Why are there 2 tides each day?

Question

I understand that the moon attracts so that would explain 1 tide but I dont get the other one

Answer 1

As the five first answer did not answer the core question of why we have TWO high tides in one period of moon transit, I dare to address the secret. The bulge of water that is forming on the opposite side of the moon is due to the Earth-Moon system is not rotating around the center of gravity of the earth but around the center of gravity of the combined system. So apart from the daily revolution of the Earth, the Earth is also "swung around" this other center of gravity that lies between the center of the earth and the surface of the earth in the direction of our moon.
This rotation around the combined center of gravity makes the point opposite to the moon more distant to the turning point of the geo-lunar system. This gives extra centrifugal force to the water bodies on that part of the globe and the water will rise there too.
To most people the mechanism of the high tide that is generated by the attraction of the moon is obvious. It's the second high tide in one revolution that is not easily explained.

The explanation that the sun creates the second tide is completely wrong.

Answer 2

We know that the Moon orbits around the Earth. As the Moon orbits the Earth its gravitational field causes a reaction on the Earth’s surface. This reaction effectively pulls upon the Earths crust causing it to bulge in two places. This first area of interaction would of course be the Earths crust located directly under the Moon. This would be the point of the greatest influence - or high tide. The second location would be the bulge on the opposite side of the Earth. This would be the point of least influence - or low tide.

Now, due to the fact that the Earth revolves faster than the Moon can orbit around it. The forward momentum of the Earth’s rotation tends to pull the high tide bulge slightly ahead of the Moon. On the other hand, the counter acting gravitational pull of the Moon on the Earth brings about a slight drag on the Earths surface. This drag causes the low tide bulge to be pulled slightly behind the Moon.

Because of this influence of the Moons gravitational pull on the Earth’s rotation. Over a period of billions of years the Earth’s rotation is slowing down. In addition to this the Earth’s gravitational pull on the Moon is gradually getting weaker while the centrifugal force of the orbiting Moon is getting stronger. This results in the Moon moving further away from the Earth, to the tune of one and a half inches per year.

Answer 3

The Moon pulls the entire mass of the Earth, not just the water. The water can move more freely than the rock, though, so it bulges more. The significant point, however, is that the Moon pulls across the entire diameter of the Earth. It doesn't just yank the surface nearest the Moon, it also pulls the centre away from the surface on the other side. The net result is elongation of the Earth, hence a high tide on both sides.

The conceptual difficulty comes from thinking of the tides in relation to the centre of the Earth, whereas you should be thinking of the planet as a whole being stretched.

Answer 4

The sun and moon affect tides in the same manner, and by about the same amount, but let's just look at the moon. The point of Earth closest to the moon draws the ocean (and land) making it bulge; The points on Earth that see the moon as just setting or just rising experience low tide at the same time. And, the point on the FAR side of Earth away from the moon also experiences a bulge. So, as Earth turns, it hits two high tides, and two low tides, in a 24 hour period.

Answer 5

OK here are three ways to look at it:

a) The oceans and the earth are both in freefall orbit around the moon. If both were small there would be no bulge because all objects in freefall/orbit fall at the same rate independent of mass.
With the earth being large, the earth falls with a freefall consistent with the moon's gravity as it is at the centre of the earth. But the surface on the side of the moon is 3000 miles closer to the moon, so the force on the water there is stronger than the earth as a whole. So the water moves up so that it develops a bulge.

On the side of the earth the force is the same, so no bulge.

However on the opposite side of the earth, the force from the moon is less. But that is less force in a downward direction, which is equivalent to a force in the upwards direction. So you get a bulge there too.



b) On the side of the earth towards the moon, the moon's gravity is strong, and pulls the ocean up.
On the opposite side of the earth, the force of the moon is weak, so the earth is accelerating quicker away so leaves the water behind.


c) You are on a bike on a circular track with a pole in the middle. You are riding no hands and have a mug of water in each hand, The mug in your left hand is attrached to a rope to the pole.
As you go round the mug in your left hand is pulled out - that's like the bulge on the side of the moon. But the mug on the right is also pulled out due to the centrifugal force as the moons gravity is less.

Bah! I can't explain a thing these days.

Answer 6

When I was a kid, this had puzzled me too. You would think that the moon's gravity would pull all the water towards it, so that we'd have only one tide a day, not two. But in fact, let's imagine that the whole earth was one huge ball of water with a heavy rock inside of it. What would happen as the earth and moon orbited about a common barycenter? In fact the liquid earth would become elongated, approaching the shape of a cigar, while the rock will fall towards the center of the elongated shape, because that's where the center of mass is. Now, let's make the rock larger, and let's also make the rock spin. After some thought, you'll see that what will happen is that the very large rock we call earth will have 2 bulges of water, one in the direction of the moon and the other away from it, both which stay put as the rock spins, and so that's why we have 2 tides a day.

Addendum: In response to Ernst's suggestion about the barycenter, in fact the barycenter could be OUTSIDE the earth and still the planet earth would be elongated. Let's imagine that both the earth and the moon were liquid and nearly of the same mass, so they are both orbiting about a point between them. Both of them would have elongated shapes like fat cigars pointed at each other. Almost anytime a massive body enters the gravitational field of another massive body, it will be subjected to internal forces, called gravitational tidal forces. This is the reason why small planetoids can fragment when flying by too close to a planet like Jupiter. And as a matter fact, the earth itself has had a dramatic effect on the moon: due to its gravitational tidal force on the moon, it has stopped its rotation relative to the earth, which is why the moon only keeps one face towards us.

Answer 7

Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide. There is about 12 hours and 25 minutes between the two high tides
Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day.
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.

Answer 8

well the moon attracts but in different angles, when a certain ocean is turned away from the moon, the gravity is not affecting it a much and leaves it a low tide. it the area is turned towards the moon ti pulls on it more and creates a high tide. (the moon is still there during the day, you just can't see it very often).

Answer 9

Gee. Noone here is even close.

You are right, the attraction of the moon alone only accounts for one tide - the one nearest the moon. The other is caused by the rotation of the Earth-Moon system about its centre of mass - this is NOT the centre of the Earth. As a result this roation is eccentric, and there is a net centripetal force on the side of the Earth opposite the moon. This leads to the second tidal bulge.

Answer 10

The moon's gravitational influence causes a bulge in the ocean that we see as the tides. A high tide occurs not just on the side of the earth facing the moon, but also on the opposite side. That's why there are two each day.