Why are there two high tides in one day?

Answer 1

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.

Answer 2

The moon exerts gravitational pull on the Earth, most affecting the water and oceans as they aren't fixed into place on the earths gravitational spin, and pulling them upwards in a kind of monumental bulge which stays fixed as the Earth rotates - and where the bulge is, the less water there is on the shores, while when the bulge moves, there is more water on the shores. This is the tides and why they are regular.

Answer 3

This is an interesting question. Think of the earth and the moon as a single "system" united by the gravitational attraction between them. We normally (when we're not being overly analytical) think of the moon orbiting around the earth, but this can't possibly be the whole truth of it. The earth and moon each orbit around their common center of gravity. If the earth and moon were of identical size and mass, you can easily see how this would be true. Their orbits as observed from high above the solar system, would each appear to be spirals. Now this is true for our earth and moon, except that the earth is much more massive than the moon. So the point around which they orbit is not half way between them. In fact, the earth is so much more massive than the moon that the point around which they both orbit, called the barycenter, is within the body of the earth itself. Because the earth is rotating on its axis, the barycenter is not a fixed point within the earth, but is moving so as to remain on the line connecting the centers of gravity of the earth and the moon. If you think about this, you will realize that this orbital force the earth experiences with the moon causes a force moving outward (toward the side opposite the moon). If you and a friend hold the ends of a rope and begin to "orbit" one another, you will feel this force attempting to pull you back, away from your friend. So the moon's gravity pulls the moon-side water toward it, and the orbiting of the earth around the barycenter tends to pull the far-side water away. There is also the fact that the moon's gravity is stronger on the moon-side of the earth, because that side is clearly closer to the moon. Also, and not insignificantly, the far side of the earth is less influenced by the gravity of the farther moon.

Answer 4

There are two major reasons for the high and low tides on opposing sides of the planet.

1) The Moon and Earth make up what we call the Earth-Moon system; they interact in a special way. It is not the moon that is doing all the orbiting while the earth is somehow 'still'. They orbit each other. Because the moon's mass is much lower than earth's, the point around which the Earth-Moon system orbits, the barycenter, is within the body of the earth itself. The barycenter is of course constantly moving in order to be in alignment with the centers of the earth and moon as the earth spins and the moon moves in its orbit. Think of the classic example of two people spinning around each other while holding onto the ends of one rope. If one person is larger, that person will still be doing some moving/orbiting, but perhaps not much depending on the difference in size. This "pushing back" of the opposite side of the earth has something to do with the high tide on Earth's opposing side.

2) The other reason is that the water on the moon-side of earth is closer to the moon, obviously, and subject to a greater gravitational effect. The water on the opposite side is farther away and therefore experiences a lower gravitational effect.

Put these ideas together, and you have two opposing high tides and two opposing low tides at the same time, at the perpendicular points of the planet.

Answer 5

High tides happen twice a day.

I think you mean highest tides,

and the reason is that during a full moon (and also a new moon) the sun, Earth, and the moon are all lined up in a straight line.

Answer 6

I'll give a qualitative answer for your question. The Moon pulls the Earth on one side, and the water "rises" towards the Moon, which we call tides. However, the Earth too moves a bit towards the Moon, "leaving behind" some water on the other end, which are the tides on the opposite side!

Edit: This is not one of my favourite questions, because even good sources give

different answers. Of course sometimes a thing can correctly be explained in more than one way.

The high tide on the side of the Earth nearest the Moon is easy to understand.

The water moves "towards" the Moon more than the land, because it's a liquid.

Of course what bothers people is the other high tide.

Here's my answer for the high tide on the far side of the Earth from the Moon. It's similar to the above answer, but importantly different.

Let's just consider the Earth and Moon and ignore the Sun, for simplicity:

The strength of the Moon's gravity is greatest on that side of the Earth facing

the Moon. It's less at the center of the Earth and least on the side farthest from the Moon. So, relative to the center of the Earth there is a force away from the Moon on the "far side" of the Earth. Water can react to this force more easily than the land, hence the tidal bulge on the far side.

Remember the important things with tidal forces are the DIFFERENCES between

gravitational forces at various places.

That's a brief, genuinely scientific, explanation. I know a lot of people prefer

an "easier" explanation, but I don't think there is one which is correct.

Answer 7

At full moon - and new moon, too - Sun, Earth, and Moon are aligned. This means that the tidal forces (differences in gravity) from Sun and Moon act in the same direction.

Answer 8

There are no tides on the moon, since there is no water or liquid of any kind on its surface, to our knowledge.

Answer 9

I think it helps thinking of Earth as being "pulled apart" by the difference in gravitation, by the Moon, on both sides of Earth (the nearer side, and the farther side).

Answer 10

This has to do with the Moon's gravitational attraction. Both on the side of Earth that is closer to the Moon and on the side that is farther away there will be tidal bulges.