Night Fishing: Newsletter Challenge (02/19/08)

Tom was forgetting that he would be dead then, right? But that is too easy, so probably not the right answer. let's go with: Tom was forgetting that they were fishing in a stream in the Rocky Mountains where there is little if any effect of the tides on fishing. Or, perhaps he forgot to refill John's drink.

I was watching a Discovery Channel special on the moon and they say that the moon actually slows the earths rotation down. So as the moon gets farther away, the earth will speed up and cause the oceans to rise with the effect of centrifical force. Ha Ha! If you believe that I have land in Florida to sell you.

Actually that special did mention that the process suggested with the moon moving away would that millions of years, not thousands.

Or will this be a discussion on global warming.

Regards JD.

i

Tom my boy, This moon is getting further away, But you are forgetting, the Sun is coming closer, Our tide may get stronger my son.

"John, old fellow, the surface of the sun may be getting closer, but its centre of gravity is receding."

Ahhh, but with the increase in the use of wave power we will introduce drag onto the moon, slow it down, reduce its cetripetal force and so it will get nearer. This will make bigger waves and tides, so accelerating the move to using wave and tide power so increaseing the drag etc. We're all doomed.

We will have cheap, clean power, but we will all need to duck once an orbit :-)

There has been discussion of astronomical effects of tidal power in another CR4 thread. In summary:

The first thing to note is that the direction of rotation of the Earth means that the effect of tidal drag is to slow the Earth's rotation and repel the moon so its orbit becomes further from the earth; therefore, if the effect of tidal power systems is to increase tidal drag, it will cause the moon to recede over time. Self-limiting in terms of extracting power, but safe.

Second, tidal losses elsewhere are at least 100 times the losses that we will be making use of to extract tidal energy - at least our contribution will be small.

Third, many tidal energy schemes will be in places where they reduce tidal resonances. In these cases, the energy loss will usually be reduced (electronic engineers will recognise the situation of overdamped coupled resonances). So, although we will be extracting power, the overall tidal drag from these locations will actually be reduced (due to the reduction in power dissipated in the remainder of the local tidal resonance). There will be local effects on the ecology - but I'm not qualified to say whether we should regard these as acceptable.

However, in the long term life on Earth is inevitably doomed - it'll get much too hot once the sun expands... But I consider that to be without our present area of responsibility

I wonder if there was a similar explanation regarding the effects of our contribution of carbon dioxide to the atmosphere, in the 1960s.

Maybe, hinting at ice, the author is suggesting the polar ice cap will sick deaper into the ocean and raise the water level. But this would not be a cyclical affect as the tide is since the oceans would just be consistantly higher. Plus the moon does not pass over the poles.

This one has me stumped.

did you mean sink deeper? why would it sink deeper? wouldn't it float higher?

I didn't think about that one, and I think it would be a good answer - but we don't know what (if anything) will be happen about global warming over the next tens of thousands of years. Even without changes in human behaviour, if the situation becomes unstable the present warming could potentially trigger an ice age; on the other hand, assuming humanity is spared long enough, I would not be surprised to see methods of climate control emerge over the coming hundreds of years. These could include space-based solar blocking (or concentration, as desired), reduction (both senses) of CO2; etc.

However, if the ice-packs do stay melted, the larger depths of the oceans will reduce the proportionate losses, which will allow water to flow more easily towards the heights of constant gravitational/centripetal potential. That would be a much more significant effect than the retreat of the moon, particularly if a significant proportion of the antarctic ice-cap were to be melted (but if it goes that far, I doubt there would be any humans left to observe it)

An interjection, if I may. The melting ice caps, if the trend continues, would increase the area of the Earth's surface covered by water, which would in turn increase the evaporation rate of water, which should increase the rainfall amounts proportionately. Also, would the added water amounts, tho relatively small, dissolve more CO2 and remove it from the atmosphere? The Pacific Ocean, combined with all it's neighboring seas, covers more of the Earth's surface than all the land areas combined. That fact will only increase as the ice caps melt. Will the reduction of ice caps cause the Earth's axis to tilt more (or less) and possibly result in a magnetic field reversal? Or even a reversal of the seasons?

