The weather guy on TV just said hurricane sandy will be especially dangerous cause it will hit land at high tide, when the moon is full, which makes tides even higher.
Is that true? Sounds a bit off to me – my understanding is a full moon is not any closer to the earth or any larger than other moons; the only difference is whether the earth’s shadow is in the way or not. Just cause you can see more of the full moon doesn’t actually mean the moon has more mass or anything.
Is there any truth to the idea that full moons cause larger tides than other types of moons?
Until someone more confident and competent comes along, let me say that it’s not that you can see any more of the moon. But, if you can see the full moon it means the sun is directly opposite it. As a result, the tides will be exaggerated compared to the sun at other positions relative to the moon.
To explain further, both the Sun and the Moon raise tides. The Moon’s effects on the tides are about twice as big as the Sun’s, so it controls when the high and low will be. But when the Sun and Moon are cooperating, as it were, the total effect is about three times as big as when they’re working at cross purposes. Tides are thus most extreme (highest high tides and lowest low tides) at full and new moon, and least extreme at half-moon.
Incidentally, since I noticed the misconception in the OP, the Earth’s shadow has nothing to do with producing the phases of the Moon. When we see the Earth’s shadow on the Moon, that’s a lunar eclipse, a relatively uncommon event. The regular phases are the result of the Moon’s own shadow.
Yes. If the Earth were completely covered by ocean, and you observed it from a distance, all tides would appear as a bulge in the waters on the side facing the Moon, and a corresponding bulge, balancing this out, on the side opposite to the Moon. That is why there are two high tides every day. (You could not really see the bulges with the naked eye for a distance far enough to take both in, but they would be measurable.) If the Sun is (more or less) on the opposite side of the Earth to the Moon, you see a full moon, and the Sun’s gravity pulls on the bulge opposite the Moon, and the bulges, and thus the tides (on both sides) are raised higher than they otherwise would be. This will also apply at the time of a new Moon, when the Sun and Moon are on the same side, both pulling the same way. On the other hand, at a half Moon, the Sun’s gravity is acting on the water more or less at right angles to the Moon, so it pulls more of the Water to the sides away from the bulges, diminishing their size, and giving rise to smaller tides. The high tide still follows the Moon (on both the side facing it and the side facing away) because the Moon, being much nearer, acts on the water more strongly than the Sun does, but the Sun still has a noticeable effect, making the tides bigger or smaller, depending on the phase of the Moon. If there were no Moon, the Earth’s oceans would still have tides due to solar gravitation, but they would be much smaller, of course.
It is not the Earth’s shadow you see when he moon is not full, it is the moon’s shadow. If you can see the Earth’s shadow on the moon then you are watching an eclipse.
…And that the ground rises and falls 22 inches a day at the equator–or am I wrong, and it slides, like the bottom layer of the ocean, because:
Laplace’s tidal equations
Ocean depths are much smaller than their horizontal extent. Thus, the response to tidal forcing can be modelled using the Laplace tidal equations which incorporate the following features:
The vertical (or radial) velocity is negligible, and there is no vertical shear—this is a sheet flow.
The forcing is only horizontal (tangential).
The Coriolis effect appears as an inertial force (fictitious) acting laterally to the direction of flow and proportional to velocity.
The surface height’s rate of change is proportional to the negative divergence of velocity multiplied by the depth. As the horizontal velocity stretches or compresses the ocean as a sheet, the volume thins or thickens, respectively.
Also, I’m having trouble with gravitational gradient and its explanation. Help?
If the sun is overhead, the tidal force acting on your body (tending to make you taller) is the difference between the sun’s gravitational force at your head and that at your feet. Your feet are further away from the sun than your head, and the force of gravity falls away with (the square of) the distance from the sun, so your feet are slightly less attracted to the sun than your head is.
Because you are not very tall (in astronomical terms) and you are quite far from the sun, this tidal force is tiny. Much, much smaller than the total force exerted on you by the sun’s gravity.
But if you were very long, or very close to a very massive object such as a black hole, the tidal force acting along your body would be substantial. You could end up spaghettified, to use the technical term.
