the tides

geordief

Is there a method that would allow me to calculate the times of the tides around the coast of Ireland? (I could of course just buy a local tide publication)

I see that there is a difference of about an hour between Galway and Dublin (not the same as the difference in times of sunrises).

What is the reason behind this ?

I know that the spin of the earth is 24,000 miles per day but the variation in the times of the tides don't seem to follow this directly.

gbee

You should find tidal times on line and there are a few apps for iPhone etc.

I would not know about actually calculating them, the tides move physically, much like a huge wave and are more influenced by the Moon, than by the rotation of the earth.

The rotation of the earth would not produce the tides as we know them, the tides are caused by the gravitation pull of the Moon which causes a 'bulge' and the earth rotates 'underneath'.

The extra high Spring tides are a combination of the Moon and the Sun pulling together.

Additionally, tides are influenced by the weather, major weather influences can increase high tides and keep tides higher than normal and delay rising tides. Tides are naturally influenced by topography, and island can divert tides until the backwash arrives giving a local port a later high tide and possibly even a 'race' tide.

John mac

http://www.pol.ac.uk/ntslf/tidalp.html

tide times for 4 weeks in advance.


couple more http://www.flaterco.com/xtide/

and here http://www.wxtide32.com/

darjeeling

geordief wrote: »

Is there a method that would allow me to calculate the times of the tides around the coast of Ireland? (I could of course just buy a local tide publication)

I see that there is a difference of about an hour between Galway and Dublin (not the same as the difference in times of sunrises).

What is the reason behind this ?

I know that the spin of the earth is 24,000 miles per day but the variation in the times of the tides don't seem to follow this directly.

There's actually a much bigger difference between Dublin and Galway - they're almost out of phase, so roughly when tide is high in one, it's low in the other. Tides vary from place to place depending on how the lie of the land and depth of the sea affect the movement of water.

UKHO EasyTide has free 6-day predictions for ports around Ireland

.

geordief

gbee wrote: »

You should find tidal times on line and there are a few apps for iPhone etc.

I would not know about actually calculating them, the tides move physically, much like a huge wave and are more influenced by the Moon, than by the rotation of the earth.

The rotation of the earth would not produce the tides as we know them, the tides are caused by the gravitation pull of the Moon which causes a 'bulge' and the earth rotates 'underneath'.

The extra high Spring tides are a combination of the Moon and the Sun pulling together.

Additionally, tides are influenced by the weather, major weather influences can increase high tides and keep tides higher than normal and delay rising tides. Tides are naturally influenced by topography, and island can divert tides until the backwash arrives giving a local port a later high tide and possibly even a 'race' tide.

well I think it must surely be a combination of the two?

As the earth spins , doesn't the "bulge " caused by the attraction of the moon "follow the moon" and so "follow the earth's spin" since the 2
are directly connected on a 24 hour basis? (I appreciate that the actual motion of the earth on its own doesn't cause the tides since it is a constant motion.)

By the way I thought the extra high Spring tides (if you mean the ones coming up at the equinox) are a caused by the sun's rays being additionally perpendicular to the equator (if I understood correctly the Orbit program that has just been shown on the BBC)

I think your last point about backwashes and topography explains why the tides don't just sweep across the globe in the same way as the sun does - but I think that the bulge caused by the moon might.

kindredspirit

Tides are very complicated.

HW and LW between Galway and Dublin are almost completely opposite in times, not an hour's difference. Remember the tide, after it hits the west coast first, has to to travel around the north coast and the south coast before it gets to Dublin.

The height of tides, which are an effect of the gravitational influence of the moon and to a much, much lesser extent of the sun, are also influenced by local factors. New moons and full moons produce Spring Tides two days later. Quarter moons mean neap tides. High and Low pressure systems can also have a significant effect on the height of tides. The length of time of a wind from a constant direction will also have an effect.

Local influences: e.g. The tide in Limerick is very much higher than Foynes because of the funneling influence of the Shannon Estuary. Courtown has almost no tide because of cancellation effect. There's weird tides off Inishboffin due to local effects. etc, etc. The seabed and shape of the coast around this country produce strange effects in places.

geordief

kindredspirit wrote: »

Tides are very complicated.

HW and LW between Galway and Dublin are almost completely opposite in times, not an hour's difference. Remember the tide, after it hits the west coast first, has to to travel around the north coast and the south coast before it gets to Dublin.

The height of tides, which are an effect of the gravitational influence of the moon and to a much, much lesser extent of the sun, are also influenced by local factors. New moons and full moons produce Spring Tides two days later. Quarter moons mean neap tides. High and Low pressure systems can also have a significant effect on the height of tides. The length of time of a wind from a constant direction will also have an effect.

Local influences: e.g. The tide in Limerick is very much higher than Foynes because of the funneling influence of the Shannon Estuary. Courtown has almost no tide because of cancellation effect. There's weird tides off Inishboffin due to local effects. etc, etc. The seabed and shape of the coast around this country produce strange effects in places.

which Inishboffin? There are more than one . There's one in Galway and another in Donegal (is there another?).
What is weird about the tides there ?

geordief

thanks
but I thought that there were also higher than average tides every time the moon was full (or empty also).
Unless I am wrong in this ,what is the coincidence that brings about these high tides ( which aren't as high as the tides at the equinox but still higher than usual-again unless I am mistaken on this) ?

darjeeling

Whenever the sun, earth and moon are in alignment you get spring tides. This happens each full and new moon. There's a nice FAQ page on tides at pol.ac.uk here that gives more info.