Yes, I agree, as I stated before about the millions of years, 10,000 years will have little affect in proportion to the tides. But why did the author tie in the consideration of ice in liquid. The question is simply confusing because if the reference was to tidal affects on ice fishing, people do not ice fish on the ocean. The only place sea ice is stable enough is at the poles and that ice is hundreds of feet thick. Ice fishing is a fresh water thing, which is not affected by the tides.

It could be that the ice was a diversion.

"Spring tide" does not refer to spring or autumn - it refers to the height of the tide. A spring tide occurs when the moon and sun are both contributing in the same direction. A spring tide has high tides that are higher than average and low tides that are lower then average. Spring (and neap) tides generally occur once per month.

You are correct (I never was that great at English linguistics). I should have referred to maximum (and minimum) tidal ranges, and omitted the irrelevant reference to hemispheres. Fortunately, at least some readers both understood what I intended to say and were tolerant of my idiotsyncrasies (sic).

What 'Physicist?' says is true, but it doesn't really provide an answer (even though it got a rating of "2" for good answer[?])

The term "Spring tide" has nothing to do with the seasons. It's certainly true that, due to precession, in about 12,000 years the Earth's North Pole will be tipped towards the star Vega (rather than Polaris); and perihelion will occur during Northern hemisphere Summer, rather than in Winter as happens now (Jan. 2, this year). But over the course of its entire orbit around the Sun, the tides will average the same then as they do now due to the Sun's gravitational pull. The spring tides that occcur in the season of 'Autumn' then, will be roughly eqivalent to the spring tides that occur during the season of 'Spring' now.

The Moon, being slightly farther away then than it is now will exert a slightly smaller gravitation pull. Since tides are proportional to the cube of (1/r) [r being the average Moon-to-Earth distance], the tides will be smaller. Just 'how much' smaller depends on how many 'tens of thousands of years' Tom is talking about.

So Tom may be forgetting that the Sun has an influence on the spring tides and that the Moon won't really be that much farther away in 10,000 years. In, say, 50,000 years the Moon will about 2 Km further away and there should be a small but measureable change in the tides. The Earth will also have slowed in its rotation on its axis which will also have a small affect on the tides.

Tom may also be forgetting about the next Ice Age, which will have a much larger affect on the tides than the increase in the Earth-Moon distance. But no one knows exactly when it will start, nor how long it will last.

I was referrring to the maximal tides over a period, not the average spring tide - see also #32 for on linguisitcs.

I believe that Earth slowing will reduce the relative effects of turbulent drag, which I think will compensate to a small extent for the recession of the moon.

If Tom and John are referring to average tides rather than to major tidal events (hard to tell - and they may not even be talking about the same aspect), then the major influences would be changes to ocean depth and continental configurations rather than anything astronomical, and we have no real way to be certain whether the oceans will be deeper (=> larger tides) or more shallow (the reverse) in tens-of-thousands of years.
Because of this, the only answer I personally have come up with was the one I gave; as I said - unless you know better...

Tom and John are indeed a strange pair. Having cocktails while night fishing? Maybe if they are on a yacht. Otherwise, I'd say that beer or coffee would be more appropiate beverages for night fishing. Obviously, this question was worded this way just to introduce the obervation of ice in a drink, so that must be a very crucial clue.

Also, Tom mentions the spring tides, and not the autumn, summer, or winter tides. Won't all tides be affected?

Okay, I'll put away my pot-stirring spoon for a while.

Ignoring the absence of a time machine to visit Earth in 10,000 years.

If the moon moves away from the earth, time for each moon orbit of earth increases and so does the tidal frequency decrease?

Decreased tidal frequency would mean the "attraction" of the moon to earth's water would be in effect for a longer period each tidal cycle causing higher tides.

Is the mass of the moon increasing (from small meteor impacts etc.) which would also lead to increased tidal influence.

Or is the real crux that being further away, the moonlight would be diminished by inverse square law and they couldn't see to mix their drinks?

