The tides

G

geordief

Valued Senior Member
I have noticed,where I am that high tides, obviously coincide with a full (or empty) moon and that they also occur around 6 am or 18.00 pm

I have done a quick check around the country (Ireland) and was a little surprised to find that this pattern seems to apply all over(N,S,E and West)

This can't apply worldwide I imagine(or does it?).

Do the times of high tides vary around the world and does the full moon concide roughly with a 6 o'clock high tide everywhere?
 
geordief said:
I have noticed,where I am that high tides, obviously coincide with a full (or empty) moon and that they also occur around 6 am or 18.00 pm

I have done a quick check around the country (Ireland) and was a little surprised to find that this pattern seems to apply all over(N,S,E and West)

This can't apply worldwide I imagine(or does it?).

Do the times of high tides vary around the world and does the full moon concide roughly with a 6 o'clock high tide everywhere?
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The time of high tide in most places* gets later by ~50 minutes every day. If it is high tide at 1800 one evening it will be high at approx 1850 the next (and, in between, high at 0625or so).

The time of the spring tides is aligned to the full and new moons, as that is when the sun, moon and earth are in line, which maximises the tidal effect on the oceans. So the high tides are highest twice a month.

I rowed for many years on the Tideway in London so got used to working this out. (Rowing is best when the river is lower, as the shingle banks are exposed, which damps out washes from river craft. Washes are the enemy of good rowing, as one has to balance a racing shell, a bit as if riding a bicycle. Not easy, especially in a single, when there are washes cutting diagonally across your path.)

* The time of high tide varies from location to location quite a lot, as the high tide is in effect a bulge or wave travelling round the coast. This is especially evident in estuaries, where it can take half an hour or more for the bulge to make its way up the inlet from the open ocean. There are also special locations where tides follow a different pattern, for instance where the bulge travels round both coasts of an island and meets up on the far side. For rowing and sailing, tide tables for a locality will show the differences at the various ports and significant locations.
 
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geordief said:
I have noticed,where I am that high tides, obviously coincide with a full (or empty) moon and that they also occur around 6 am or 18.00 pm

I have done a quick check around the country (Ireland) and was a little surprised to find that this pattern seems to apply all over(N,S,E and West)

This can't apply worldwide I imagine(or does it?).

Do the times of high tides vary around the world and does the full moon concide roughly with a 6 o'clock high tide everywhere?
Click to expand...
Today is the full moon, and high tides in Dublin are at 11:02 and 23:38.

www.tidetime.org

Dublin Tide Times | 7 Day Tide Chart | TideTime.org

7 day tide chart and times for Dublin in Ireland. Includes tide times table and graph, moon phases and current weather conditions.
www.tidetime.org www.tidetime.org

You may want to re-evaluate your observations.
 
gmilam said:
Today is the full moon, and high tides in Dublin are at 11:02 and 23:38.

www.tidetime.org

Dublin Tide Times | 7 Day Tide Chart | TideTime.org

7 day tide chart and times for Dublin in Ireland. Includes tide times table and graph, moon phases and current weather conditions.
www.tidetime.org www.tidetime.org

You may want to re-evaluate your observations.
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Yes,you are right.

I was mistakenly looking at the low tide time.(which was the by chance almost same as the high tide time on the West Coast)

There seems to be a 6 hour difference between the West Coast and the East Coast of Ireland (which made the misreading more convincing to me)

I was expecting there to be a difference and was surprised when I misread the two times to be almost the same.

Where I am high tides concide with 6 o'clock and I can check when the high tide is due ,approximately just by looking at the moon.

We can't walk on the beach when the tide is particularly high so it can be handy to anticipate.
(also the tides can be even higher if there are strong winds close to shore and the roads are flooded occasionally)
 
geordief said:
Yes,you are right.

I was mistakenly looking at the low tide time.(which was the by chance almost same as the high tide time on the West Coast)

There seems to be a 6 hour difference between the West Coast and the East Coast of Ireland (which made the misreading more convincing to me)

I was expecting there to be a difference and was surprised when I misread the two times to be almost the same.

Where I am high tides concide with 6 o'clock and I can check when the high tide is due ,approximately just by looking at the moon.

