Did You Know the Moon Isn’t the Only Thing That Affects Earth’s Tides?
When we think about ocean tides, the Moon is usually the first thing that comes to mind. And for good reason: the Moon is the primary celestial body responsible for Earth’s tides. But did you know that the Moon isn’t the only object affecting the tides?
The Sun also plays an important role in Earth’s tides. Although the Sun is much farther away from Earth than the Moon, its enormous mass gives it a powerful gravitational influence.
The interaction between the gravitational forces of the Moon and Sun, combined with Earth’s rotation, creates the changing pattern of high and low tides that we see around the world’s coastlines.
But what happens during a full Moon? Why can tides become higher and lower than usual?
Let’s take a closer look.
What Causes Ocean Tides?
Ocean tides are primarily caused by gravity.
The Moon’s gravitational pull affects Earth’s oceans, causing seawater to bulge toward the Moon. At the same time, another tidal bulge develops on the opposite side of Earth because of the Earth-Moon system’s orbital motion.
As Earth rotates, different coastal areas move through these tidal bulges. This produces the familiar cycle of high tide and low tide.
Although the oceans are the most obvious place where tides occur, Earth’s solid surface also experiences small tidal movements. The atmosphere is affected by tidal forces as well.
The Moon Has the Biggest Influence
The Moon is the most important celestial influence on Earth’s ocean tides.
One reason is its relatively close distance to Earth. Tidal forces are especially sensitive to distance, so even though the Moon is far smaller than the Sun, its proximity allows it to produce a stronger tidal effect.
This is why the Moon’s position in relation to Earth has such a noticeable effect on ocean tides.
The Moon’s gravitational influence doesn’t simply create one high tide per day, however. Because Earth rotates while the Moon moves along its orbit, many coastal locations experience approximately two high tides and two low tides during a lunar day, although actual tidal patterns vary considerably depending on geography.
But the Sun Affects Earth’s Tides Too
Here’s the fascinating part: the Sun also creates tides.
The Sun is vastly more massive than the Moon, so it has tremendous gravitational influence. However, because the Sun is much farther from Earth, its tidal effect is weaker than the Moon’s.
Scientists estimate that the Sun’s tidal-generating force is roughly 46% as strong as the Moon’s.
The Sun’s gravity therefore works together with the Moon’s gravity to influence the height and timing of Earth’s tides.
Sometimes their effects reinforce each other, while at other times they partially oppose each other.
What Are Spring Tides?
When the Sun, Earth, and Moon are approximately aligned, their tidal forces work together.
This happens during both the new Moon and full Moon.
The result is known as a spring tide.
Despite its name, a spring tide has nothing to do with the season of spring. The word “spring” refers to the idea of tides “springing forth.”
During a spring tide:
- High tides are higher than average.
- Low tides are lower than average.
- The difference between high and low tide is greater.
- The combined gravitational effects of the Sun and Moon produce a stronger tidal range.
Why Are Tides Stronger During a Full Moon?
During a full Moon, Earth is positioned approximately between the Sun and Moon.
This arrangement places the gravitational influences of the Sun and Moon along roughly the same line.
Instead of working against each other, their tidal effects reinforce one another.
The Moon produces its tidal bulges, while the Sun contributes additional tidal forces. Together, they produce a larger difference between high tide and low tide.
This is why coastal areas can experience particularly large tidal ranges around a full Moon.
However, it is important to understand that a full Moon does not automatically mean that every beach or coastline will experience a dramatic flood.
The actual height of a tide depends on many factors.
New Moon Tides Are Also Strong
Interestingly, the full Moon isn’t the only time when strong tides occur.
New Moons also produce spring tides.
During a new Moon, the Moon is positioned between Earth and the Sun. The Sun and Moon are again approximately aligned, allowing their tidal forces to reinforce each other.
So both:
New Moon → Spring tide
and
Full Moon → Spring tide
can produce larger tidal ranges.
What Happens During a First or Third Quarter Moon?
The situation changes during the Moon’s quarter phases.
During the first-quarter and third-quarter Moon, the Sun and Moon are positioned approximately at right angles relative to Earth.
Their tidal influences partially oppose one another.
This produces a smaller tidal range known as a neap tide.
During a neap tide:
- High tides are lower than average.
- Low tides are higher than average.
- The difference between high and low tide is smaller.
So the Moon’s phase provides an important clue about the expected tidal range.
The Full Moon Doesn’t “Pull the Ocean Up”
A common misconception is that the full Moon somehow pulls the entire ocean upward.
The reality is more complicated.
The Moon’s gravity creates differences in gravitational acceleration across Earth. These differences are called tidal forces.
The oceans respond to these forces, producing large-scale changes in sea level.
The shape of coastlines, the depth of the ocean, underwater geography and the shape of bays can then greatly modify the tides experienced at particular locations.
This is why two coastal areas can experience very different tides even when they are under the same Moon.
Why Doesn’t Every Location Have Two Identical High Tides?
If the Moon is responsible for the tidal bulges, you might expect every location on Earth to experience exactly the same tidal pattern.
But Earth isn’t a perfectly smooth ball covered by a uniform ocean.
Continents interrupt the oceans, while underwater mountains, continental shelves, islands, bays and narrow channels influence how tidal waves move.
As a result, different places can have very different tidal patterns.