The gravitational pull of the moon has its effects on all life on earth. Maybe John's worried about what effect it will have on his favorite beverage.

I am curious as to the full and correct answer. Certainly in the past the tides were both higher and more frequent when the moon was closer. Just imagine how they must have affected the rates of erosion, especially before there were plants.

So, I am anxious to hear what Tom was forgetting and all of us so far seemed to have missed as well.

Your Chapman's should have an illustration of tide cycles over the year - kind of interesting, since there are a couple of gravitational influences at play.......

From: Spacecraft Attitude Determination and Control - Ed by James R. Wertz - ISBN 90-277-1204-2 "this precession of the equinoxes has a period of about 25,700 years."

This is given in "Fundamentals of Astrodynamics - Bates/Mueller/White- ISBN 0-486-60061-0 as 26,000 years. Also discussed here is "Nutation" with a period of about 18.6 years. This nutation is a small perturbation of the precession of the equinoxes caused by the precession of the moon's orbital plane that has this same 18.6 year period and is caused by solar perturbation of the moons orbit.

This is a significant difference from the 21000 years as given in an earlier post; if indeed this is what was being referenced.

Precession of the equinoxes should not be confused with the effect of the "anomalistic year" in reference to our calendar. The "anomalistic year is 5 minutes longer than the sidereal period of our orbit which makes it about 25 minutes longer than the "tropical year.

The anomalisitic year is the period between earth's perihelion passage.

The tropical year is the time it takes for the earth's equatorial plane to cross the ecliptic from south to north and is the basis of our calendar as it defines our seasons. This relationship is probably more complex than intuition would lead us because of the varying reflectivity between the equator and poles, as well as other variables.

If my math is correct, using 25700 years as the period of precession of the equinoxes; in one period of equinox precession perihelion passage would change by about 128,500 minutes. Perihelion passage would cycle through about 1 year and 81 days relative to the tropical year.

Spring tides occur when the moon is at opposition or inferior conjunction (in syzygy relative to the earth and sun.) Would these tides also be affected by earth's true anomaly?

And last but not least there is lunation which I can best describe as the cyclic occurrence of lunar perigee passage relative to syzygy. If perigee passage corresponds to syzygy then it would also have an affect on tides. I think Wik says this occurs about every 7 1/2 lunar periods.

Wik says the current rate at which the moon's orbital radius is increasing is 3.8 cm per year. Will this rate be constant or will it increase or decrease exponentially? At this constant rate the semi-major axis would increase by less than one kilometer in 25700 years.

What effect does the centripetal acceleration caused by earths orbit about the earth/moon system barycenter have on tides?

I am going to agree with whoever said the biggest difference won't be the tides but the condition of ice in the glass. That in the time frame referenced the change in the orbital dynamics of the earth/moon system would have little more effect on the tides than the total heat of fusion of the ice in the glass would have on the local temperature of their immediate environment.

However, if there is any interaction between asteroid 3753 Cruithne the other three near-Earth asteroids, (54509) 2000 PH5, (85770) 1998 UP1 and 2002 AA29, which exist in orbits similar to Cruithne's, or other as yet large undiscovered interlopers; then all bets are off.

Gavilan

Ok, it sounds like you swallowed a dictionary. Mind farther defining a few things like what is "syzygy". You just tossed that in for general confusion.

Actually I did look it up on Wikipedia and it is a real word. See:

http://en.wikipedia.org/wiki/Syzygy

Syzygy is the point where an imaginary line would cross the orbit of a planet when drawn from that planet directly through the middle of the sun. In simple terms anyway.

You need to be more careful when defining the "Precession of the Equinoxes". The period of the precession with respect to the so-called fixed stars is indeed as you state; but the ellipse of the Earth's orbit is not constant, and the axis of this ellipse moves in the opposite sense to the axis of the Earth's spin, and with a period greater than 100 thousand years. The result is that the repeat period for the spring equinox relative to the Earth's distance from the sun is slightly less than 21000 years.
Yes, there is also a much shorter period for maximum spring tides of about 7.5 years, caused by the relative alignment of sun and moon, and there are other cycles with intermediate times. But neither these nor the 21000 year timescales can be pertinent to the "tens of thousands of years" timescale of the challenge. The variation of the ellipticity of the Earth's orbit, however, is in the correct order as stated above.

