Why Does The Moon’s Gravity Cause Tides On Earth But The Sun’s Gravity Doesn’t?

If you have ever lived close to the sea or worked on the oceans, you will understand the concept of tides. Twice every day, the sea levels will rise and fall which is known as the tides. 

Why does the moon’s gravity cause tides on earth but the sun’s gravity doesn’t?

It has long been known that the gravitational pull of the moon influences the tides that we experience on Earth. This is because the tides and the moon rising work at similar times.

It is not just the moon though, that has an impact on the tides found on Earth. The sun’s gravitational pull also influences the tides, however, this was not understood as quickly as the moon’s influence.

Sir Isaac Newton

The ocean tides seen on Earth are a direct result of the combination of the sun and the moon’s gravitational gradient. This concept was not fully understood until 1687 when Sir Isaac Newton explained the idea of gravity and gravitational attractions on water.

His law of universal gravitation states that the gravitational attraction between 2 bodies is directly proportional to their mass and inversely proportional to the square of the distance between them.

In layman’s terms, the larger the mass of the objects and the closer they are located to one another, the larger the gravitational pull (or attraction) between them is.  

In terms of tide levels, the proximity of the object to the Earth’s surface is more important than the mass of the object.  

Our sun is 27 million times the size of our moon. This means that if tidal gravitational forces were solely based on mass, the tides produced by the sun would be 27 million times larger than those generated by the moon.

In terms of distance, the sun is about 390 times as far from the Earth as the moon is. This means that the pull of the tide generating forces is dropped by around 3903  or 59 million times less than the moon. This ultimately evens out to solar forces being about half the strength of lunar forces.  

Image from: https://oceanservice.noaa.gov/education/tutorial_tides/tides02_cause.html

Lunar days

As the moon is rotating around the Earth in the same direction as Earth rotates around its axis, the lunar day is slightly longer than the 24-hour solar day. It takes roughly an additional 50 minutes for the Earth to catch up with the moon, creating a lunar day.  

As the tides move in accordance with the moon due to its increased gravitational force, we see high and low tides roughly every 12 hours and 25 minutes. This means that it takes half of a lunar day to complete one tide cycle.  This is known as a semidiurnal cycle. 

This is not the case everywhere on the planet. Places along the Gulf of Mexico have been recorded by the National Ocean Service as only having one tide per day – a diurnal cycle.  

The moon’s pull

As both the Earth and the moon are spherical (roughly) there is no constant distance between the two surfaces. The curvature of both surfaces means that not all points on the Earth are an equal distance away from the moon, or even in the same direction from the moon. The uneven surface of the Earth combined with this angularity means that there are differential forces upon the surface.

Differential forces are differences in the attraction between the moon and varying areas of the Earth’s surface. This causes the Earth to change shape slightly as the forces on the moon side are much greater than on the side facing away from the moon. This means that the Earth is slightly stretched to resemble a football shape. This is known as a prolate spheroid, and as you would expect, the longer edge faces the moon.  

With this knowledge, we can understand that if the Earth was entirely water, the spherical distortion amounts could range up to 1 meter. As we have a lot of solidity on the Earth’s surface, the actual distortion rate is much reduced to only about 1/3.  

Estimates suggest that the tidal distortion of the solid portions of the Earth only reaches about 20 cm. This means that the gravity on Earth is not balanced with the moon’s differential forces as there is not enough distortion. This means that the solid structures (such as land) are not impacted by the moon’s gravitational pull. As water distorts more easily, this is how the moon’s pull impacts tides.

These rising tides present as bulges in the water covering the Earth. The water on the side of the Earth closest to the moon pulls towards the moon, creating a tidal rise. This also happens on the opposite side of the Earth. As the day passes and the Earth rotates, we shift our position relative to the moon. This is why we experience 2 daily surges and falls in the tides. 

Image from: https://courses.lumenlearning.com/astronomy/chapter/ocean-tides-and-the-moon/

The sun’s pull 

The sun is much further away from the surface of the Earth, meaning that it has a weaker gravitational gradient to the Earth. This is estimated to be less than half as forceful as the moon’s impact on rising tides.  

There are 2 special types of tides to be aware of, called Spring and Neap tides. Spring tides occur when the sun and moon are aligned, as in the case of a new or full moon. These tides are particularly high as their gravitational gradient fields are added together, resulting in a stronger effect.  

Neap tides occur when there is a right angle between the positions of the sun and moon. This results in much lower tides than normal as the gravitational gradients more or less cancel one another out. The gravitational gradient of the sun will never be able to cancel out the moon’s completely, hence why there are still tides at these times.  

The orbit of the moon around the Earth takes roughly one month. This means that Spring and Neap tides are seen about every 2 weeks. 

Image from: https://courses.lumenlearning.com/astronomy/chapter/ocean-tides-and-the-moon/

Gordon Watts

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