Some locations experience two high tides and two low tides each day. Others experience one high tide and one low tide per day, while some have mixed patterns.
Local geography is extremely important.
The Moon’s Orbit Also Matters
The Moon doesn’t travel around Earth in a perfect circle. Its orbit is slightly elliptical.
That means its distance from Earth changes.
When the Moon is closer to Earth, its tidal influence can be stronger. When it is farther away, its tidal influence is somewhat weaker.
The Moon’s closest approach is called perigee, while its farthest point is called apogee.
When a full or new Moon occurs relatively close to perigee, the resulting spring tides can be especially large. These events are sometimes popularly called king tides when unusually high tides occur.
The Sun’s Distance Also Changes
The Earth’s orbit around the Sun is also slightly elliptical.
That means Earth’s distance from the Sun changes throughout the year.
When Earth is closer to the Sun, around early January, the Sun’s tidal influence is slightly stronger than when Earth is farther away, around early July.
This effect can combine with the Moon’s orbital position to produce variations in tidal ranges.
Weather Can Make Coastal Water Levels Even Higher
Gravity isn’t the only factor that determines how high water gets along a coastline.
Weather can have a major influence.
Strong winds can push water toward the coast, while low atmospheric pressure can cause sea levels to rise somewhat. During storms, these effects can produce a storm surge.
This means an unusually high water level during a full Moon isn’t necessarily caused entirely by the Moon.
A combination of:
- Astronomical tides
- Moon-Earth distance
- Sun-Earth distance
- Wind
- Atmospheric pressure
- Coastal geography
- Ocean currents
- Storm conditions
can determine the actual water level observed at a particular coastline.
Full Moon vs. New Moon: What’s the Difference?
From a tidal perspective, both full and new Moons can produce spring tides.
The main difference is their position relative to Earth and the Sun.
During a full Moon, Earth is approximately between the Sun and Moon.
During a new Moon, the Moon is approximately between the Sun and Earth.
In both cases, the Sun, Earth and Moon are approximately aligned, allowing their tidal forces to combine.
Why Are Spring Tides Not Always Exactly on the Full Moon?
Another interesting fact is that the strongest observed tides don’t necessarily occur at the exact moment of the full Moon.
There can be a delay between the astronomical alignment and the highest local tidal response.
Ocean basins don’t respond instantly or uniformly to gravitational forces. Water has to move through enormous ocean systems, and local geography strongly influences how tides develop.
This means the date of the largest tide can vary from place to place.
Does the Moon Cause Tides on the Other Side of Earth?
Yes.
One of the most interesting aspects of tides is that a tidal bulge forms on the side of Earth facing the Moon, while another develops on the opposite side.
The far-side bulge isn’t simply caused by the Moon “pulling water away” from Earth.
Instead, tidal forces arise because the Moon’s gravitational pull is slightly different at different locations across Earth. The orbital motion of the Earth-Moon system also plays an important role in creating the overall tidal pattern.
Tides Are a Team Effort
So, the next time you look at the ocean during a full Moon, remember that you’re watching the result of an enormous gravitational interaction.
The Moon is the dominant influence, but the Sun is an important contributor.
When the Sun and Moon line up during a new or full Moon, their tidal effects reinforce one another and create spring tides, producing a larger difference between high and low tide.
When the Sun and Moon are positioned roughly at right angles during the first and third quarter phases, their tidal effects partially counteract each other, producing neap tides.
And that’s only part of the story. The Moon’s distance from Earth, Earth’s distance from the Sun, ocean depth, coastline shape, underwater geography, winds and atmospheric pressure can all affect the tides we actually experience.
Final Thoughts
The next time someone says, “The Moon causes the tides,” you can tell them they’re right—but there’s more to the story.
The Moon is the primary driver of Earth’s tides, but the Sun also contributes significantly.
During a full Moon, the Sun and Moon are approximately aligned with Earth, allowing their tidal forces to reinforce each other. This creates a spring tide, with higher-than-average high tides and lower-than-average low tides.
The same basic effect occurs during a new Moon.
So Earth’s tides are really the result of a fascinating gravitational partnership between the Moon, the Sun and our planet, with Earth’s oceans and coastlines determining how that gravitational dance appears from shore.
Frequently Asked Questions
Is the Moon the only thing that causes tides?
No. The Moon is the strongest tidal influence, but the Sun also produces tides. Earth’s rotation and the geography of the oceans and coastlines influence the tides we observe as well.
Does the Sun affect ocean tides?
Yes. The Sun’s tidal-generating force is significant, although it is weaker than the Moon’s because the Sun is much farther away.
Are tides higher during a full Moon?
A full Moon produces a spring tide, which generally means a greater tidal range—higher high tides and lower low tides. However, the actual water level depends on local geography and other factors.
Are tides also stronger during a new Moon?
Yes. New Moons also produce spring tides because the Sun and Moon are approximately aligned relative to Earth.
What is a neap tide?
A neap tide occurs around the first- and third-quarter Moon phases. The Sun and Moon’s tidal influences partially oppose each other, resulting in a smaller difference between high and low tide.
What is a king tide?
“King tide” is a popular term for an unusually high astronomical tide. These can occur when several factors, including the positions and distances of the Earth, Moon and Sun, combine to produce a particularly large tidal range.
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