Alternatively, in the absence of future "corrective" action by humankind, the explaination that warming will cause deeper oceans, and that the oceans' surface can conform closer to the gravitational equipotential looks more than plausible.

The History Channel last night was talking about the wobble of the Earth and that in a few thousand years Polaris will no longer be the pole star. Just goes to show that nothing is a constant.

Dear Physicist?:

Thank you for your thoughtful and informative reply to my post.

"but the ellipse of the Earth's orbit is not constant,"

Are you saying that the eccentricity of orbit changes? What causes this change in eccentricity? How much does it vary and how was it determined?

"and the axis of this ellipse moves in the opposite sense to the axis of the Earth's spin, and with a period greater than 100 thousand years."

How is the "axis of this ellipse", that you refer to, related to "the line of apsides?"

"The result is that the repeat period for the spring equinox relative to the Earth's distance from the sun is slightly less than 21000 years."

I will refer back to my earlier post – "The anomalistic year is 5 minutes longer than the sidereal period of our orbit which makes it about 25 minutes longer than the "tropical year."

Doing the math shows that the vernal equinox would indeed correspond with perihelion passage about once every 21000 years but I need more time and study to wrap my mind around how this would square or relate to a 100,000 rotation of the line of apsides which you give. Could you help me with this?

Gavilan

Significant cyclic influences on the Earth's orbit include the gravitational effects of the other planets and relativistic precession. I hesitate to recommend Wikipedia**, but at the present time the entry on "Milankovitch cycles" gives a reasonable (and more more extended) precis of these cyclic effects.

The ellipse has two axes, the one that is usually referenced corresponds exactly to the line that joins the apsides.

Basically, angular periods are proportional to the inverses of angular velocities, so I'm inverting, adding, and inverting the answer.
I'll just add this example in rough numbers: in 21,000 years, the line of apsides will have rotated by about 1/5 of a cycle, and the Earth's axis of rotation by about 4/5 of a cycle. As they are rotating in opposite directions, their relative alignments will indeed repeat about every 21000 years.

Fyz

**As it can be subject to virtually random alterations.

Spring Tides are associated with the Sun, not the Moon, silly .

Actually Spring tides occur when the Sun and Moon are in Line, ie, New and Full moons and nothing to do with the seasons (as far as I can tell).

Spring - "to jump" or "to leap up" (ripped from the Wikipedia). You are absolutely right.

If the influence of the moon decreases (which works in opposition to the forces applied by the Sun), then there will be effect from the solar influence.

What Tom is forgetting is that John's cocktail is empty and needs refilled. John doesn't care about 10,000 years in the future. He is night fishing and having cocktails. He cares that his cocktail is empty which is obvious to him as he stares at the ice. Anyone who has ever done any serious night fishing would know this obvious answer! Damn it Tom! Get John a drink!

I support Fyz remarks with regard to Milankovitch cycles, and offer the following as an explanation as to why an ice block in a glass has some reference to the above.

http://ocean.mit.edu/~cwunsch/papersonline/tzipermanetal2006.pdf

Fyz takes it a bit further than the above, but is still relevant.

Regards JD.

The Milankovitch cycles were proven bogus in the recent supreme court ruling; Kovitch VS. Milan. No one believes any of that anymore.

Ha ha ha ho ho ho he he he. No cancel that - groan goran roang roang wrong

I was unaware of Kovitch & Milan paper that cosmic dust particles could be considered as the reason for cyclic ice-ages, I have only looked at a few Google searches, but will follow it up.

Regards JD.

What do Kovitch + Milan together make? (Hint: change the order of the names and remove the gaps. I think you will find that this spells HOAX)

Thank you. Ice reduction associated with M cycles** is certainly a possibility. So, unless man survives to tame the elements over that period, John is likely correct

**Surely that should be Q cycles?

As a X drinker and night fisherman One can just dream about 20,000 year old scotch!!!

