Peter Conway asked:
If the tides are caused by the gravity of the moon, why is there a high tide on the side of the Earth furthest from the moon as well as on the closest side?
Dominic - Tides certainly are caused by the gravity of the moon which is always pulling the Earth very gently towards the moon. Now, the side of the Earth which is faced directly towards the moon is slightly closer to the moon from the middle of the earth. That means it feels a slightly stronger pull because gravity decreases with distance from the moon. And so, that's being pulled more strongly towards the moon and so, you can understand why you get a high tide there. Water is being pulled there more strongly towards the moon.
Now, on the far side, the pull towards the moon is weaker than anywhere else on the Earth, just because it’s further away from the moon. And that means the moon is pulling down on that water on the far side less strongly than elsewhere. And so, it rises up away from the moon in the opposite direction from the moon to form this second high tide. So, you've got two, one on either opposite side of the Earth.
Chris - As the planet turns, it’s turning through both of those bulges of water, so you get high tide number 1, then it takes 12 hours to get round to the other side which is half a rotation, half a day, and there's the second bulge, second high tide.
Dominic - Exactly, so. Those two bulges stay in the same place in space more or less on the line through the Earth to the moon, and the Earth is rotating once every 24 hours, so we move through one of those two bulges every 12 hours as you say.
Chris - And just very briefly, Dominic, the difference between a spring and a neap tide. How does that happen and why?
Dominic - The sun also produces tides. They're about half the strength of the lunar tides. So, that's another signal on top of the lunar tides and sometimes the sun and moon tides coincide and sometimes they don't. They coincide at full moon and new moon, and then you have tides which can be 30%, 40% higher than at other times of the month when the moon and the sun are ninety degrees apart in the sky when you have what are called neap tides which are much lower.
The tidal force exerted on the earth causes a stretching effect on the earth. This causes the water to be pulled towards the moon on one side of the earth and pushed away from it on the other. This results in two bulges. As the earth turns each location on earth passes through the two bulges causing two tides. Pmb, Sat, 7th Dec 2013
Land barriers get in the way of that though and make it much more complicated, so it's really a case of oceans and seas sloshing around in resonance with the ideal pattern (for a landless globe that's all sea). The result is that in most places you get two high and low tides a day, though with times that vary from place to place far out of sync with the ideal case (when the two bumps would pass over), and in a few places there are four high and low tides a day instead of two, while in others there is only one high and low tide a day. David Cooper, Sat, 7th Dec 2013
I've never understood how there can be a high tide on each side of the earth, but I do know that the Sun also creates a tide some days they act in opposition and there's no tide at all. Never heard a satisfactory explanation. A Davis, Sun, 8th Dec 2013
If you take a single large mass (say the Moon, which has a stronger tidal pull than the Sun due to its closer distance), the Moon's gravity effectively tugs on the center of Earth's mass, ie the center of the Earth follows an ideal elliptical orbit, and this point feels no tide.
That's close to it. If you think about every particle of the Earth trying to follow its own orbit round the sun, only a tiny proportion of them are going to be on the line of the orbit that the centre of the Earth follows. The rest want to do other things. The particles on the Earth nearest the sun are orbiting too slowly and want to behave as if they are at aphelion, so their natural course to follow would take them closer in towards the sun and away from the Earth (if the Earth's gravity wasn't holding them back). The particles on the opposite side of the Earth want to behave as if they are at perihelion, so their natural course to follow would be to take them further away from the sun and thereby away from the Earth too.
I've always understood it as this:
I think I understand now, so if I drew a perfect elipse representing the earths orbit around the Sun there would be a monthly ripple on the orbit towards and away from the sun every two weeks, does this effect show up in any other measurement say the earths gravity or astronomical measurement. A Davis, Tue, 10th Dec 2013
There's nothing that happens every two weeks that isn't happening all the time. The part of the Earth furthest from the sun is being forced to orbit faster than the course it's following should allow, but its attached to the rest of the Earth and can't move about much. The water on top of it there can move more easily though, so it lifts off slightly, and that's why you get a bulge in the sea there. On the opposite side the opposite applies: the part of the Earth there is being forced to orbit more slowly than the course it's following should allow, so it wants to fall more towards the sun. Again it can't move much, but the water floating around on top of it can, so you get a bulge there too. David Cooper, Tue, 10th Dec 2013
It does happen all the time and the calculaton is extremely complicated, I've found ten variables in the calculation already there may be more. I was hoping that someone would answer the questions I posed, but I have answered one of them for myself after calculating the shift in the earths center of mass towards the moon. A Davis, Sun, 15th Dec 2013
It struck me last night that I'd only half switched my brain on when posting in this thread, due to pressure of time - I dash through here once a day to see what's new and am always in a hurry to get offline as quickly as possible. What I said before applies to the smaller component of the tide, that part caused by the sun, but I completely forgot to look at the larger component of the tides involving the moon.
