauts experience zero gravity. That's not because there's no gravity up there. It's because the ship and its occupants are constantly falling toward Earth while also moving sideways around the planet. This sets up a perpetual freefall, or zero-g.


Like the orbiting spaceship, the center of the Earth is in free-fall around the barycenter of the Earth-Moon system.


Here's the kicker: On the side of Earth opposite the Moon, the force of the Moon's gravity is less than at the center of the Earth, because of the greater distance. It can actually be thought of as a negative force, in essence, pulling water away from the Moon and away from Earth's surface -- a second high tide.


Our planet rotates under these constantly shifting tides, which is why high and low tides are always moving about, rolling in and rolling out as far as observers on the shore are concerned.


The Sun, too, has a tidal effect on Earth, but because of its great distance it is responsible for only about one-third of the range in tides. When the Earth, Moon and Sun are aligned (at full or new Moon), tides can be unusually dramatic, on both the high and low ends. When the Moon is at a 90-degree angle to the Sun in our sky (at first quarter or last quarter) tides tend to be mellower.



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Woops, maybe this is why:


The side of Earth nearest the Moon always gets tugged more than the other side, by about 6 percent.


Thus, the night time tides would be greater.

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