Why two tides

[DPH]

Can anyone explain why the UK get's two tides a day and yet other parts of the world (I think New Zealand for example) only get one a day.

As the tides are caused by the moons gravitational pull I'd expect one a day.

 

[Skyva_2]

The main cause of tides is the gravitational pull of the moon and sun. When we have a high tide the opposite side of the earth also has a high tide. Similarly when the opposite side has a high tide so do we in the UK - hence two tides are the norm in one day. I didn't know that NZ has only one, but as the books say, 'tidal patterns are complex due to the Earth's spin, geography of Ocean basins, shallow water, funnelling, etc, etc,

Regards,

Keith

ps having checked at www.ofu.co.nz/graph/tides.htm it would appear that NZ has two tides as normal. There may be other places with one.

 

[hlb]

Then of course, there Poole with four tides per day. Just to confuse!!

Have you sent off Your Stamped. SAE and cheque. Yet.

Haydn

 

[byron]

Everywhere in the world has a tide approximately every 12 hours. Even the so called tideless Med has a small rise and fall. Some places i.e. Poole appears to have a double tide, this is caused by the flow returning before the ebb has finished due to the land mass of the I.O.W. trapping it.

ô¿ô

 

[jfm]

There was a thread on this about a year ago. I have never seen a satisfactory explanation of why there are two, not one. Skyva's explanation is along the lines of what many textbooks etc say, but with repsect to Skyva he does not explain WHY. He, like all the books I have read on this, say it hapens because it happens.

So, anyone know?

This is a separate form the detailed tide effects as found in Poole etc

 

[byron]

In short it happens because what goes up must come down. The Moon's gravity drags the water one way then loses it as it gets shielded as it wanes thus allowing it to fall back only to be caught again when the moon returns. This is why we have Spring Tides as the Moon is higher on the horizon it exerts more gravitation pull.

ô¿ô

 

[gjgm]

http://www.sfgate.com/getoutside/1996/jun/tides.html

will probably explain it most clearly.....!!

 

[boatone]

Right...here goes....try and get your heads round this...............
The Sun and the Moon both exert gravitational pull on the seas as they revolve around the earth. When the Sun and Moon are both inline on the same side of the Earth or on opposite sides of the earth their gravitational pulls are acting together and we have spring tides - ie highest high waters and lowest low waters. When the Sun and the moon are at right angles to each other their pulls are acting against each other and we have neap tides - lowest high waters and highest low waters.
Effectively, as I understand it, the Sun exerts the primary force which creates the tides and the Moons contribution determines whether they are springs or neaps.

TonyR
boatone@boatsontheweb.com
www.boatsonthethames.co.uk

 

[jfm]

Gjgm:
This explanation is plainly wrong. I have quoted the relevant bit and added comments in capitals.

"As the earth spins, this water is balanced evenly on all sides by centrifugal force THAT'S NONSENSE. The moon has a gravitational pull on this layer of water as it orbits the earth. CORRECT. This pull causes the water to bulge toward the moon. Because the earth is spinning there will be a bulge on the opposite side of the earth as well. NONSENSE

 

[jfm]

Tony, that's not correct. The moon is the primary force, the sun has a much smaller effect - it's 83m miles away!

What we need is an explanantion of why there are 2 tides a day, not one. Can anyone explain that?

 

[jfm]

Byron:
that's not an explanation There's no such thing as gravitational shield.

The reason why the high tide closest to the moon occurs is easy. The moon simply pulls the water towards it. But why does the hight tade farthest from the moon happen? Why does the weater there move away from the moon's gravitational pull? (It is most certainly not shielded from the moon's gravity by the mass of the earth).

I think it's something to do with the fact the earth itself is free floating. I mean, if the earth were connected to the moon on a rigid spoke, then I think there would be one tide, closest to the moon. But as all of the water, and the earth, and the moon are freefloating and impacted only by gravity (there are no spokes), the gravitational pull results in an elongation of any fluid body, so the earth/sea gets elongated, with a high tide closest to the moon and another farthest from the moon

 

[boatone]

Absolutely righ. sorry, got them the wrong way round but the relationship to springs and neaps is still correct. Will have to get my books out re the two tides issue!


