The Gravitational Connection
Tides are primarily caused by the Moon's gravitational pull on Earth's oceans — but the mechanism is more subtle than most people realize. It's not simply that the Moon "pulls" water toward it. What actually creates tides is the difference in gravitational force across the Earth's diameter.
The side of Earth facing the Moon is about 12,742 km closer to it than the far side. Since gravity weakens with distance (following an inverse-square law), the water on the near side feels a slightly stronger pull than the center of the Earth, while the water on the far side feels a slightly weaker pull. This differential — what physicists call the "tidal force" — stretches the ocean along the Earth-Moon axis, creating two bulges: one facing the Moon, and one on the opposite side.
That second bulge on the far side confuses people. Why would water bulge away from the Moon? Think of it this way: the Moon's gravity pulls the solid Earth away from the far-side water more than it pulls the far-side water itself. Relative to Earth's center, the far-side water is left behind, creating a bulge. It's a differential effect, not a direct pull.
Spring Tides: When Moon and Sun Align
During new and full moons, the Sun, Earth, and Moon are approximately aligned — a configuration astronomers call syzygy (one of the great words in the English language). During these alignments, the gravitational tidal forces of the Sun and Moon combine, creating especially strong tides called "spring tides."
The name "spring tide" has nothing to do with the season. It comes from the German word springen, meaning to leap or surge. Spring tides produce both higher high tides and lower low tides than average — the overall tidal range increases significantly. In places like the Bay of Fundy in Nova Scotia, which already has the world's most extreme tides due to its funnel-shaped geography, spring tides can push the difference between high and low water to over 16 meters (53 feet). That's a five-story building's worth of water movement twice a day.
The Bay of Fundy's extreme tides happen because the bay's natural resonant period — the time it takes water to slosh from one end to the other and back — almost perfectly matches the tidal period of 12.4 hours. It's the same principle as pushing a child on a swing: push at the right rhythm, and the amplitude builds dramatically.
Neap Tides: When Forces Oppose
During first and last quarter moons, the Moon and Sun are at roughly 90° angles relative to Earth. Their tidal forces partially cancel each other — the Sun's tidal bulge is oriented perpendicular to the Moon's. The result is "neap tides," with a noticeably smaller range between high and low water.
The word "neap" is old English and may derive from a word meaning "without power" — fitting, since neap tides are the weak ones in the cycle. The typical tidal range during neap tides can be 40-50% smaller than during spring tides. For fishermen, boaters, and coastal engineers, the difference between spring and neap conditions matters enormously.
The Sun's Role: Stronger Than You'd Think
The Sun's tidal force on Earth is about 46% of the Moon's. Given that the Sun is 27 million times more massive than the Moon, that might seem surprisingly low — but tidal forces depend on the gradient of gravity, not just the total pull. The gradient weakens with the cube of distance, and the Sun is about 389 times farther away than the Moon. Mass wins on total gravitational force; proximity wins on tidal force. The Moon wins the tidal battle because it's so much closer.
During spring tides, the Sun and Moon work together and their tidal forces add up. During neap tides, they partially oppose each other. This interplay — the Moon's 29.5-day cycle modulating the twice-daily tidal rhythm — creates the complex patterns that tide tables document.
Why There Are Two High Tides Per Day
Earth completes one rotation in about 24 hours, but during that time the Moon has moved slightly forward in its orbit — roughly 13° along its path around Earth. This means a complete tidal cycle takes about 24 hours and 50 minutes, not exactly 24 hours. That's why high tide comes about 50 minutes later each day, and it's why most coastal locations experience two high tides and two low tides in each tidal day.
I say "most" because the reality is messier than the textbook model. Continental landmasses get in the way. The ocean floor has ridges and basins and varying depths. Some places — like parts of the Gulf of Mexico — experience only one high tide per day (diurnal tides) because the geometry of the basin suppresses one of the two bulges. Other locations have "mixed" tides where the two daily highs are dramatically different heights.
Perigean Spring Tides: The King Tides
The Moon's orbit isn't circular — it's elliptical. At its closest approach (perigee), the Moon is about 356,500 km from Earth. At its farthest (apogee), it's about 406,700 km away. That 14% difference in distance translates to roughly a 30% difference in tidal force, because tidal force scales with the inverse cube of distance.
