Tides

At the beach, the sea creeps up the sand.

Hours later, it slides far back. Rocks and little pools come out of the water. This rise and fall is called the tide. High tide is when the sea is high. Low tide is when the sea is low.

Spend a whole day at the beach and you may see the sea move. At one time the water may cover the rocks; hours later it may have slipped far back, leaving wet sand and tide pools. This slow rise and fall of the sea is the tide.

NOAA, the U.S. ocean agency, calls tides one of the most reliable things in the world: just as the Sun rises in the east, the sea keeps rising and falling along our shores. When the water is highest, it is high tide. When it is lowest, it is low tide. The difference in height between the two is called the tidal range.

NOAA describes tides as very long-period waves that move through the ocean in response to forces from the Moon and the Sun. They begin in the open ocean and travel toward coastlines, where they appear as the regular rise and fall of the sea surface. When the crest of this wave reaches a place, it is high tide; when the trough arrives, it is low tide. The height difference between them is the tidal range.

Tides are among the most dependable events in nature, and people have measured them for a long time: NOAA's tide station in San Francisco has been running for more than 150 years. Knowing the time and height of the tide matters to anyone who works on the water, from people who fish to the crews who guide large ships through shallow ports and channels.

The Moon pulls on Earth. It pulls on the sea too.

The sea makes a big bump on the side near the Moon. There is another bump on the far side of Earth. The bumps are high tides.

Gravity is the pull that objects have on each other. The Moon is close to Earth, and its gravity tugs on the oceans. The pull is strongest on the side of Earth facing the Moon, so the water there heaps up into a bulge toward the Moon.

There is a second bulge on the opposite side of Earth. There the Moon's pull is weakest, and the water tends to keep going in a straight line, away from Earth. NOAA calls this tendency inertia. The two bulges are high tides. In between, the water is lower, and those places have low tide.

NOAA's tides tutorial credits Isaac Newton with explaining, in 1687, that ocean tides result from the gravitational attraction of the Sun and Moon on Earth's oceans. The attraction is strongest where Earth faces the Moon, so water on that near side is drawn toward it, forming one bulge.

On the far side the Moon's pull is weaker; there, NOAA explains, inertia exceeds the gravitational force and the water tends to keep moving in a straight line away from Earth, forming a second bulge. Over the rest of the globe the two are roughly balanced. Because water is fluid, the bulges stay aligned with the Moon as Earth rotates. NASA adds two details: the Moon also stretches the solid Earth very slightly, into a shape a little like a football, and Earth's spin carries the bulges a little ahead of the Moon rather than directly beneath it.

Earth spins around once a day.

The two sea bumps stay lined up with the Moon. So most beaches pass through both bumps. That makes two high tides and two low tides.

Earth spins while the bulges stay lined up with the Moon. So a beach turns through one bulge, then a low part, then the other bulge, then another low part. That is why most coasts get two high tides and two low tides.

A tide day is a little longer than an ordinary day. The Moon moves along its orbit in the same direction that Earth spins, so Earth needs about 50 extra minutes to catch up with it. This tide day, called a lunar day, lasts 24 hours and 50 minutes. High tides come about 12 hours and 25 minutes apart, and the water takes about 6 hours to go from high to low.

A solar day is the 24 hours a spot on Earth takes to turn from beneath the Sun back to beneath it. A lunar day, or tidal day, measures the same thing against the Moon and lasts 24 hours 50 minutes, because the Moon orbits in the same direction that Earth rotates. Rotating through two bulges each lunar day, many coasts get two highs and two lows: high tides 12 hours 25 minutes apart, with about 6 hours 12.5 minutes from high water to low.

Continents complicate this. They block the bulges from sweeping freely around the globe, so each ocean basin develops its own pattern. NOAA names three: semidiurnal (two highs and two lows of about equal height, more typical of the U.S. East Coast), mixed semidiurnal (two highs and lows of different heights, typical of the U.S. West Coast) and diurnal (one high and one low each day, as in many parts of the Gulf of Mexico).

The Sun pulls on the sea too. Its pull on the tides is smaller than the Moon's.

When the Sun and Moon line up, high tides are extra high. When they pull from different sides, the tides are smaller.

