Equinoxes and tides.

In today’s Science Times (NY Times Tuesday science section) there is an item that claims that the largest tides (it is about tidal bores in the Amazon) come when full moon coincides with one of the equinoxes. It is clear that the sun and moon are exerting maximal tidal forces at full and new moon (since they coordinate rather than oppose) but what would this have to do with the equinox? I would expect that the strongest tides would happen when the full or now moon happens to coincide with both perihelion and perigee. Now perihelion comes in early January and I guess there is a perigee every month.

Can anyone explain why the equinox has anything to do with this?

According to NOAA it has to do with the Sun’s declination from the equator.

I feel stupid. Following that link and moving to the next page there is this animation:


Why is there a rising tide when the moon is on the opposite side of the planet (for the second high tide)?

Because, in addition to having more force on the side closest to the moon, there is less force on the side farthest away.

Or to put it another way, the moon pulls on the center of the Earth in an average sort of way. It pulls more strongly on the near side, which bulges toward it. It pulls less strongly on the far side, which seems to bulge away (because it is pulled less strongly than the middle). The oceans are even squishier than the earth, so they bulge more than the earth, both on the near and far sides.

This is a layman’s view. I think tides can be modeled in several different ways.

Imagine 3 balls evenly spaced separation 1mile apart on a long string, orbiting the earth, with the string pointing to the center of the earth all the time. The whole setup orbits at the speed of the center ball, say Xmph. The one closest to the earth should be going faster to maintain its orbital distance, but it isn’t… so instead it is pulled inward, it’s going too slow for its orbit, so it would be falling inward if it weren’t for the string. The one on the outside, it’s going too fast for its orbit, so it wants to fly away thanks to centrifugal “force”.

that’s why the fluids on an earth are tidal about the center of the earth-moon orbital center or the erath-sun orbital center.

I read this explanation from Isaac Asimov- he is like a god to me.

I read this explanation from Larry Niven- he is like a god to me.
I’ve always kinda hoped they were mathematically (or at least morally) equivalent.

The problem with 99% of the explanations you see for how tides work is that they fail to explain why oceans have tides and ponds don’t.

Here’s a much more detailed explanation from PBS. https://www.youtube.com/watch?v=pwChk4S99i4 Long story short, it’s not true that the tides happen because the water directly under the moon (and on the opposite side of Earth) is lifted up. It’s because the water everywhere else is squeezed toward those spots so the water directly at those spots has nowhere to go but up. Tides are not about up and down motion, they are about water sloshing sideways from one part of the oceans to another. That’s why you don’t see tides in ponds, because there’s a limit to how far the water can slosh around, so the tidal bulge is too tiny to notice.

Anyway, watch the video. It’s long but it really explains the topic well.

That’s true for visible tides, but the force acts on the entire earth, not just the oceans. The amount of tidal force one feels does depend on physical size (or at least length, in the tidal direction), so small things like people and ponds don’t feel it much, while large things like the earth and its oceans do.

And to be clear, the earth feels (“is subjected to”? Not trying to anthropomorphize) just as much tidal force as the oceans do. It’s just that the rocky planet itself doesn’t get deformed as much because it isn’t a liquid like water.

Or at least, it’s a much more viscous liquid. Give it time, and the Earth would settle in to a tidal shape, too. And the process of trying to settle into such a shape is causing the Earth’s rotation to gradually slow.

While we’re at it, if the Earth were nothing but a uniform ocean with a uniform floor, the tidal bulge of the oceans would also be very slight. Most of the noticeable tides comes from that sloshing running up against barriers and piling up.

Gravity falls off with distance, as we know. So the side closest to the Moon gets the strongest pull, the center of the Earth gets a little bit less and the far side gets even less still.

Like this:

Moon:              near     center   far    
O              <---------   <------  <---

So if you look at the next (difference) forces from Earth’s point of view, it looks like this:

Moon:                   near center far
O                       <----  <>  ---->

Tidal forces are all about the differences in gravitation pull.

You quoted the wrong poster, standingwave. Kinthalis had the basic question about tides (which you answered), Hari Seldon had a more complicated question that was answered in the first reply.

Equinox refers to a time when day and night are of equal length. I don’t doubt that it could affect the tides.

My bad.

I am afraid I don’t find the answer convincing. I don’t think that, in general, there is any correlation between the equinoxes and the tides. But the mouth of the Amazon is near the equator and therein lies the explanation. In general the moon is near the ecliptic (very near right now since there was a recent eclipse) and so they pull in the same line, more or less. If I were located at 23.5 N, I would expect my high tides to happen at the summer solstice when the sun is directly overhead and the moon is either directly overhead or directly opposite; that is new or full. But the mouth of the Amazon is 2 S, so the sun is directly overhead right around the equinoxes.

As I said, the other influences are how close the moon is to perigee, how close the earth is to perihelion and how far from the ecliptic, the moon is.

If this explanation is not correct, please show me why.

Well yes - Neutron Star and The Integral Trees - tidal effects figure in both those stories.

technically, yes, the bigger the pond, the bigger the slosh. It’s not like everything fluid is immediately pulled up 6 to 12 feet or more. (In fact, IIRC, the almost enclosed Mediterranean has minimal ties.) it’s a cumulative effect, and the spill happens from the sides; as you see from diagrams showing the exaggerated tidal bulges. What happens is the water pressure balances the tidal gravitational difference. Gravity “pulls” the close part of the oceans more (and less on the far side, so centrifugal force balances there) until the bulge creates additional pressure that balances, as with every force.

In a lake, say a Great Lake only 200 miles or so wide, the differential between gravity one end directly under (or opposite) the moon in Detroit, vs. 200 miles away in Buffalo, is pretty much minimal. In a global ocean, the sloshing has plenty of time to slowly balance out.

if you ever watched wave motion - except for the breakers on shore, things in motion pretty much stay where they are. If you float on the waves you go up and down in a circular motion. The water of the tides don’t follow the moon or sun; the bulge does, but only borrows from its immediate neighbourhood.

The tidal bulge simply becomes exaggerated when it hits an obstacle, and the fill/drain process becomes exaggerated, just as with waves. (Or tidal waves - a small tsunami and become very exaggerated in the right shore conditions).

Continuing in this vein …

Now that the Moon is tidally locked to Earth it is becoming distinctly asymmetric. Nothing we can see with the naked eye, but it’s real obvious from satellites orbiting the Moon that it’s lumpy and distinctly heavier on the Earthward side.

That’s not an ongoing process, but rather that the Moon settled in the position where it’s heaviest side faces the Earth. That’s a lower energy position than any other, so it was virtually certain to happen when the Moon stopped rotating.