How Does The Moon Cause Tides?

Ocean Tides Formation

Astronomers call tides a subtle but relentless force that has shaped most objects in the Universe. The center of mass of an object is the average position in an object of all its mass. For an evenly distributed sphere, that’s its center.

Gravity is the force that pulls everything in the universe towards everything else. Even though the moon is much smaller than the Sun, the moon’s pole has a much more pronounced effect on the Earth’s ocean. That is because it is closer than the Sun.

The moon exerts around two and a half times more force than the Sun. On the earth’s side facing the moon, the pull of gravity causes oceans to bulge outward. On the other side of the earth, the moon’s pull on the solid ground causes the oceans to bulge.

The Earth rotates on its axis, so this bulge constantly changes its location. The moon thus tends to stretch the earth slightly along the line connecting the earth to the moon. The solid earth deforms slightly, but the fluid ocean water is free to move much more in response to the tidal force.

Where the Bulge is bigger, it’s high tide. Where the water doesn’t bulge, it’s low tide. The moon’s orbit around the earth also causes tidal changes. Most places get two high tides and two low tides each day. Approximately twice a month around the new moon and full moon.

How does the moon cause tides?

Many factors create the tides here on earth, but the big three are the moon, the sun, and the earth. The gravitational pull of all three of those bodies interacting creates the tides. The biggest factor is the moon. It exerts about 2.2 times more power on the tides than the sun.

What causes tides? Tides are the periodic rise and fall of sea level at a particular place. When the sea level rises to its greatest height. It is known as a high tide, and when the sea level drops to its lowest height, it is known as a low tide.

Tides are primarily caused due to the gravitational force of the moon. But how? Part of the earth facing the moon experiences a stronger gravitational pull towards the moon than the earth’s center. So, the part facing the moon is pulled away from the center, creating a bulge. Thus it increases the sea level and causes a high tide.

The earth’s center experiences a stronger gravitational force towards the moon. Then the part facing away from the moon. Hence, this part is pulled away from the center, resulting in the opposite side’s high tide. In addition, the places in between the two high tides, where the sea level drops, experience low tides.

When the Sun, Moon, and Earth form a line, the tidal force due to this alignment is maximum. The range of the tides is also at its maximum. This is called springtime. When the moon is in the first quarter or third quarter, the Sun and Moon are separated by 90 degrees, and the solar, tidal force cancels the moon’s tidal force. At these points in the lunar cycle, the tides range at their minimum. When the Sun and Moon are at right angles, their gravities smooth out the bulges. The less extreme differences are called a neap tide.

Logically, water would be drawn toward the moon because of its gravitational pull, causing a tidal bulge. A tidal bulge also forms on the opposite side of the planet. So what’s going on? The moon’s gravity isn’t pulling on the water. It’s pulling on the whole earth too.

Water closest to the moon gets drawn towards it hardest. The Earth gets tugged toward the moon a little less vigorously, and water on the far side of the earth is pulled much less. So essentially, it’s being left behind and forming its little lump.

Our moon has a slightly elliptical orbit that varies how far away it is by about 50,000 km each month. Tides are much more pronounced when closest to or in perigee than when it’s the farthest at apogee. The continents keep the water from freely flowing, complicating tidal patterns.

  • Most places get two alternating high and low tides a day. If the highs and lows are about the same, it’s called semidiurnal.
  • If they’re different, it’s a mixed semidiurnal pattern. If they get one high and one low tide a day, it’s diurnal.

The moon is the biggest factor at work, but the Sun’s getting in on the action, too, with solar tides being about half as big as lunar tides.

  • When the Sun and the moon are aligned twice a month, we experience extreme highs and lows called spring tides because their powers combine.

The tidal force depends on several factors. For one thing, it depends on how strong the gravity is from the first object. The stronger the gravity, the more tidal force affected the object. It also depends on how wide the affected object is. The wider it is, the more the force of gravity from the first object changes across it, and the bigger the tidal force.

Finally, it depends on how far the affected object is from the first object. The farther away the affected object is, the lower the tidal force. Tides depend on gravity, and if gravity is weaker, so is the tidal force. The overall effect of the tidal force is to stretch an object.

Moon is 380,000 kilometers away from earth. So the gravitational force it has on you is pretty small. You’re small compared to that distance, a couple of meters long from head to foot. But the Earth is big! It’s nearly 13,000 kilometers across. That means the Earth’s side facing the Moon is about 13,000 kilometers closer to the Moon than the other side of the Earth. It is a big distance, enough for tides to become important.

The Earth’s side facing the Moon is pulled harder by the Moon than the other side of the Earth, so the Earth stretches. It becomes ever so slightly football-shaped, like a sphere with two bulges, pointing toward the Moon and one away.

The tidal force is strongest on the Earth’s sides, facing toward and away from the Moon, and weakest halfway between them on each side. Much of the Earth is covered in water, responding to this changing force. The water bulges up where the tidal force is strongest, on opposite sides of the Earth.

The tidal force stretches the solid Earth by about 30 centimeters. The moon spins once per orbit, so it creates tidal locking. Also, it’s worked on nearly every big moon in the solar system.

When the Moon is in the first quarter, the tidal bulge is 90° around the Sun; high tide overlaps low tide from the Sun. We get a slightly lower high tide and a slightly higher low tide. The pattern repeats when the Moon is full. The tides are so strong near a black hole, where the gravity is incredibly intense. Astronomers call this the spaghettification effect. Tidal waves are not related to gravitational push and pull. They’re usually the result of earthquakes.


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Sources:

Hubbard, Richard. Boater’s Bowditch: The Small Craft American Practical Navigator.
“Tidal lunar day.” NOAA. Do not confuse with the astronomical lunar day on the Moon.
Mellor, George. Introduction to physical oceanography. Springer.
“Glossary of Coastal Terminology: H–M.” Washington Department of Ecology, State of Washington.
“Definitions of tidal terms.” Land Information New Zealand.
“A tutorial on Datums.” National Oceanic and Atmospheric Administration (U.S.).

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher. I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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