Two High Tides

[Mothy]

Puzzle for you all.

The earth spins on it's axis once every 24 hrs and the moon takes 28 days to rotate around the earth - hense the moon is overhead at a different time each day and the times of the tides shift a small amount each day.

The ocean tides are said to be caused by the gravitational effect of the moon acting on the earth.

If the earth spins only once per day, why are there two high tides each day?

[Jim Richards]

Plenty to read about the Moon and tides...

http://www.aerospaceweb.org/question/astronomy/q0262.shtml

[ckissick]

I'm a geologist. I'll take this one.

As you know, the gravitational pull between the earth and moon is what causes the tides. Over the billions of years that the earth and moon have been around, a stable "tidal wave" has developed. On the side of earth that faces the moon, you have your highest of the two daily high tides.

It seems odd, I know, but there is another, lower, high tide on the side of earth that is facing away from the moon. This is because the "tidal wave" is a normal ocean wave with a wavelength equal to one-half the earth's circumference. It has two peaks and two troughs, and it follows the moon as the moon does its orbit.

I suppose you could ask why the wavelength isn't equal to the entire circumference of the earth, with one peak and one trough. I have some ideas, but I'd have to look that one up.

In any case, it is the manifestation a true tidal wave. That's why we prefer to cause seismic waves tsunamis instead of tidal waves. Ironically, tsunami is Japanese for tidal wave, or so I hear.

[Zeke]

Originally Posted by ckissick

I'm a geologist. I'll take this one.



I suppose you could ask why the wavelength isn't equal to the entire circumference of the earth, with one peak and one trough. I have some ideas, but I'd have to look that one up.

No, I won't ask. I can see where the reciprocal tide is necessary to keep the planet somewhat in balance. I wouldn't want to live where I live if the thing wobbled. We'd have an earthquake every day.

[Hugh R]

That's easy. The sun and the moon pull on the Earth as if it were a single point. The Earth shifts slightly and the water on both sides of the earth sort of bulges out a little. Hence two tides/day.

[cgarr]

I am still trying to figure out if I am right on the equator and flush my toilet which way does the water go?

[Jim Richards]

Down, silly.

[gassy]

Cuz your mama jumped into the ocean twice.

[Zeke]

Originally Posted by cgarr

I am still trying to figure out if I am right on the equator and flush my toilet which way does the water go?

Goes the same regardless of your location. Check it out on snopes.

[Heel n Toe]

Originally Posted by milt

Goes the same regardless of your location. Check it out on snopes.

Yup... it goes whatever way the toilet is designed to make it go... most I've seen shoot the water in a clockwise direction.

cgarr, the question you're thinking of "is which way does the water in a sink or tub spiral when the plug is removed?"



That does go a different direction depending on whether you're above or below the equator. On the equator, I dunno.

[cgarr]

This is cool, Now I just need to load it up and try it!!

[Pazuzu]

Originally Posted by ckissick

I'm a geologist. I'll take this one.
Over the billions of years that the earth and moon have been around, a stable "tidal wave" has developed.

I'm a physicist, and as much as I hate to say it...you're wrong. Sorry.

The term "tidal" and "tide" come from gravitational tidal forces. Any gravitational source puts several different stresses on any other object...it doesn't just pull it.

One of the forces is a squeezing in the transverse direction. Another is a lengthening in the radial direction. These two force together cause the ocean to form an oval (more properly, and ellipse) around the Earth (which is also being squeezed and lengthened into an ellipse, but on a MUCH smaller scale).

The math behind it is pretty complex, but the result is well understood. It's not because of an ancient stable tidal wave traveling around the planet.

[Pazuzu]

I didn't look at the link in the second post, it basically says the same thing, without the big words

[Jim Richards]

Laplace is laughing...

[ckissick]

Originally Posted by Pazuzu

I'm a physicist, and as much as I hate to say it...you're wrong. Sorry.

The term "tidal" and "tide" come from gravitational tidal forces. Any gravitational source puts several different stresses on any other object...it doesn't just pull it.

One of the forces is a squeezing in the transverse direction. Another is a lengthening in the radial direction. These two force together cause the ocean to form an oval (more properly, and ellipse) around the Earth (which is also being squeezed and lengthened into an ellipse, but on a MUCH smaller scale).

The math behind it is pretty complex, but the result is well understood. It's not because of an ancient stable tidal wave traveling around the planet.

I've read my explanation in many places. It depends on how you look at it. A physicist, maybe it was Brian Greene, even went to so far to say that there is no such thing as a gravitational force. It's just an illusion caused by the warping of the fabric of space-time.

In any case, this link gives a pretty good explanation of the two tides.

http://www.pbs.org/wgbh/nova/venice/tides.html

[ckissick]

Originally Posted by Jim Richards

Laplace is laughing...

Funny you should mention Laplace. I used to work on earthquake prediction research in which we used groundwater levels in deeps wells to measure earth strain. If the strain changed anomalously, the groundwater would rise or fall, and an earthquake would soon follow. That's the theory, anyway.

Because of earth tides, groundwater rises and falls twice a day as the earth compresses and contracts. To filter out these earth tide effects, we modified the data with a Laplace Transform.

[DARISC]

[HardDrive]

I was SO waiting for a physicist VS. geologist expletive filled pissing contest. Take off the thinking caps and let the hate flow pencil necks.

[DARISC]

Originally Posted by HardDrive

I was SO waiting for a physicist VS. geologist expletive filled pissing contest.

If you ain't seen a topless female physicist versus geologist mud wrasslin' match, you ain't seen nothin"

[Jim Richards]

Getting my popcorn...I'll be right back.