TonyR
boatone@boatsontheweb.com
www.boatsonthethames.co.uk

 

[DPH]

I agree matter can’t shield gravity, however, gravitational pull does lessen with distance but I can’t see that effect causing two tides.

Is it some sort of natural resonance caused by the size of the earth? E.g. if the earth was bigger then you’d get three, four or whatever tides a day.

When there is a new moon then the moon and sun are in line from the earth and the additional gravity forces add up to create a larger pull. So why do you then get a spring tide when there is a full moon? The more I think about this, the more puzzled I’m getting! Someone here must know for sure.

 

[KevB]

The tides which we see in the oceans are due to the pull of the Moon and the Sun. The simplest explanation is that the water on the side of the Earth closest to the Moon is pulled, by the Moon's gravitational force, more strongly than is the bulk of the Earth; whereas the water on the side furthest from the Moon is pulled less strongly than the Earth. The effect is to make bulges in the water on opposite sides of the Earth. The effect of the Sun's pull is similar and the tides that we see are the net effect of both pulls.

When the pull from the Sun adds to that of the Moon the tides are large and we call them Spring tides whereas when the pulls are at 90 degrees the tides are small and we call them Neap tides. The heights of spring tides are governed by the distance of the Moon from the Earth, being largest at Perigee (when the Moon is closest to the Earth) and smallest at Apogee (when the Moon is at its furthest).

Because the Sun's pull is aligned with that of the Moon at New Moon and Full Moon these are the times when Spring Tides occur. The pull of the Sun is less than half that of the Moon and so the frequency of the tides is determined by the apparent passage of the Moon around the Earth which takes just over a day. We, therefore, in most places on the Earth have two tides a day with the time of each becoming later from one day to the next by just under an hour a day. (The actual period is, of course, determined by the rotation of the Earth and the orbit of the Moon.)

The height of the tide at any one place is determined by the shape of the coastline and of the nearby continental shelf. The presence of shelving land masses and bays gives much greater range to the tides than is seen in mid-ocean. A phenomenon which is generally not realised is that the air and solid landmasses also move up and down due to the tidal forces. Although the movement is much less in the land than that in the sea it can amount to a metre of vertical shift. It might be expected that the time of high tide would be when the Moon is on the meridian. This is not so. The reason is that, because of the Earth's rotation and friction, the tidal bulge gets left behind a little. The effects near complex coastlines such as in Britain are very difficult to compute.

http://www.rog.nmm.ac.uk/leaflets/tides/tides.html


<P ID="edit"><FONT SIZE=-1>Edited by KevB on Wed Mar 27 13:42:37 2002 (server time).</FONT></P>

 

[paulineb]

So why ...

... is the sky blue ? ;-)

Pxx

 

[trev]

The basic reason we have two tides per day is that the moons gravitational pull causes one bulge towards the moon and another, on the other side of the Earth, away from the moon (the equal and oposite forces principle) - thus the fundamental tidal period would be around 12 hours.
Springs and Neaps are combinations of the moon's distance from the Earth (which varies up to 30,000 miles) and the direction of pull in conjunction with the sun.
That's very basically it.
The four tides a day at Poole are, like those in the Solent, caused by the ebb down the English channel causing a second 'flood' into the local areas.

Trev

 

[DPH]

Thanks Kev. I think that's covered it.

 

[KevB]

Re: So why ...

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

http://www.ask.co.uk/

 

[jfm]

disagree, trev

Trev, you said...

"The basic reason we have two tides per day is that the moons gravitational pull causes one bulge towards the moon and another, on the other side of the Earth, away from the moon (the equal and oposite forces principle) - thus the fundamental tidal period would be around 12 hours."

This is merely stating what happens, not WHY it happens, imho

 

[DepSol]

Look guys if you want JFM to beleive you make sure its factual and not anecdotal please. Any half hearted attempt at explaining things will end up in a huge thread of queeries and retort.

Dom