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Robert Tulip
2011-Dec-27, 06:39 AM
Recently visiting the beach, I was surprised to see low tide with the waning moon high in the sky at meridian. I had thought that moon at meridian always matched to high tide.

1. How does the moon relate to tides?

2. How many high tides are there in a lunar month?

3. At new and full moon, do the sun and moon match to high tide at zenith and nadir, and low tide at horizon?

4. At neap tide (moon square sun), is the tide always at a specific time of day?

astromark
2011-Dec-27, 09:12 AM
Pick any coastal location and set about observing the timing of tides.. and the extent of the tidal hight.

Having worked with shipping and off coast sounding equipment for fifteen years

I have a first hand knowledge of tidal motion.

It is directly effected by the lunar position

and the daily rotation of the Earth in regard to the Sun..

Coastline shape and volumes of open water directly affect tidal motion..

Local knowledge and tidal charts will quickly show that some delay is apparent

from moon rise time to overhead position.. are NOT the same as high and low tide times..

Calendars are available that show local tidal times.. and heights..

You might see that a eleven hour cycle is apparent..

That moves to higher and lower tides as the lunar cycle continues..

This has NO relation to the lit portion of the moon cycle.

To your tides, full and new moon are the same.

*A neap tide and a king tide are not always at the hour of lunar position or alignment...

*How does the moon affect tides.. ? Gravity.

*How many a month.. ? 64..ish.. roughly..( I would need to look that up.)

*No they do not match full and new.. but tide hight is affected yes..

.. *and No,. The Tides are a hour or so earlier every day. Roughly..

Your local Newspaper should have this information daily.. Mark.

CaptainToonces
2011-Dec-27, 09:41 AM
"affected" mark, things are "affected" by other things. That is the verb affect, when you use "effect" it is for the noun form, an effect that happens.

Anyways, to answer the OP:
1. The tides are caused by both the moon and the sun, who exert roughly equal tidal force on the Earth. As the Earth rotates through its 24-hour rotation period, the tidal bulge caused by the moon will reach a maximum twice for a given location on Earth's surface (once when the moon is most overhead, and once when it is most underneath your feet). The same goes for the sun, whose tidal force will cause a bulge to pass through Earth daily that is largest at noon and midnight for a given place on Earth's surface. At the two times in the day when the sum of these two forces is greatest, you have high tide.

2. Since a lunar month is 29 days, there are 58 high tides in a lunar month.

3. The difference between high tides and low tides will be greater during new moon and full moon, because the sum of the of the moon and sun's tidal effects will be more in line, just as two waves in harmony produce a stronger additive wave rather than canceling each other out.

4. When the moon and sun are in a perpendicular fashion twice a month, the difference between low and high tide will be at minimum, because the effect of sun and moon will be close to canceling each other out. You will almost always be able to define a low and high tide though because it's difficult for the waves to match up exactly, so when you take the sum of them over the course of a day, there will usually be two maximums and two minimums per day.

As Mark says the local coastline geometry can play a role in tides too as it can lead to bunching up of the water mass due to the complications of coastal geometry, such as how shallow or deep the ocean is and if it is inside a bay or out on a peninsula.

Further complications are caused by the fact that the Earth's axis of rotation is 20 degrees offset from the ecliptic where the sun and moon reside, and the fact that the Earth is orbiting the sun in a yearly fashion, causing the interplay of sun and moon to be just a tad different than the lunar month would have you believe if the Earth and Sun were stationary relative to each other.

astromark
2011-Dec-27, 10:15 AM
I did not say lunar month did I.. Month.. and I added that I would need to look it up.. ( I did not.)

Joe average has no idea what a 'Lunar month' Is.

Most of them do not know the orbital period of the Moon..

and you are wrong.. At full moon or new moon the tides are not full at midday.. are they ?

and I can be churlish also.. You used the word 'who' when talking of the Sun or Moon.. should have been 'which'..

'Who' should only apply to people.. who are sometimes wrong.. I should know, I'm good at it..

and gracious enough to accept the slap when it comes.

Paul Beardsley
2011-Dec-27, 10:33 AM
and I can be churlish also.. You used the word 'who' when talking of the Sun or Moon.. should have been 'which'..