The biggest springs occur when a spring tide coincides with one of the equinoxes. When this happens, the sun appears directly above the equator, and the moon nearly so (NOAA tide pdf). The reason this causes abnormally high springs is hard to root out of the internet, but there's a brief explanation on astroprofspage.com (link):

astroprofspage.com wrote:

Near the equinoxes, the Sun appears to be right over the equator. A Full Moon or New Moon will also be over the equator at the equinox. And, it is easier to pull the tides when you are pulling with the rotation of the Earth [rather than at an angle to it], so that makes the tides slightly more extreme when you get a Full Moon or a New Moon near the equinox than an ordinary spring tide

Maybe someone here will know more?

Cú Giobach

geordief wrote: »

thanks
but I thought that there were also higher than average tides every time the moon was full (or empty also).
Unless I am wrong in this ,what is the coincidence that brings about these high tides ( which aren't as high as the tides at the equinox but still higher than usual-again unless I am mistaken on this) ?

I deleted my first post, it was "slightly" misleading.
Here have a look at this.
Check out "Inclination" at the end. When the forces that create these diurnal (once a day) tides are at a minimum, which is around the equinox, because as the moon is passing over the equator the Sun is also above it, the semi-diurnal (twice a day tides) are at a maximum.

gkell2

geordief wrote: »

well I think it must surely be a combination of the two?

As the earth spins , doesn't the "bulge " caused by the attraction of the moon "follow the moon" and so "follow the earth's spin" since the 2
are directly connected on a 24 hour basis? (I appreciate that the actual motion of the earth on its own doesn't cause the tides since it is a constant motion.)

By the way I thought the extra high Spring tides (if you mean the ones coming up at the equinox) are a caused by the sun's rays being additionally perpendicular to the equator (if I understood correctly the Orbit program that has just been shown on the BBC)

I think your last point about backwashes and topography explains why the tides don't just sweep across the globe in the same way as the sun does - but I think that the bulge caused by the moon might.

The empiricists such as John Wallis and Robert Boyle were extremely well rounded individuals who could discuss anything and everything,not by flinging around reckless assertions but rather taking note of observed tidal fluctuations in tandem with planetary and lunar dynamics and applying loose analogies when appropriate.It no longer happens today due to the fact that 20 years after Wallis wrote his treatise on tides in a letter to the Irishman Robert Boyle,empiricism imploded by overreaching the link between astronomy and experimental sciences through the predictive convenience of Ra/Dec,the same system that predicts tidal levels as days and dates within the calendar system just as it predicts lunar and solar eclipses.That in itself is a long and complicated story.

While the daily and monthly cycles indicate daily rotational and lunar orbital influence on tides,the third orbital influence is the one that catches the eye as the observant Wallis noted that these cycles are linked closely to the variations in natural noon cycle.The motion of the Earth is not constant as it speeds up and slows down at different points in its orbit and I assure you that only with today's imaging power and the ability to make planetary comparisons can the natural noon variations be explained properly whereas at the time of Wallis they rightfully conceded they didn't know.In any case,the original papers are well worth reading even if they eventually went astray and stuck too closely to the clockwork solar system.



"According to the second cause, if operating singly, we should have the longest daies at the two Solstices in June and December, and the two shortest at the Æquinoxes in March and September; which would at those times give occasion of four Annual High-waters.
But the true Inequality of the Natural Days, arising from a Complication of those two causes, sometimes crossing and sometimes promoting each other though we should find some increases or decreases of the Natural daies at all those seasons answerable to the respective causes (and perhaps of Tides proportionably thereunto) yet the longest and shortest natural daies absolutely of the whole year (arising from this complication of Causes) are about those times of Allhalontide and Candlemas; (or not far from them) about which those Annual High-tides are found to be: As will appear by the Tables of Æquation of Natural daies. And therefore I think, we may with very good reason cast this Annual Period upon that cause, or rather complication of causes. For (as we before shewed in the Menstrual and Diurnam) there will, by this inequality of Natural daies, arise a Physical Acceleration and Retardation of the Earths Mean motion, and accordingly a casting of the Waters backward or forward; either of which, will cause an Accumulation or High water.
'Tis true, that these longest and shortest daies, do (according to the Tables, some at least) fall rather before, than after Allhallontide and Candlemas (sometimes) those high Tydes: And it is not yet so well agreed amongst Astronomers are all the Causes (and in what degrees) of the Inequality of Natural daies; but that there be diversities among them, about the true time"
John Wallis

http://en.wikisource.org/wiki/Philosophical_Transactions/Volume_1/Number_16

Today it is nearly impossible to find an individual who can affirm that the daily temperature fluctuations keep in step with daily rotation let alone deal with tidal issues or orbital ones and that is a great shame.

kindredspirit

geordief wrote: »

which Inishboffin? There are more than one . There's one in Galway and another in Donegal (is there another?).
What is weird about the tides there ?

To understand a little more about local tide anomalies this is a good read (from a kayaker's point of view)

http://www.iol.ie/~dwalco/oileain.htm

You'll have to dissect it for the tide info unfortunately.