When a spring tide coincides with lunar perigee — when the Moon is both aligned with the Sun and at its closest to Earth — you get what's called a "perigean spring tide" or, more colloquially, a "king tide." These are the highest tides of the year, and they're when coastal flooding risks are greatest, especially if a storm surge coincides with the astronomical high water.
King tides have become an important concept in climate science. Coastal communities use king tide events to photograph and document what routine flooding will look like as sea levels rise. Miami, for instance, already sees king tide flooding on sunny days — streets in Miami Beach fill with seawater several times a year during these events, offering a preview of what higher sea levels will bring.
Tides in Enclosed and Semi-Enclosed Seas
The Mediterranean Sea has remarkably small tides — typically less than 30 cm (one foot) in most locations. That's because it's a semi-enclosed basin connected to the Atlantic through the narrow Strait of Gibraltar. The strait acts as a bottleneck, preventing the full Atlantic tidal wave from propagating into the Mediterranean. The same effect occurs in the Baltic Sea and, to a lesser extent, in the Gulf of Mexico.
By contrast, some river estuaries amplify tides dramatically. The Severn Estuary in the UK has a tidal range of up to 15 meters. The Amazon River experiences a tidal bore — a wave that travels upstream at high tide — called the pororoca, which can be several meters high and travel inland for hundreds of kilometers.
Practical Implications: Who Needs to Know About Tides
Tidal knowledge isn't just academic. Surfers time their sessions around tidal patterns — many breaks only work at certain tide stages. Fishermen know that fish feed more actively during tidal changes, especially during spring tides. Coastal engineers design seawalls and drainage systems for king tide scenarios. Port authorities schedule large ship movements around high water to ensure adequate depth under the keel.
If you're planning a beach trip, checking the tidal schedule alongside the weather forecast can make the difference between a wide sandy beach and a narrow strip against a seawall. A spring low tide exposes tide pools and rock formations that are normally underwater — some of the best beachcombing happens during these events.
Frequently Asked Questions
Do moon phases cause tides?
Yes. Tides are primarily caused by the Moon's gravitational pull on Earth's oceans. The Moon's phase determines its alignment with the Sun, which affects whether their gravitational forces combine (spring tides during full and new moons) or partially cancel (neap tides during quarter moons).
Why are tides highest during a full moon?
During a full moon, the Sun, Earth, and Moon are aligned in a straight line (syzygy). The combined gravitational pull of the Sun and Moon creates spring tides with the highest high tides and lowest low tides. The same effect occurs during a new moon.
How many high tides occur each day?
Most coastal locations experience two high tides and two low tides approximately every 24 hours and 50 minutes. The extra 50 minutes accounts for the Moon's orbital movement, which is why tide times shift later each day.
What are spring tides?
Spring tides are extra-high tides that occur during new and full moons when the Sun and Moon are aligned with Earth. The name comes from the German word "springen" (to leap), not the season. Spring tides produce both higher high tides and lower low tides than average.
What are neap tides?
Neap tides are weaker tides that occur during first and last quarter moons, when the Moon and Sun are at 90-degree angles relative to Earth. Their gravitational forces partially cancel each other, resulting in a smaller difference between high and low water levels.
Does the Sun also affect tides?
Yes. The Sun exerts about 46% of the Moon's tidal force on Earth's oceans. While the Sun is much more massive than the Moon, it is also much farther away, so its tidal influence is roughly half that of the Moon. The Sun's effect is most noticeable when it aligns with or opposes the Moon.
Why do some places have only one high tide per day?
Some locations, such as parts of the Gulf of Mexico and Southeast Asia, experience diurnal tides (one high and one low tide per day) due to the shape of the coastline, ocean floor topography, and the tilt of the Earth's axis relative to the Moon's orbit. These geographic factors can suppress one of the two daily tidal bulges.
Sources
- NOAA Ocean Service: Tides and Water Levels
- NASA: What Are Tides? (science.nasa.gov)
- Pugh, D.T. (1987). Tides, Surges and Mean Sea-Level. John Wiley & Sons.