LarryLakes

Tom is forgetting that wine should not be served with ice cubes in it.

They won´t be there when that happens, so don´t worry. Gonzalo g.velilla@yahoo.com

Is Tom forgetting one thing, or three things? Sun gets bigger, Earth spin slows, Moon moves away, aligned in a Summer could be the tide rises instead of falling.

I actually am interested in knowing if tides in the future will be "milder" or more severe.

Hence I look forward to the secret revealed seriously on the 26th.

HI Aidan Hart here, lateral thinker, capetown sa.

The tidal effects would not be significant because as the moon gets further away, earth's gravitational pull on the oceans will be stronger, thus cancelling out any difference??

Enjoy this beautiful day...

As I understand it, the Spring Tide (as pointed out earlier - thanks LaMarTEK!), is the result of the Sun pulling on one side and the Moon pulling on the other.

Can we assume that the change in the Sun's gravitational force is negligible over the time period being considered?

This suggests that the tides would increase on the side facing the Sun, and decrease on the side facing the Moon.

The sun will be negligibly lighter (primarily energy loss due to radiation)

True but, as yoiu say, negligible. The rates of change are considerably different.

Why should the Earth's pull increase? In any case, increasing Earth's gravity would reduce tide heights (Earth gravity tries to keep the surface spherical)

The center of gravity of the Earth-moon system is under the surface of the Earth. The tidal bulge on the side of the Earth which faces the moon is due to the gravitational attraction of the moon, but the bulge away from the moon is due to centripetal force. If the moon moves farther away from the Earth the CoG will move more toward the surface toward the moon but farther away from the opposite side. This shift will cause an increase in centripetal force and increase the tidal bulge away from the moon as well as a decrease in the centripetal force on the moon side and decrease in the tidal bulge.

The bulge toward the moon also shrink due to a lessening of gravitational attraction(as well as the centripetal force component). With the moon-side bulge shrinking and the away-from-the-moon bulge growing the question then becomes will the shrink be greater than the growth or visa versa? I would suggest that the proportional increase in the distance to the bulge opposite the moon is much greater than the proportional increase in the earth moon distance and would therefore have a greater impact.

The next question to consider is which of the bulges is the larger at present. If the moon-side bulge is the larger then the bulges would move closer to the same size and the tidal range would decrease and spring tides would be smaller. On the other hand if the away-from-the-moon bulge is larger than the difference in size would be exaggerated and the spring tides would be larger.

Bottom line Tom was forgetting the movement of the center of gravity of the Earth moon system

Sounds good but the fact that the moon is moving away from the earth by 2 inches a year, in 10,000 years thats only 2000 feet which compared to the size of the earth is insignificant. I am not sure this would have the expected impact. Maybe after a million years, but not 10,000.

You wrote: "with the moon-side bulge shrinking and the away-from-the-moon bulge growing":
As the moon gets further away, the centre of gravity of the Earth-moon system gets closer to the centre of gravity of the Earth. So the surface of equi-gravitational potential becomes closer to circular. The result is that both tidal bulges decrease.

He forgot the notice "Do not drink and drive a fishing boat"

Due to pollution and global warping - the Tom's of the future will only drink on fishing trips.

Hendrik,

They are ice fishing which means they are standing on a frozen lake fishing through a hole in the ice. So you need to say "Do not drink and drink their truck".

When the moon decreases its tangential velocity around the earth, the orbit diameter will decrease too (angular velocity will increase). Forces are proportional to inverse of (orbit diameter^2). Then "tens of thousands of years in the future, spring tides shouldn't be milder".