Churlish and wrong. "Whose" is the possessive form of both "who" and "which", so Captain Toonces used it correctly. (ETA Oops, just realised "who" is used for sun and moon, although as the meaning is clear, I would not complain.)

Also, whereas "king tide" is an actual term, it does not mean "spring tide".

Robert Tulip
2011-Dec-27, 01:10 PM
Does that mean that when I saw a low tide with the moon overhead, contrary to the expected gravitational pattern, it was purely due to shape of the coastline? This was at Coffs Harbour on 17 December.

I looked this up on wikipedia (http://en.wikipedia.org/wiki/Tide#Principal_lunar_semi-diurnal_constituent), simply because I was so intrigued to see a low tide and high moon at the same time. It says high tides basically track the moon around the planet over a 12.25 hour period. It seems the ocean is slightly more oblate than the earth and its oblation spins once every ~25 hours with the moon. So I am still mildly perplexed as to how the local tide can be completely out of phase from the gravitational driver.

This map of global tide patterns (http://upload.wikimedia.org/wikipedia/commons/5/5e/M2_tidal_constituent.jpg) shows global cotidal lines and tidal amplitudes, with eastern Australia basically sitting on east-west cotidal lines that throw the phasing out from the moon position.

If earth was a perfect spheroid, with ocean of constant depth, would the cotidal lines be purely north-south?

Swift
2011-Dec-27, 02:50 PM
"affected" mark, things are "affected" by other things. That is the verb affect, when you use "effect" it is for the noun form, an effect that happens.



and I can be churlish also.. You used the word 'who' when talking of the Sun or Moon.. should have been 'which'..

'Who' should only apply to people.. who are sometimes wrong.. I should know, I'm good at it..

and gracious enough to accept the slap when it comes.

Churlish and wrong. "Whose" is the possessive form of both "who" and "which", so Captain Toonces used it correctly. (ETA Oops, just realised "who" is used for sun and moon, although as the meaning is clear, I would not complain.)

OK, enough English lessons from everyone.

Hornblower
2011-Dec-27, 03:58 PM
There is some lag even in the middle of the ocean as a result of drag and the inertia of the sea water. In shallow coastal waters that lag increases, and the high tide bulge can take many hours to go relatively short distances in places like Chesapeake Bay in the vicinity of Washington and Baltimore.

MAPNUT
2011-Dec-27, 06:05 PM
Mayber an easier way to explain that is that the tide is a wave phenomenon, and waves take time to travel from an open body of water into a confined one. Also the speed of wave travel is dependent on depth, and is slower in shallow water.

Also some coasts have diurnal tides, that is, only one high and one low tide per day.

grapes
2011-Dec-27, 06:13 PM
2. Since a lunar month is 29 days, there are 58 high tides in a lunar month.The lunar month is close to 29.5 days long, and since the moon orbits the earth during that time, you lose one relative rotation, so it's about 57, instead of 59.
Does that mean that when I saw a low tide with the moon overhead, contrary to the expected gravitational pattern, it was purely due to shape of the coastline? This was at Coffs Harbour on 17 December.What were you doing up at 4:30 in the morning? :)

Tide tables for Coffs Harbour: http://tides.willyweather.com.au/nsw/mid-north-coast/coffs-harbour-beach.html
That says high tide was at 1:30am, and low tide at 7:15am

So, it's not exactly low tide, but it wasn't high tide either.

So I am still mildly perplexed as to how the local tide can be completely out of phase from the gravitational driver.
Some places even have a tide cycle that is closer to 24 hours, because of resonance.

CaptainToonces
2011-Dec-28, 04:53 AM
It says high tides basically track the moon around the planet over a 12.25 hour period. It seems the ocean is slightly more oblate than the earth and its oblation spins once every ~25 hours with the moon. So I am still mildly perplexed as to how the local tide can be completely out of phase from the gravitational driver.

You need to include the position of The Sun in your calculations. As I said, the high tide tracks the low and high point of the sum of the effects of The Sun and The Moon much like when you play two notes at the same time it creates a harmonic note that is different from the individual waves.

