Tides

[Steve Senseney]

As the storm was hitting the east coast, and every one was talking about tides, I looked on Wikipedia to understand all of this better.

One illustration shows that the larger planet would have tides pulled toward the satellite (Moon) but also shows that it would have tides on the opposite side of the object.

I understand that at a "full Moon" the Sun and the Moon would be on opposite sides of the earth. But with just a single object pulling on the planet, why would the tide be high on the opposite side of the larger object?

[Dave Stewart]

+1 Steve. Whenever I hear someone say the "full moon high tide is higher" my gut reaction is wait a minute, how can that be... the moon hasn't changed position?! No doubt someone will come along with a simple explanation.

[Chuck Tweedy]

This link may help.
I didn't have time to read, but it looks like the ocean sloshes in a direction I didn't understand before.

http://home.hiwaay.net/~krcool/Astro/moon/moontides/

[Dave Stewart]

Good link Chuck......that explains it.
"When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon. "

[Steve Senseney]

Dave--

I understand when the Sun and the full Moon are aligned, that you get the tides on each side of the earth at their higher levels.

When they are on the same side of the earth (as at a time of eclipse) would there be a high tide on the far side of the Earth?

As I was thinking of this, imagine a single moon around a planet with water. If the size of the moon was increased, you would get a bigger bulge of water toward the moon. As it increased a lot more, eventually you might even pull water away from the planet toward the moon. As this occurs, you would not expect to have a "high tide" effect on the opposite side of the planet, with water drawn away from the far surface of the planet. There is no force pulling the water away on the far side of the planet.

[Dave Stewart]

huhhh.....you talkinta' me??
Steve, as I understand from the link, sun & moon act in concert at "spring" tides. During new moon (both on same side), they both exert extra pull on the tides.... ie higher. During full moon (opposite sides), the moon pulls the water towards it & the sun pulls the EARTH away from it...ie higher.
I'm happy to accept this & go back to sleep!.

[Jason Rodgers]

I like this bit of info:

Why are ocean tides so different everywhere?
"Because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans have vibratory modes that get stimulated in changing ways from minute to minute."

Can't put glitter on that, but I wonder if that's what clouds do?

[Simon Magennis]

I puzzled about tides for awhile. Eventually I came up with my own very over simplified explanation which some people think is crazy daft but it cleared up the issue for me.

First off I stopped thinking about the tides rising and falling. In my simple model they do not. Imagine a continuous skin of water over the globe. The water is much more elastic than the globe itself which mean that if the sun is pulling at one side and the moon on the other the skin of water becomes ellipsoidal rather than spherical. In other words the moon and the sun are pulling up the skin of water a little. The globe is more rigid and rotates pretty quickly so efective it rotates while the "humps" caused by the sun and moon stay in position. So in stead of visualizing vast bodies of water moving, think of the water as being stationary but the earth/globe underneath the water moving. The effect is more or less the same. Once I came up with this idea, the whole concept cleared up in my mind and I finally grasped the idea of spring tides and neap tides and so on.

Draw a circle inside an ellipse and visualise the circle rotating. Draw the sun and moon in different relevant positions and see what it would do to the ellipse and then visualise the circle rotating. Worked for me.

[Chuck Tweedy]

Works for me.