It doesn't work like that - as the moon is driven away by tidal forces the potential energy increases but the velocity reduces, so the centripetal acceleration continues to match the inverse square law attraction by the Earth (for circular motion, a satellite's speed is approximately proportional to 1/r^1/2)

Gentlemen,

I am not on the same level of competence in astronomical issues as any of you, but I wanted to offer this simple observation. After reading the question again, Tom and John are "night fishing", not ice fishing (I don't think you'd be drinking something with ice in it if you were ice fishing but I also have never been ice fishing). Second, Tom's time line referred is "tens of thousands" not 10,000. It's arbitrary enough to be interpreted as a time period that's long enough to have an impact of significant change in lunar orbital diameter. I also have to agree that since Tom's only reference is to the Spring Tides, that this reference is not related to the season they are fishing in, but it could be. Perhaps the answer is that, according to references from others about ice ages, ice will have covered their fishing spot and negate the tidal affect completely. Beyond this, you're past my knowledge but it fun to read.

I can't wait to read the answer, but some great discussions have been offered. Great job!

Tony

What we're all forgetting is the fact that the sun is losing combustibility every year,so in those "tens of thousands" of years the sun's power will be diminishing and who can say that this little change alone wouldn't or would have a global impact? Also we forget that its human nature to destroy ourselves so we could also already have annihilated the human race with nuclear war.Sad to say but true.If your a religious person you would have to take into consideration the "Armageddon" and the coming of the "Antichrist",if thats the case who knows what the world would be like and maybe all mankind's wrongs would be righted.

The one small overlooked item is simply that "Spring Tides" are more a function of solar tidal effect than lunar and the differences would therefore be miniscual.

Tom is forgetting about gravity. Specifically, gravitational force which is dependent upon the masses of the two objects and their distance from each other. Although the sun is much farther away, it contains almost 90% of the mass of the UNIVERSE! The tides are not only governed by the moon's gravity, but also by the sun's gravity, although this is a simplified explanation. The effects of the sun's gravitational pull on the tides is almost half that of the moon.

Hope you meant solar system and not universe. Some of the less kind members might challenge this in the open forum.

Like where you're coming from though. Some others chase convoluted solutions to simple problems of understanding.

Yes, you are correct; I did mean solar system. OOPS. If you look at the gravitational forces, the moon & earth will always be "attached" to each other in orbit, unless the mass of one or both of them changes significantly. The moon may move "away" from the earth, but it will reach a point of maximum distance. My understanding is when/if this occurs eons from now, the moon will only be visible from one side of the earth.

We don't need to be unkind to each other to challenge answers or ideas. In fact, I joined this web site to learn; I have never in my life learned anything from anyone who agreed with me.

I try to follow the KISS principle of problem solving; thanks for the comment.

Sure, the sun contributes half as much to the tides as the moon. Proportionate to the separation, long-term changes in the Earth's average orbital distance from the sun are even slower than in the changes between the Earth and the moon - the more significant (short-term) effects of sun-Earth separation are generally described by the term "Milankovitch cycles".

If the moon orbits further away from the earth, then it has to also orbit more slowly. This will give the ocean tides more time to flow and accually rise higher. But, since the gravitational pull of the moon will be less, the tides will flow more slowly. The net effect is the tides may accually not change at all.

This is an interesting insight, but incomplete (and the proposed consequence at odds with historical fact). If all that happens is that the moon orbits less frequently, the tidal period would in fact reduce - because the moon's orbit is in the same direction as the Earth's rotation. However, the same tidal effect causes a reduction in the rotation rate of the Earth, and the net effect should** be an increase in the tidal period. However, the reduction in the distortion of the equipotential surface (relative to the spherical) is more than enough for the reduction in losses (caused by the small change in period) to be submerged (sic). In actuality, even the reduction in losses (caused by the increased period) may not give the effect we predict above - it may be that reduction of the damping of the conformation of the Earth's crust has greater effect on residual tidal range than than reduction of the water damping; but this is a complex matter and way outside my area of expertise.

**Over "short" time periods such as we are considering here, changes in the moment of inertia of the Earth have a much larger effect on orbital period than do tides. At the present time, the shape of the Earth is still relaxing after the last ice age (the time constant for this is about 4000 years). Failing asteroidal interaction or similar(!), over the coming 10000 years the Earth's shape will inevitably continue to relax so that its rotational period reduces. Unless instabilities trigger another ice age, that reduction in period will persist.