CaptainToonces
2011-Dec-28, 05:02 AM
and you are wrong.. At full moon or new moon the tides are not full at midday.. are they ?

I should very much think that they are. As I said, there are geographic issues at play. But as a general rule yes I should think that near midday during a full or new moon, the bulge of the ocean would be near maximum.

astromark
2011-Dec-28, 08:14 AM
I should very much think that they are. As I said, there are geographic issues at play. But as a general rule yes I should think that near midday during a full or new moon, the bulge of the ocean would be near maximum.

... and you know this to be true do you ?

Go get a tidal chart / calendar. Look at it.

Only to add, that your position of observation is pivotal to tidal times..

chrlzs
2011-Dec-28, 09:40 AM
May I just add.. Tidal calculation/prediction is incredibly complex, and there are more factors than those outlined above that determine the actual tide (which is not the same - in either timing or amplitude - to what is the calculable 'gravitationally induced' theoretical tide. Harmonic and non-harmonic factors come into play, like geography (including the nature (eg silt, rocky, weedy bottoms will give different frictional effects) as well as the depth, length and shape of the sea bottom, nearby outfalls, etc), atmospherics and oceanics (eg prevailing winds and waves, even interference from the 'backwash' of adjacent tidal waves), weather conditions (not just locally - storms and high/low pressure systems at huge distances can affect the local tide) and so on. Even the mean sea level (which varies seasonally) is a factor. And it goes on..

Some of that can be fairly accurately measured/predicted and thereby incorporated in tide tables, but some obviously cannot so the predictions have to be used with care. And I'll wager almost all published tide tables across the world have been developed from both forecasting and hindcasting, which allows the tables to have what might be rudely called 'fudge factors' that represent how the tides have been measured in the past to vary from what would be their theoretical value.

MAPNUT
2011-Dec-28, 02:17 PM
As far as I know tide tables are very accurate, except when there are severe winds. What you call fudge factors I call coefficients, which can be calibrated very accurately if there is 20 years of records. Tide calculation models include the effects of not just the moon and sun but also the larger and nearer planets. The "theoretical values" include all of these, but bathymetry is the most important.

Chew
2011-Dec-28, 05:09 PM
2. Since a lunar month is 29 days, there are 58 high tides in a lunar month.


Only in locations were the tides are diurnal. Regions that experience semi-diurnal tides get 2 highs and 2 lows per lunar day. Then there are areas of "mixed" tides where you get both diurnal tides and semi-diurnal tides through the lunar month depending on the Sun-Earth-Moon configuration.

http://i.imgur.com/SCCnH.png

The difference in the meridian passage of the Moon and the tide is called "priming" and "lagging" and is quickly demonstrated in this graphic:

http://i.imgur.com/GCaIc.jpg

If tides were easy to understand then Galileo wouldn't have gotten it wrong.

AutoBoof
2011-Dec-29, 03:09 PM
Here is the answer to number 1 in video...

http://www.youtube.com/watch?v=l37ofe9haMU

Chew
2011-Dec-29, 04:14 PM
Here is the answer to number 1 in video...




That shows why we get neap and spring tides but doesn't address why the tides don't strictly correspond to the position of the Moon, i.e. the priming and lagging.

Swift
2011-Dec-29, 04:29 PM
Here is the answer to number 1 in video...

http://www.youtube.com/watch?v=l37ofe9haMU
AutoBoof

I changed your embedded video to a link. We much prefer that users not embed videos, for a variety of reasons, but link to them instead. Rule 8 now explicitly states this. It is also considered common practice to give a little summary of what is at a video link (at least more than "the answer") - a lot of people don't like clicking on blind links.

Thanks,

CaptainToonces
2011-Dec-29, 09:16 PM
That shows why we get neap and spring tides but doesn't address why the tides don't strictly correspond to the position of the Moon, i.e. the priming and lagging.

I think that there are just a lot of factors at play, including the fluid dynamics of the ocean, the gravity of Sun, Moon, and even other planets, shoreline, ocean floor depth, altitude, time of year, subtle fluctuations in the moon's orbit.

