Ocean Tides

Why are there two high tides in one day?


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2013-03-01 00:42:01
2013-03-01 00:42:01
Reason for Two TidesTides are caused not simply by gravity but by the gravity gradient or in other words the difference in gravity in different places. The side of the earth that is closer to the moon is more strongly accelerated than the side that's far away. The water that's near the moon gets pulled toward it (that's obvious). The not so obvious part is that the water on the far side is getting "left behind" because the Earth is getting pulled away from it.

The phenomenon of tidal force gets really significant around things with stronger gravitational pulls, say a black hole. If you tried flying into a black hole you'd never make it. The front of your ship would get pulled in faster than the back, ripping it apart before you arrived.

Same as above with a different 'spin'.When I got the following idea, I got the "high tide" opposite the moon. For a moment, in your mind, imagine that the earth and the moon are exactly the same size and mass. Which one is orbiting around the other? In fact, they would each be orbiting around a point in space right between them. Looking from high above in space, we'd see them both basically spiraling around one another. This should be obvious, since there would be absolutely no reason why one of them should (or could) be more 'stationary' than than the other.

The above idea holds true now for the real earth and moon, even though the moon is smaller. We would observe some perturbation, or scalloping of earth's orbit around the sun because of the moon's presence, and we would see the same kind of thing if we observed the moon's "orbit" around the sun.

What about that theoretical "point" around which they both orbit? Interestingly, the "point" is within the body of the earth. And, of course, the point is constantly moving, as the earth rotates on its axis. So, in a sense, whatever part of the earth's surface is opposite the moon, is in fact swinging out behind the moon-side surface of the earth! Thus, the push outward of the tides on the side opposite the moon. This is happening as a continuous movement as the earth rotates and the moon orbits. You can imagine in your mind's eye that there are therefore 2 high tides and two low tides on the planet, at any given point in time. Since we rotate through all of that "tugging and pulling" in 24 hours, we experience all of those tides.

The rotation of the earth is so much faster than the moon's orbit (relatively speaking) that the rotation has the effect of dragging the tide along with it a little, in advance of the moon, which has some interesting effects beyond the scope of this answer.

Another Viewpoint: So that's two different explanations. The first is based on

"differential gravity" and the second is based on "centrifugal" effects. Unfortunately, there are several "explanations" you can find for this tidal

phenomenon. Even scientists can give different answers. I tend to believe what mathematicians say about this question. From my reading on the subject, I believe that "differential gravity" is the preferred explanation when the problem is analysed mathematically.


Related Questions

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There are two high tides and two low tides a day.

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The three types of tides are:DiurnalSemi-diurnalMixedDiurnal tides have one high and one low tides per day.Semi-diurnal tides have two high and two low tides per day.Mixed tides go from extreme high to extreme low, then medium high and then medium low.Here is the distribution of the different tides

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Actually, there are TWO high tides and TWO low tides, on almost every day.

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There are two high tides and two low tides every day.

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