That. Let me add to Hibernicus’s explanation the fact that the tidal force falls off as the cube of the distance (to a first approximation). That is because the gravitational force is proportional to 1/d^2 and the derivative of that is -2/d^3. The minus sign means that the tidal force is repulsive rather than attractive and that explains the “spaghettification”. One result of this is that the tides are also stronger when the moon is closer to the earth. The earth/moon distance varies by, IIRC, about 10% and this causes the tides to vary by about 30%. So you get the higher tides at a new or full moon that coincides with apogee. And if it happens in early January, around perihelion, you get the highest possible tides. I think we are an average distance from the sun right now and I am too lazy to look up the moon’s distance.
The tide on the side toward the moon is a little higher than the one opposite, because the opposite one is farther from the moon where the gravity gradient isn’t as large. The same is technically true for the solar tides, but the earth is so small compared to it’s orbital radius, that there is very little difference in gradient due to the minuscule difference in distances to the sun.
I find it kind of cool that the solar and lunar effects add when the sun and moon are on the same side of the earth (new moon) and ALSO add when they are on the opposite sides (full moon).
This I don’t. I’m still not long; is the gravitational force (huge) such that it amounts to (effects smaller portions?) of my body, which is no longer “short”? By spaghettification? But that’s quid pro propter, to my mind.
My mental image, then, is that the gravitational force is more “focused,” able to work on teenier sections, sort of like the curve is divided up in tinier portions.*
*This is my dullard way of expressing a term in calculus, which, surprise, I don’t know.
The force of gravity decays as the square of distance. If you plot gravitational force vs. distance, you will have a parabola. A parabola is very sharply curved in the middle, and nearly straight as you go out on the arms. The tidal force depends on how sharply curved it is where you are. For strong tides, you need to be very close, or you need a huge mass to scale everything up. A black hole allows both.
It’s all about the differences in the force, hence the calculus. But I think I can show this with pictures. If you look at the Moon pulling on the Earth, we can all agree that the side nearest the Moon receives the most force, the center of the Earth receives less and the side opposite the Moon less still. In the picture below the length of the arrow roughly represents the strength of the gravitational pull:
Moon En Ec Ef
o <=== <== <=
En: Earth side nearest Moon
Ec: Earth center
Ef: Earth side furthest from Moon
When you look at the resulting net difference in the forces we have:
Moon En Ec Ef
o <= == =>
Resulting in the tidal bulge pulling the Earth in both directions.
Although a yachtie for 20 years, and critical of the reporting in the media, I thought I’d check:
An inspection of tide tables for the Queensland coast shows that the highest full-moon high tide might be a little higher than the highest new moon high tide. It’s the range that’s larger - the difference between low and high tide. The periods of low high tides are called “neaps”, when the range between high and low is smallest.
NOTE: SOME full moon high tides are not very high. Check out your local tide graphs for an easy appreciation of the patterns.
It’s really a news outlet’s need for making all events exceptional and exciting that has caused them to make such a silly statement. I heard today on ABC radio (Aust) “high tide AND full moon”, as if the moon was a newly discovered factor.
Also confused in Aust news reports is the concept of tidal surge - repeatedly being described as a wave. It’s not!!! it’s simply the higher sea level which occurs like a dome underneath a revolving tropical storm (hurricane, cyclone, typhoon). This id due to the atmosphere weighing less (“low pressure system”) hence allowing the sea level to rise underneath it, compared with surrounding seas which bear the “normal” weight of atmosphere.
The significance of storm surge on the coast is that is lifts the height of the surface of the sea so that it carries the storm’s destructive wind-driven wave action inland beyond the normal “coast”. This influence can extend for many hundreds of kilometres along a coast.
On our Queensland East Coast, the SW quadrant of the cyclone is the “dangerous” quadrant. One factor is that as the cyclone closes on the coast, wind driven water in this sector runs into the coast and can pile up the water in addition to the expected sea surge. Coastal contours can make a difference of metres in sea surge height at sites only a few kilometres apart.
There are about a dozen influences on the tides in any place; and yes, there is a lag on gravitational effects. It does take time for the water to move (hence “tidal flow” as distinct from “currents” due to other causes). Effects can be additive or subtractive. Hence some places may have more or less than the more usual twice-a-day tides.
The inability to predict tides across the world was important during WWII - the difference between troops landing at high tide (most convenient and safest) or landing at low tide with a wide area of mud and sand to cross while fully exposed to the enemy and the covering ability of ships’ armaments being a long sway off in deep water.
