How the Moon & Sun Cause Tides: Sixth Grade Science
Along ocean coasts worldwide, seawater rises and falls in a rhythmic, predictable dance that floods coastal estuaries before receding hours later to expose mudflats, driven by the gravitational pull of our celestial neighbors.
The Restless Pulse of Earth's Oceans
If you spend a day at the seashore, you will notice that the boundary between land and water never stays motionless. Over approximately six hours, the water level creeps steadily up the beach, submerging rocks and sandbars at high tide. Then, over the next six hours, the water slowly drains away, uncovering tidal pools, barnacle-covered piers, and sandy flats at low tide. This perpetual cyclic rise and fall of coastal sea levels is called the tide. While early civilizations viewed the tides as the breathing of the sea, seventeenth-century physics revealed that tides are enormous, planetary-scale waves generated by the gravitational attraction of the Moon and the Sun acting upon Earth's rotating hydrosphere.
THE TIDAL BULGE CONCEPT
(Exaggerated view of Earth's oceans)
Sublunar Bulge
(Near Side)
.----.
.'-/ \-'.
.' / \ '.
<=== Low Tide ===| | EARTH| |=== High Tide ===> ( MOON )
'. \ / .'
'.-/ \-.'
'----'
Antipodal Bulge
(Far Side)
The Gravitational Mechanics of Tidal Bulges
To understand why tides occur, we must examine Newton's Universal Law of Gravitation, which establishes that gravitational pull weakens as distance increases.
Differential Gravity Across Earth
Earth has a solid diameter of about 12,742 kilometers. Because of this vast size, the side of Earth facing directly toward the Moon is approximately 12,742 kilometers closer to the Moon than the opposite side. Consequently, the Moon exerts a stronger gravitational pull on the near side of Earth than it does on Earth's center, and a stronger pull on Earth's center than on the far side. This variation in gravitational force across the diameter of a planet is known as differential gravity or tidal force.
DIFFERENTIAL GRAVITATIONAL PULL ACROSS EARTH
Near Side (Close) Center (Mid) Far Side (Distant)
Strongest Moon Pull Average Moon Pull Weakest Moon Pull
======> ===> =>
(Water pulled (Earth pulled (Water left behind:
toward Moon) toward Moon) inertia creates bulge)
The Near-Side Bulge (Sublunar Bulge)
On the near side of Earth facing the Moon, the Moon's gravitational attraction is at its maximum strength. This pull draws the flexible liquid water of the oceans toward the Moon, causing water to pile up into a massive, broad wave known as the sublunar tidal bulge. This produces a high tide on the side of Earth facing the Moon.
The Far-Side Bulge (Antipodal Bulge)
Remarkably, a second high-tide bulge forms simultaneously on the opposite side of Earth, pointing directly away from the Moon. Why does this antipodal bulge occur? Because Earth's solid, rocky core is closer to the Moon than the far-side water, Earth itself is pulled more strongly toward the Moon than the distant oceans are. In essence, the Moon pulls the solid Earth out from under the water on the far side, leaving the water behind. Forward inertia causes this ocean water to bulge outward away from the Moon, creating a second high tide.
Between these two high-tide bulges, water is drawn away to supply the crests, creating two intervening troughs of low water known as low tides.
The Tidal Day and Daily Rhythms
Because Earth rotates on its axis once every 24 hours, an observer on the coastline is carried through both high-tide bulges and both low-tide troughs each day.
24 HOURS AND 50 MINUTES: THE TIDAL DAY
Time 0:00 Time 24:00 Time 24:50
Earth aligns with Moon Earth completes 360 deg Earth catches up to
rotation; Moon moved! Moon's new position!
.---. .---. .---.
( E ) ----> ( M ) ( E ) ( M ) ( E ) ----> ( M )
'---' '---' '---'
Why a Tidal Day is 24 Hours and 50 Minutes
Most coastlines experience two high tides and two low tides each day. However, high tides do not occur at the exact same clock time each day; they occur approximately 50 minutes later every day.
This delay occurs because while Earth is rotating, the Moon is simultaneously orbiting Earth in the same direction (counterclockwise/eastward). In 24 hours, Earth completes one full 360-degree rotation, but the Moon has moved roughly 13 degrees forward along its orbit. Earth must rotate for an additional 50 minutes to bring the coastline back into direct alignment beneath the Moon. This period—24 hours and 50 minutes—is called a tidal day (or lunar day). Consequently, the interval between consecutive high tides is roughly 12 hours and 25 minutes.
Tidal Patterns: Semidiurnal, Diurnal, and Mixed
Depending on coastline geometry, ocean basin shape, and geographic latitude, coastal regions experience one of three tidal patterns:
- Semidiurnal Tides: Two high tides and two low tides each tidal day of approximately equal height (common along the Atlantic coast of North America and Europe).