Perhaps a better reading of the gravitational effects could be obtained simply by weighing a stone with a very accurate scale at different times of day. That way you eliminate the fluid dynamics of a liquid body like the ocean.

astromark
2011-Dec-30, 04:08 AM
My days of boating are at last over and I have quickly become a 'land lover'.. Oh Arrr... with Parrot.

However many years of coastal shipping and the need to enter river harbourage soon teaches facts.

Along the western coast of New Zealand the high tide times are at minimum an hour behind the clock.

and by clock. I am talking of the actual mid day point of the sun being highest, moment..

and that, that is often as late as 1pm.. or as early as 11.30 am..

The same story applies to the midnight tides.. Sitting at anchor waiting for water at the bar...

NZ has a great deal of open water around it. Unhindered by land mass shapes..

The small Island nation of Australia being some 1200 miles west... does not affect our tides..

The not too distant Wellington Port has a tide time 45 mins early'r and in the bay 'Port Nicholson'

Its over a hour latter. All of this gripping reality has given me some authority on this subject..

and I am almost amused by the string of not true facts I have seen here..

Not wanting or intending to offend.

I do understand the science of tides..

But in reality things are variable for many reasons..

I do not claim to know them all.. but that they are is enough.

Hornblower
2011-Dec-30, 01:15 PM
I think that there are just a lot of factors at play, including the fluid dynamics of the ocean, the gravity of Sun, Moon, and even other planets, shoreline, ocean floor depth, altitude, time of year, subtle fluctuations in the moon's orbit.
My bold. I would feel safe in ignoring the other planets. Their contribution to the gravitational gradient is so slight that even a moderate amount of wind would mask any differences they would make in the tides.

By "subtle fluctuations in the Moon's orbit", do you mean the monthly and annual perigee/apogee pattern, or do you mean longer term variations?

Perhaps a better reading of the gravitational effects could be obtained simply by weighing a stone with a very accurate scale at different times of day. That way you eliminate the fluid dynamics of a liquid body like the ocean.

That would not tell us anything about the gravitational field that we do not already know from observations of the motions of the Earth, Moon and Sun.

Robert Tulip
2011-Dec-30, 01:58 PM
I would feel safe in ignoring the other planets. Their contribution to the gravitational gradient is so slight that even a moderate amount of wind would mask any differences they would make in the tides.

I did some calculations on this a few years ago. Tides are calculated by mass divided by the cube of distance.

If the tidal effect on the earth of the moon is 1, tidal effects of other bodies are

Sun 31.6%
Venus 0.0050607%
Jupiter 0.0006120%
Mars 0.0000956%
Mercury 0.0000324%
Saturn 0.0000208%
Uranus 0.0000003%
Neptune 0.0000001%

Even considering that these daily gravitational patterns have been stable for all of the trillion days the oceans have existed, and the large quantity of water in the oceans (about two billion teralitres), the signal to noise ratio is so bad that any tides caused by other planets will be unmeasurable, and probably are below the threshold of theoretical existence.

Theoretically, we might say using these numbers that if the moon moves all the water of the sea every tide, then for example Neptune moves two cubic kilometers of earth water per tide. Venus would move 100,000 km3 of earth water per tide. But detecting that signal would be like listening to a local radio station on the other side of the planet, completely drowned in the static of stronger signals. Even when all the planets are lined up, the tidal influence from the moon is nearly 20,000 times greater.

grapes
2011-Dec-30, 02:37 PM
I did some calculations on this a few years ago. Tides are calculated by mass divided by the cube of distance.

If the tidal effect on the earth of the moon is 1, tidal effects of other bodies are

Sun 31.6%
Can that be right? I thought the sun was closer to half.

When I submit this to wolframalpha, I get 0.54:

(mass of sun/(distance to sun)^3)/(mass of moon/(distance to moon)^3)

Robert Tulip
2011-Dec-30, 09:42 PM
Apologies, I used an incorrect value for the mass of the sun. However, when I re-did the formula, I got a value of 46%. http://www.physicsforums.com/showthread.php?t=8121 confirms this answer.


Moon Sun Ratio
Mass kg 7.3477E+22 1,989,000,000,000,000,000,000,000,000,000 27,069,695
Distance: km 384400 149,600,000 389
Tide 1293603.79 594073.2571 0.45924