While force of gravity follows an inverse square law with distance, the tidal force follows an inverse cube law with distance (being the difference of two points on an inverse square law curve).
Thanks - that's a good link. It also agrees that the simplest explanation in this case is indeed the right one. David Cooper, Sat, 28th Dec 2013
Evan-au said "the center of the Earth follows an ideal elliptical orbit", but that's not quite true. The Earth and moon are tied together into a system by gravity, and it is the centre of mass of that system that goes around the sun in a smooth orbit. Well, if you want to be sniffy about it, there are lots of other influences on this system, and the highest tides of all occur when the Earth, Sun, and all planets are in a line - but that doesn't happen often. However, ignoring these lesser influences and concentrating on the Earth-moon system, it is the centre of mass of the system that orbits the sun, not the centre of the Earth. I believe that the centre of mass of the system is, in fact, inside the Earth, but not at its centre. If the moon is directly over the UK, then the UK is closer to the centre of mass of the Earth-moon than New Zealand is, and whether "centrifugal" is a dirty word or not, it is helpful to think of water close to New Zealand as being thrown out by centrifugal force.
Chris and Dominic seem to be saying the high tides are close to the moon and opposite the moon. That way of thinking of it is just wrong. If it were true, then high tides would occur simultaneously along lines of longitude, but in fact they don't. Have a look at this: http://www.seafriends.org.nz/oceano/tides.htm Bill, Mon, 4th May 2015
1) “Centrifugal” force isn NOT a forbidden word whatsoever: it is just a poorly understood and poorly explained force. I´m not going to deliver now any further explanation about why I say so, perhaps in another post if found convenient.
In Reply #21 a wikipedia link is given. It surprises me the don´t mention the centrifugal forces originated by Earth´s rotation around the Moon/Earth rotation axis, as I exposed at Reply #20.
I scanned the section of Gravity From the Ground Up by Bernhard Schutz which covers ocean tides. It's on my companies website at http://www.newenglandphysics.org/Science_Literature/Journal_Articles/schutz_tides.pdf PmbPhy, Fri, 29th May 2015
In pdf linked in reply #23, I can see that Schutz also mentions only the different gravitatory pulls from the Moon …
I know centrifugal force can be considered as an inertial, actually kind of ficticious force … But not only that way.
Sorry. An "n" is missing at "But the object does**´t suffer that reaction force". It is clear, isn´t it? rmolnav, Sun, 31st May 2015
After my last posts I´ve found my point can be put in a simpler way:
I HAD NOT SEEN Reply # :
If anybody think I was utterly wrong at #30, as said at #31, please kindly google "tidal earth crust deformation" ...
No reply in two days ... I do hope somebody could be considering my point, without seeing clearly wether I´m right or wrong.
Sorry, but what said in #35 is NOT true. F. e., THREE times I have previously asked:
A couple of things more.
Two days more and … I´m going to post a “divertimento” on the issue , hoping it may be interesting to some folks.
rmolnav - Who are you talking to? I lost interest in this thread myself.
Looking for some information relative to local solid earth effects on ocean tides, I´ve seen in wikipedia an article titled "Tide".
Perhaps yesterday I was kind of too assertive when saying what quoted from wikipedia was a “physical nonsense”. I´ll try to explain why I said so.
Just a short post to put another argument/evidence supporting "my" theory ... as far as I can see.
Thank you, evan_au. I was just drafting something more, but relative to what I said about Io ...
Sorry. I said "outer Io bulges must get a little bigger ...". I mean both bulges: if an (or two) outer satellite pull outward, Jupiter has to compensate increasing its inward pull ... Otherwise Io would get out of orbit ... Well, that is kind of simplification. Those phenomena must be really complex. rmolnav, Thu, 18th Jun 2015
Who "we"? Southampton has four high tides per day, some Mediterranean ports have almost no tidal range at all. alancalverd, Thu, 18th Jun 2015
Recently I´ve seen a Wikipedia article where arguments similar to mines on #26 and 28 are given, when explaining differences between centripetal and centrifugal forces at different points of a rotating solid. In this case it is kind of a cable/elevator, with a "counterweight" rotating around our planet over the equator ...
yes the space elevator is science fiction.. however i dont see what it has to do with tides.. which are caused by gravitational attraction from the sun and moon... for something that has been known for centuries, if not millenia, I have a book that accurately tells me in advance the heights of all the tides in a year and the times.. and are accurate within reason (weather effects have a great deal to play in some areas)
(Sorry I wrote #49 instead of #51 in last post first line)
Apart from the issue of the wobbling I referred to in my last post, I previously exposed the other "leg" of my argument: the reality of centrifugal force.
This answer is wrong. The bulge on the opposite side of the earth is caused by a centripetal force created by the earth's rotation, which is in effect trying to throw the oceans off into space. Wiz, Thu, 28th Jul 2016