An easier puzzle than deep waves

[Foxy]

Here's an easier one for those who don't already know, but who like a puzzle.... "Why is there a high or low tide on opposite sides of the world at the same time, why not high on one side and low on the other?" Admittedly, not a really tricky one, but interesting if you don't know the answer! And you get a new word to use in the pub!.....


/forums/images/graemlins/smirk.gif

 

[bilbobaggins]

Syzygy.....

 

[bbg]

Does it have anything to do with tidal nodes?

 

[damo]

One bulge (ie high tide) is due to the moon's gravitational pull, the opposite one is due to the 2-body rotation of the earth-moon system /forums/images/graemlins/grin.gif

example of explanation

 

[AngusMcDoon]

So how many tides a day in the South China Sea, and why?

 

[saltwater_gypsy]

Damo has got it!!
The centre of rotation of the Earth-Moon system is about 1/4 of the way below the Earths surface. This eccentric axis of rotation causes the water bulge out on the opposite side.
Damo's link has good diagrams to explain this.
Local tides and currents are often due to distortions of the tidal bulge by land masses.
Just look close to home at the Solent tidal system.

 

[Guest]

Similar explanation in fewer words:
The sea's level is raised on the side of the Earth nearest the Moon (principally the Moon). This tends to cause a small shift in the Earth's Centre of Gravity directly away from the Moon, but in so doing permits an equal rise in sea level on the other side of the planet, thus restoring the Earth's normal Centre of Gravity.

It might also be worth mentioning that the tidal bulges are stationary - it's the rotation of the Earth under them which gives us the impression of their movement around the Earth.

 

[saltwater_gypsy]

re. SAE 140.
Not just more words but INCORRECT ones!!!
The tidal bulge does not cause a sufficiently significant shift in centre of gravity to induce a tide on the opposite side. The flaw in that argument is that the Moon would decrease the tide on the far side.Check out Damo's link for a full explanation.
Indeed the bulges are stationary. A good analogy is a "ruck in the carpet runner "where the ruck can be moved from one end to the other without there being a significant movement of the whole carpet.

 

[Guest]

[ QUOTE ]
re. SAE 140.
Not just more words but INCORRECT ones!!!
The tidal bulge does not cause a sufficiently significant shift in centre of gravity to induce a tide on the opposite side. The flaw in that argument is that the Moon would decrease the tide on the far side.Check out Damo's link for a full explanation.
Indeed the bulges are stationary. A good analogy is a "ruck in the carpet runner "where the ruck can be moved from one end to the other without there being a significant movement of the whole carpet.

[/ QUOTE ]

You are quite correct that my explanation was incorrect - I wrote it too quickly and didn't check that the words matched my thinking !
A tidal bulge towards the moon would cause a shift in the earth's C of G towards the moon, thus relaxing the attraction on the opposite side of the earth by an equivalent amount. Got the direction wrong - a senior citizen moment ....