We can't walk on the beach when the tide is particularly high so it can be handy to anticipate.
(also the tides can be even higher if there are strong winds close to shore and the roads are flooded occasionally)
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This doesn’t make sense. If it is high at 6 one day it will not be the next day. It gets later by nearly an hour a day.
 
The sidereal month would be the relevant period to tides, IF it were just the moon alone, but since it also depends on the sun, which is shifting wrt the vernal equinox, it's the slightly longer synodic month that's important to the tidal lineups. And that 29.5 days, not the 27.3 days of the actual orbit. So, quick math here ....divide 24 hours by 29.5, you get roughiy 50 minutes later per day. Not like anyone asked, but it's fun to work these things out.
 
TheVat said:
The sidereal month would be the relevant period to tides, IF it were just the moon alone, but since it also depends on the sun, which is shifting wrt the vernal equinox, it's the slightly longer synodic month that's important to the tidal lineups. And that 29.5 days, not the 27.3 days of the actual orbit. So, quick math here ....divide 24 hours by 29.5, you get roughiy 50 minutes later per day. Not like anyone asked, but it's fun to work these things out.
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I doesn't work out like that if you look at the charts.The time progression from one day to the next varies quite a bit although it must average out to what you said.

(I did that math in the past but I counted from 28 days )
 
geordief said:
I doesn't work out like that if you look at the charts.The time progression from one day to the next varies quite a bit although it must average out to what you said.

(I did that math in the past but I counted from 28 days )
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It varies because the moon's orbit is elliptical. So yeah, there's the average, and then there's the moon moving faster in its orbital path near perigee and slower near apogee. (Also oceans take some time to react and "follow")

Elliptical orbits, like Earth around the Sun, can give you headaches as certain markers shift. Like the Sun lagging behind solar noon in the winter throws off calculation of what's the earliest sunset (it's NOT Dec.20-21). E.g. where I'm located, the earliest sunset is Dec. 10th. Damned ellipses.
 
exchemist said:
Not really, unless I'm misunderstanding you. There is nothing about the tides that will be at the same time each day.
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I think this is what I said that you are questioning(?)

"Where I am high tides concide with 6 o'clock"

So I meant that the sequence of high tides peak when they occur at 6 am or 6 pm.

Like today we have a full moon (I watched it moving up from behind the mountain -it looked like a flaming ball rolling down the slope)

And because it is the full moon I know that the high tide is around 6.

But I also know that it is the highest high tide until 2 weeks time when the moon will be empty and since the previous "empty moon" 2 weeks ago-so "peak".

That is what I mean by the highest tides coinciding with (the ) 6 o'clock (tide)
 
geordief said:
I think this is what I said that you are questioning(?)

"Where I am high tides concide with 6 o'clock"

So I meant that the sequence of high tides peak when they occur at 6 am or 6 pm.

Like today we have a full moon (I watched it moving up from behind the mountain -it looked like a flaming ball rolling down the slope)

And because it is the full moon I know that the high tide is around 6.

But I also know that it is the highest high tide until 2 weeks time when the moon will be empty and since the previous "empty moon" 2 weeks ago-so "peak".

That is what I mean by the highest tides coinciding with (the ) 6 o'clock (tide)
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But that's not true. During spring tides (full moon or new moon) the high tide doesn't have to be at 6. It can be at any time of day, because, as I say, the time of high tide goes back 50min each day. So over 14 days, which is the interval between successive spring tides, it goes back 14 x50 = 700 mins = 11hrs 40mins. So it will seem to drift back by 20 mins each set of springs and after 3 months will have gone from 6pm to 8pm (I think?).
 
exchemist said:
But that's not true. During spring tides (full moon or new moon) the high tide doesn't have to be at 6. It can be at any time of day, because, as I say, the time of high tide goes back 50min each day. So over 14 days, which is the interval between successive spring tides, it goes back 14 x50 = 700 mins = 11hrs 40mins. So it will seem to drift back by 20 mins each set of springs and after 3 months will have gone from 6pm to 8pm (I think?).
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Maybe so ,but I have observed it to be the case for quite a while now..

I am quite settled in my opinion but will try to check the historical charts to see if there is any deviation.