The Sun is far bigger than the Moon, but it is also much farther away. So its pull on the tides is only about half as strong as the Moon's. The Sun does not make a separate set of tides; it makes the Moon's tides bigger or smaller.

At new moon and full moon, the Sun, Earth and Moon are nearly in a line. Their pulls add up, so high tides are a little higher and low tides a little lower. These are spring tides, a name that has nothing to do with the season: it comes from the tide "springing forth." About a week later, at a quarter moon, the Sun and Moon pull at right angles. Their pulls partly cancel, and the tides are gentler. These are neap tides.

NOAA notes that the Sun has about 27 million times the Moon's mass but is about 390 times farther away. Tide-generating force varies with the inverse cube of distance, so the Sun's is reduced by 390 cubed, about 59 million times, leaving it about half the Moon's. Solar tides appear as changes in the lunar tide, not as a separate set.

At new and full moon the two add, giving spring tides with slightly larger ranges; they happen twice each lunar month in every season. At first and third quarter the Sun and Moon act at right angles and partly cancel, giving neap tides. Distance matters as well: the Moon's distance varies by about 31,000 miles over a month, and tidal ranges are above average near perigee (closest approach) and below average near apogee. Ranges are also enhanced around January 2, when Earth is closest to the Sun, and reduced around July 2. The What If? labs let you test these effects in a model.

Tides are not the same size everywhere.

In one bay in Canada, the sea goes up and down more than 15 meters. Islands far out in the ocean have tiny tides. Wind can push the sea, too.

NOAA explains that the shape of the coast can make tides much bigger or smaller. When the tidal bulges reach the wide undersea edges of a continent, the tides can grow taller. Funnel-shaped bays can squeeze them even more. The Bay of Fundy in eastern Canada is the classic example: NOAA says it has the highest tides in the world, over 15 meters.

Islands in the middle of the ocean, far from any continent, usually have small tides of 1 meter or less. Weather matters too. Strong winds blowing toward the shore can pile water up, and winds blowing out to sea can push it away, uncovering more of the shore at low tide.

Astronomy sets the rhythm, but geography sets the size. NOAA notes that tides can be magnified where the bulges meet wide continental margins, while mid-ocean islands not near continental margins typically have tides of 1 meter or less. Funnel-shaped bays can alter tides dramatically: the Bay of Fundy is the classic case, with what NOAA calls the highest tides in the world, over 15 meters. Narrow inlets and shallow water have the opposite effect; parts of Laguna Madre in Texas and Pamlico Sound in North Carolina are classified as non-tidal even though they have ocean inlets.

Weather adds its own push. Offshore winds can move water away from the coast, exaggerating low tide; onshore winds can pile it up. High-pressure systems can lower sea level, bringing exceptionally low tides, while low-pressure systems are typically linked to tides higher than predicted. In rivers such as the Delaware and the Columbia, strong spring river flows can mask the incoming tide.

When the tide goes out, tide pools appear.

Barnacles, snails, crabs and sea stars live on the shore. Birds walk on the wet sand to find food. When the tide comes in, seals and fish swim in to eat.

The strip of shore between high tide and low tide is called the intertidal zone. It can be a rocky ledge, a sandy beach or a muddy flat hundreds of meters wide. Living things here must cope with being underwater and then out in the air.

NOAA explains that hardy animals such as barnacles, mussels, limpets and shore crabs live high up, where the sea reaches only at high tide. Lower down, where the tide comes and goes twice a day, live more kinds of plants and animals, including sea stars and anemones. At low tide, shorebirds search the shore for food; at high tide, seals, sea lions and fish come in to feed.

NOAA divides the intertidal zone into four bands. The spray zone is dampened by spray and high waves and is underwater only in very high tides or severe storms. The high intertidal floods at the peaks of high tide; its hardy residents, such as barnacles, marine snails, mussels, limpets and shore and hermit crabs, withstand pounding waves. The middle intertidal, over which the tide ebbs and flows twice a day, holds a greater variety, including sea stars and anemones. The low intertidal is almost always submerged except in the lowest spring tides, and life there is more abundant.

People plan around tides too. On June 14, 2002, four giant cranes from Shanghai were carried on San Francisco Bay under a bridge on their way to the Port of Oakland. With detailed knowledge of the tidal cycle and skillful piloting, NOAA reports, they cleared the bottom of the bridge by about 6 feet.