With Sun, Earth and Moon in a line
The tides will roll round just fine
How high they will pour
In forty-thousand and four
We have no way to opine

_{(with apologies for broken scansion)}

we know that tides are formed due to the gravitational pull of the moon on the earth. similarly the gravitational force of the earth also attracts the moon and we also know that the gravitational force of earth is 6 times greater than that of the moon. so taking this point in view, how can the moon go away from the earth as the gravitational force of the earth is constantly acting on the moon?

Where to start? Here perhaps?

[No, I don't always trust entries in Wikipedia, but this one was helpful (i.e. basically correct) at the time I submitted this post]

How was calculated that differences of 3,8 cm of the moon's orbit?

That is based on readily available information.

There are different sources for the calculations for this information: the most direct (and almost certainly most accurate) is based on long-term averages of the moon's period; observation of the sizes and locations of the bulges on the surface of the Earth (coupled with other geophysical data) gives one estimate of the force that the Earth exerts on the moon; and the slowing of the Earth's rotation (combined with calculations of the relative tidal forces exerted by moon and sun) gives an independent estimate of the force on the moon. Fortunately, the three sources of data give results that are consistent (within the level of detail/precision of the various measurements).

I forgot what is now regarded as the most accurate measure - the ranging system that uses a corner mirror left on the moon by the Apollo astronauts. That gives the 3.8-cm/year, whereas the other estimates are somewhat higher. BTW, the present rate of recession is known to be anomalously high, because over geological timescales the orbital period can be assessed from variations in paleontological data.

Do you honestly believe that a precision laser ranging system is outdone by mathematical computations of bulges on the earth that are millions of years old? GET REAL!

No need to be rude - especially as what you apparently thought you read is not what I wrote.

I accidentally omitted the laser ranging initially, but stated that as the most accurate measure of the current rate of recession. BTW, although laser ranging gives extremely accurate data for the instantaneous distance of the moon, this does not convert as easily to a rate of recession as you appear to suppose. This is primarily because of (astronomically speaking) short-term perturbations of the moon's orbit by the other planets.

Regarding your adverse comment on the possibilities of modelling: it turns out that the error bars on the laser-ranging measurement of orbital distance over moderate periods (over which our detailed knowledge of the tidal values do not add significantly to the error) include the orbital positions modelled using the known orbits of the planets and using general relativistic models. In fact, it is this good correlation that allows us to calculate the rate of recession based on the ranging results.

Unfortunately, laser ranging can give no indication of the moon's orbit before 1969. So we need different sources for longer-term information. This historic data is based geological evidence from sediments, supported by modelling based on known alignment of the continents (itself based on magnetic data from geological strata).
Sedimentary data is not accurate over short periods, but gives a good indication of how the rate of recession has varied over time. The tidal calculations based on continental alignments correlates rather well.

Hi Physicist?,

My apologies. I had read that you said the laser system was reading higher than the other methods. That is NOT what you said. You apparently said that the moon is receding anomalously faster now. That makes no sense to me, and others in this thread. Can you tell us how many percent faster you think the moon is moving now to say a billion years ago? (define anomalously faster) Can you give us links to the other methods that are used?

Regards,

S

Apologies accepted.

The direct evidence is in oceanic deposits, from which you can deduce the number of tidal cycles that occur each year - this implies the period of the moon's orbit. For a long time this appeared inconsistent with the expected tidal losses that determine the rate of recession. This inconsistency was eventually resolved as a result of analysis of residual magnetism in rocky strata, which indicates that, for most of the Earth's history, the continents have been coalesced into a single land mass. Roughly speeking, the reason this makes a difference is as follows:
The rate of recession depends on the surface shapes of the Earth being different from what they would be if the Earth was not rotating. When there was only one continent, the oceans could flow relatively freely, so the drag on the Earth's rotation (and matching accelerating torque on the moon) was relatively small.

In answer to your question, I believe (purely from memory, and therefore unreliable in detail) that one billion years ago the rate of recession was about 60% of its present value.