When I lived in Mersea I took the bus every day to CRGS and it had to cross the Strood ,the causeway that separated the island from the mainland.

When the Strood was flooded the bus nearly always got through (it would be around 8 in the morning and 5 in the evening)

But if we had to go into town (Colchester) around midday the flooding would be much worse and we might have to wait up to an hour possibly before we could drive through.

So that is a regularity I was familiar with and the pattern lasted some 10 years.

Now,here the very high tides seem to come around 6 .

I will try to find the tide charts that go back a few years (that might be difficult) and let you know if I still think I am in the right.
 
OK I downloaded the Dublin Port tides for 2025 and looked for the first full moon in Jan 2025

High Tide is listed as 11 am. (Jan 13 2025)
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I followed the full moons over the later months and the corresponding high tides did advance a bit (about an hour perhaps) but when it got to December it had gone back again to 10.30 am.

So it didn't change more than half an hour over the year and seemed to advance and then retreat but staying pretty close to what it was at the beginning of the year.

That is one whole year.I don't know if we would need to look at earlier years or if the pattern is clear enough to say that the times of the highest tides (corresponding to the full moon) stays fixed to a particular time of day for any set location.


(I think earlier historical charts might have to be paid for ,but perhaps they could be found somehow on the net)

Edit Dublin Port's page

Tide Tables - Dublin Port

Use our tide tables to find out the tidal movements around Dublin Port. We update this regularly for your convenience.
www.dublinport.ie www.dublinport.ie

Re-edit: it was not not too hard to get Dublin Port's 2024 charts

They are almost identical to 2025...(so the pattern lasts 2 years at least)
 
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geordief said:
So it didn't change more than half an hour over the year and seemed to advance and then retreat but staying pretty close to what it was at the beginning of the year.
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I'm puzzled then. The synodic lunar cycle is around 29.5 days, so the average period between spring tides is 14.75 days. How would that get high tides being punctual at the same hour?
 
TheVat said:
I'm puzzled then. The synodic lunar cycle is around 29.5 days, so the average period between spring tides is 14.75 days. How would that get high tides being punctual at the same hour?
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Well ,I checked out the Mersea Island tides since I have the pattern in my memory from some 60 yrs ago -having lived there then.

I looked at both 2025 and 2023 and the times of the spring tides (as they are apparently called) were consistently /roughly in the midday region (11or 12 am or 1pm

(just as I remembered from being held up in traffic and cut off from the mainland by the high tides at or after midday)

I don't think it can be coincidence and ,unless I find a tide chart that shows different times of the spring tides (they always coincide with a full or empty moon) then I will continue to believe that they occur at roughly the same time from one lunar month to another

(not calendar month ,of course .Perhaps I wasn't clear about that? Could that have been the misunderstanding?)
 
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OK, sure. Spring tides will tend to occur in middle of the day, particularly when Sun and Moon are highest in the sky.
 
DaveC426913 said:
OK, sure. Spring tides will tend to occur in middle of the day, particularly when Sun and Moon are highest in the sky.
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But not where I am .It is always around 6 am or 6 pm here.(West Coast of Ireland)

East of England (Mersea Island) looks like 12 am or 12 pm (give or take an hour or so - and allowing for changes in the clocks in applicable regions)

I was under the impression that the times of spring tides correlate very closely with the moon being full or empty .

(Here we seem to get slightly higher tides when the moon is empty than when it is full)
 
geordief said:
But not where I am .It is always around 6 am or 6 pm here.(West Coast of Ireland)

East of England (Mersea Island) looks like 12 am or 12 pm (give or take an hour or so - and allowing for changes in the clocks in applicable regions)

I was under the impression that the times of spring tides correlate very closely with the moon being full or empty .

(Here we seem to get slightly higher tides when the moon is empty than when it is full)
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I’ve been thinking more about this and also found this link, which confirms you are quite right and I was wrong: https://ntslf.org/tides/about-tides/tides-faq

High water at the peak spring tide does indeed occur at approximately the same time of day, just as you say. The funny thing is that it does not necessarily occur at midday or midnight, when one would think the sun would add maximum pull to that of the moon. It varies by location, due to wave effects as the tidal bulge travels round complex coastlines.

So I’ve learnt something here.
 
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