- Diurnal Tides: One high tide and one low tide each tidal day (common in the Gulf of Mexico).
- Mixed Semidiurnal Tides: Two high tides and two low tides each day, but with significant differences in height between successive crests and troughs (common along the Pacific coast of North America).
The Influence of the Sun: Spring Tides and Neap Tides
Although the Moon is the primary tidal driver, the Sun also exerts gravitational pull on Earth's oceans. The Sun is 27 million times more massive than the Moon, but because it is 400 times farther away, its tidal generating force is only about 44 percent as strong as the Moon's. Depending on whether the Sun and Moon act together or against each other, the tidal range (the vertical difference between high tide and low tide) varies on a two-week cycle.
SPRING TIDES (Sun, Earth, Moon Aligned)
Highest High Tides & Lowest Low Tides
.---. .---. .---.
( SUN ) <============ ( M ) <============ ( E )
'---' '---' '---'
(New Moon or Full Moon)
Tidal forces combine: Solar Bulge + Lunar Bulge = Maximum Tidal Range
-------------------------------------------------------------------------
NEAP TIDES (Sun and Moon at 90 Degrees)
Moderate High Tides & Modest Low Tides
.---.
( M ) First Quarter Moon
'---'
^
|
.---. .---.
( SUN ) <============ ( E )
'---' '---'
Tidal forces oppose each other: Solar pull cancels Lunar pull
Spring Tides: Maximum Tidal Range
When the Sun, Earth, and Moon align in a straight line in space (an alignment known as syzygy), the gravitational forces of the Sun and Moon combine. This occurs twice each month: during the new moon (when Sun and Moon pull from the same side) and during the full moon (when Sun and Moon pull from opposite sides).
During these times, the solar tidal bulge reinforces the lunar tidal bulge. The result is spring tides (derived from the German word springen, meaning to leap up). Spring tides produce the highest high tides and the lowest low tides of the month, resulting in the maximum tidal range. Spring tides have nothing to do with the season of spring; they happen every two weeks all year long.
Neap Tides: Minimum Tidal Range
When the Moon reaches its first quarter and third quarter phases, the Moon, Earth, and Sun form a 90-degree right angle (an arrangement known as quadrature).
In this perpendicular configuration, the gravitational pull of the Sun pulls water at right angles to the gravitational pull of the Moon. The solar tidal bulge works against the lunar tidal bulge, partially cancelling each other out. The result is neap tides. During neap tides, high tides are unusually low and low tides are unusually high, resulting in the minimum tidal range of the month.
Tidal Friction and Planetary Evolution
The motion of tidal bulges across shallow coastal seas generates substantial friction between moving water and the solid sea floor. This tidal friction acts as a continuous brake on Earth's rotation, slowing our planet's spin rate by approximately 2.3 milliseconds per century. Hundreds of millions of years ago, an Earth day lasted only 21 to 22 hours.
At the same time, conservation of angular momentum causes the energy dissipated by tidal friction to be transferred to the Moon's orbit. As a result, the Moon is slowly spiraling outward away from Earth at a measured rate of approximately 3.8 centimeters per year.
Exercises and Solutions
Exercise 1: Explain why Earth experiences two tidal bulges simultaneously on opposite sides of the planet rather than just one single bulge facing the Moon.
Solution 1: Earth experiences two tidal bulges due to differential gravity. On the near side facing the Moon, the Moon's gravitational pull is strongest, drawing ocean water toward it to form the sublunar bulge. On the opposite far side, the Moon's gravitational pull is weakest, while Earth's solid body experiences an average pull that tugs the planet toward the Moon. Solid Earth is pulled forward away from the distant ocean water, and inertia causes this water to bulge outward away from the Moon, forming the antipodal bulge.
Exercise 2: Contrast spring tides with neap tides in terms of celestial alignment, lunar phases, and effect on tidal range.
Solution 2: Spring tides occur when the Sun, Earth, and Moon align in a straight line during the new moon and full moon phases. The gravitational forces of the Sun and Moon combine to produce the highest high tides, lowest low tides, and maximum tidal range. Neap tides occur when the Sun, Earth, and Moon form a 90-degree angle during first quarter and third quarter phases. The gravitational forces pull at right angles and partially counteract each other, producing moderate high tides, modest low tides, and minimum tidal range.
Exercise 3: Why does high tide occur approximately 50 minutes later each day rather than at the exact same clock time?
Solution 3: High tide occurs about 50 minutes later each day because the Moon orbits Earth in the same direction that Earth rotates. While Earth completes one 360-degree spin in 24 hours, the Moon advances roughly 13 degrees along its orbit. Earth must rotate for an extra 50 minutes to bring the coastline directly back under the Moon's gravitational alignment, creating a tidal day of 24 hours and 50 minutes.