Unfortunately, I don't know of a really good reference on the web (I admit I haven't looked that hard), but you might find something if you start here, in spite of odd minor inaccuracies of implication in the article itself.

Do you have any links to sites that discuss the cycle times of the earth's orbit being more and less circular? If it is tied to ice ages, I would be very interested in the details.

PLEASE GO BACK 50 BILLION YEARS WITH THE MOON 3.8CM CLOSER EACH YEAR AND

WHAT DO YOU HAVE??

LJC

Oh, this is fun, let me try......

THE DARK PART OF THE MOON IS THE SIDE WE DON'T SEE, WHAT IS IT HIDING?

THE NEWTONIAN DYNAMICS MYTH IS DISPROVED BY THE 5 DIMENSIONAL HYPERSPHERE THAT CONNECTS THE EARTH TO NEPTUNE.

1000000000000000000 YEARS AGO THERE WAS ENERGY AND NOW ITS GONE WHERE DID IT GO????

well first off 50 billion years ago we did not have an earth or sun as the earth is only about 4 billion years old. But seeing where you are heading IF the change was constant, which it would not be, the change over 4 billion years would have been 152,000 km or 40% closer. Now THAT would have made some real tides.

However, I do not think we REALLY KNOW WHEN the earth captured the moon or where the orbit started. But think about how much faster erosion would have been with the moon closer and before there was vegetation. Also, as we go back in time the earth was rotating faster and the days were shorter and thus there were more tides per year. While the original teachings of geology were that the changes were slow and constant we now realize that has not been the truth and that all things are not constant.

"IF" YOU DO NOT KNOW! YOU ASSUME IT WOULD NOT BE CONSTANT.

WHAT WOULD CHANGE IT. A TOUCH FROM THE HAND OF GOD??

LJC

"WHAT WOULD CHANGE IT. A TOUCH FROM THE HAND OF GOD??"

Yes, as seen through the window of science and mathematics; and revealed by the inverse square law.

Gavilan

That is an intriguing question considering the universe is only about 14 billion years old. Maybe there was another universe in this location at that time. Of course, i guess we would need some fixed point of reference to measure our location from that was present and fixed through the entire period.

I disagree with the 'official' answer. The following is an abridged version of a review of the article in "Science" from 1997.

http://www.lbl.gov/Science-Articles/Archive/ice-age-sediments.html

Astronomical Theory Offers New Explanation For Ice Age

By Jeffery Kahn, JBKahn@LBL.gov

July 11, 1997

Recent ice ages -- ten periods of glaciation in the past million years -- are caused by changes in the tilt of the Earth's orbit, according to research published in the July 11 issue of Science magazine. The new analysis also presents strong evidence that another long prevailing theory does not account for these ice ages.

Researchers Richard A. Muller of the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab), and Gordon J. MacDonald of the International Institute for Applied Systems Analysis, Austria, are co-authors of the Science article.

Muller and MacDonald report that cyclical changes in the location of the Earth's orbit cause differing quantities of extraterrestrial debris to come into the Earth's atmosphere. This, in turn, results in variations of climate on the planet.

Milankovitch said the ice ages are caused by variations in sunlight hitting the continents. In his theory, the ice ages are linked to "eccentricity," a very gradual, cyclic change in the shape of the Earth's egg-shaped orbit around the sun that completes a cycle roughly every 100,000 years. Eccentricity changes the Earth's average annual distance from the sun and slightly alters the amount of sunlight hitting the Earth.

Muller and MacDonald's analysis yields "spectral fingerprints" which can be compared to the predictions of the two theories. Their analysis shows a clear pattern: The fingerprints of the ice ages show a single dominant feature, a peak with a period of 100,000 years. This precisely matches their theory. The fingerprints do not match the expected trio of peaks predicted by the Milankovitch theory.

Fishing?! Ice?! What would happen to the ice in the drink because of the moon?!?! C'mon, guys, can I just say: "Get a grip!" In a hundred years none of us is going to care! Please. Keep it REAL!

Pete

Exactly, and this is why i don't get overly concerned about global warming.