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Observation of Tides and Causes Behind Them

Grade 8
Jun 12, 2023

Observation of Tides


The motion of the Moon around the Earth:

The gravitational force between Earth and the Moon causes the Moon to orbit Earth. The Moon also rotates on its axis.

The motion of the Moon around the Earth

The gravitational force between Earth and the Moon causes the Moon to orbit Earth in an elliptical path.


moon orbit and the earth

At perigee: Moon is closest to the Earth.

At apogee: Moon is furthest to the Earth.


Earth is affected by the gravitational attraction of both the Moon and the Sun. The Earth’s surface develops bulges or bumps due to these gravitational pulls.


Gravitational pull of the sun and the moon on earth

The Moon is much smaller but closer to Earth than the Sun.

The Sun’s influence on the planet’s surface is significantly less than that of the Moon due to its greater distance from Earth. This pull on the rocky portion of the Earth’s surface is so small that humans are unaware of it. However, in large amounts of water bodies, like the oceans, the pull can be felt. The ocean’s surface rises and falls because of this force.

Twice a day, the water reaches its highest point on the shore and then recedes to its lowest point on shore.

Tides in ocean

Over weeks and months, the heights of the highest and lowest levels vary. Also, the times at which the high and low levels occur change throughout the year.

These daily changes in the height of the ocean are the result of the Moon affecting the Earth. These changes are caused by the gravitational forces that the Sun and Moon exert on Earth.

Ocean water by sun and the moon

Think again of the beach and how the level of the sea rose and fell during the day. This rise and fall in sea level are called a tide. A huge wave produced by the gravitational pull of the Sun and Moon causes a tide on Earth.

The ocean wave (or tide) formed due to gravitational pull has a height of only 1 or 2 m, but it has a wavelength of thousands of kilometers. As the crest of this large wave approaches the shore, the level of the water in the ocean rises.

tides in ocean

High tide refers to this surge in sea level. Six hours later, when the wave’s trough approaches, the sea level drops, causing a low tide.

Cause of Tides:

The attractive gravitational force exerted by the Moon on the water in an ocean depends on the distance of the water from the Moon. The closer the Moon is to the water, the stronger the Moon’s pull.

Causes of Tides

As a result, water is pulled on the side of Earth that faces the Moon more strongly toward the Moon than water on Earth’s opposite side. This causes two tidal bulges on Earth. One is on the side facing the Moon. The other is on the side opposite the Moon. The term “tide” refers to the daily cycle of rising ocean levels. Every 24 hours, the Moon makes one overhead pass. Thus, you would anticipate that the tide would only rise once every 24 hours.

But the oceans on the side of Earth that faces the Moon aren’t the only ones that rise; the oceans on the side of Earth that is directly opposite to the Moon rise as well.

The Moon does not orbit Earth as if it were stationary at the center of a circular orbit. Instead, Earth and the Moon revolve around a common center of mass.

Consider holding Earth and the Moon in place with a huge see-saw. Even though the fact that Earth is significantly heavier than the Moon, there comes the point where the see-saw balances. The Earth-Moon system’s center of mass is located at that location.

center of mass

When you take a sharp turn at a curved corner in a car, your body slides to the outside of the curve, away from the center. This happens because your body wants to move in a straight line in the direction it was going before the turn. A pseudo force called centrifugal force acts on your body.

Centrifugal force

Similarly, the tide on the side of Earth, which is on the opposite side of the Moon, is somewhat pulled outward by the Earth’s inertial forces as it spins around the Centre of Mass. This pull is because of Centrifugal Force.

Forces that influence the tides

The centrifugal force generated when the Earth and Moon revolve around this common center of mass is always pointed away from this common center of the revolution.

resultant bulges of water

The centrifugal force generated at any location on or within the Earth will always be directed away from the Moon because the Earth’s center of mass is always on the other side of this common center of revolution from where the Moon is located.

resultant tidal force on the earth

This fact is demonstrated by the thin arrows in Fig. 1 and the common direction of the arrows (indicating the centrifugal force Fc) at positions A, C, and B.

mood and earth

Magnitude of the forces  present

As Earth rotates and the Moon revolves, different locations on Earth’s surface pass through the high and low tides.

The Sun is much more massive than the Moon, but it also is much farther away; because of this, the Moon has a greater effect on Earth’s tides than the Sun.

Tides of earth

However, the Sun does affect Earth’s tides: it can strengthen or weaken the Moon’s tidal effect.

Spring Tide:

When the Moon and the Sun are lined up, they exert gravitational pull together; as a result, the high tides are much higher, and the low tides are much lower. This is called a spring tide, as shown in the Figure.

spring tide

Neap Tide:

However, when the two are at right angles to each other, the high tide is not as high, and the low tide is not as low, producing a neap tide.

neap tide

Observation of Tides


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