Tide facts for kids

The Bay of Fundy at high tide

The Bay of Fundy at low tide

Schematic of tides showing (exaggerated) high tides at the sublunar and antipodal points. The diagram shows the hypothetical case of an ocean of constant depth with no land. There would also be smaller, superimposed bulges on the sides facing toward and away from the sun.

A tide is the periodic rising and falling of Earth's ocean surface caused mainly by the gravitational pull of the Moon acting on the oceans. Tides cause changes in the depth of marine and estuarine (river mouth) waters. Tides also make oscillating currents known as tidal streams (~'rip tides'). This means that being able to predict the tide is important for coastal navigation. The strip of seashore that is under water at high tide and exposed at low tide, called the intertidal zone, is an important ecological product of ocean tides.

The height of tides varies somewhat with the phases of the Moon. At New Moon and Full Moon, tides are higher because the Sun's tidal force adds to the Moon's. This is called "spring tide".

Two tides a day

In most places, there are two tides a day. They each have a high point (the high tide) and a low point (the low tide). We speak of a "flood tide" coming in towards high tide, and an "ebb tide" going out towards low tide.

The period of the tide is about 12 hours and 25.2 minutes, exactly half a tidal lunar day. The lunar day is longer than the Earth day because the Moon orbits in the same direction the Earth spins. This is analogous to the minute hand on a watch crossing the hour hand at 12:00 and then again at about 1:05½ (not at 1:00).

The Moon orbits the Earth in the same direction as the Earth rotates on its axis, so it takes slightly more than a day—about 24 hours and 50 minutes—for the Moon to return to the same location in the sky. During this time, it has passed overhead once and underfoot once, so in many places the period of strongest tidal forcing is the above mentioned, about 12 hours and 25 minutes.

Because the gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker force on the opposite side. The Moon thus tends to "stretch" the Earth slightly along the line connecting the two bodies. The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally. As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium—there is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.

Related pages

• Aquaculture
• Tidal energy

Images for kids

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  In Maine (U.S.) low tide occurs roughly at moonrise and high tide with a high moon, corresponding to the simple gravity model of two tidal bulges; at most places however, moon and tides have a phase shift.

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  Illustration by the course of half a month

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  Animation of tides as the Moon goes round the Earth with the Sun on the right

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  Low tide at Bangchuidao Scenic Area, Dalian, Liaoning Province, China

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  Low tide at Ocean Beach in San Francisco, California, U.S.

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  Low tide at Bar Harbor, Maine, U.S. (2014)

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  The M2 tidal constituent. Amplitude is indicated by color, and the white lines are cotidal differing by 1 hour. The colors indicate where tides are most extreme (highest highs, lowest lows), with blues being least extreme. In almost a dozen places on this map the lines converge. Notice how at each of these places the surrounding color is blue, indicating little or no tide. These convergent areas are called amphidromic points. The curved arcs around the amphidromic points show the direction of the tides, each indicating a synchronized 6-hour period. Tidal ranges generally increase with increasing distance from amphidromic points. Tide waves move around these points, generally counterclockwise in the N. Hemisphere and clockwise in the S. Hemisphere

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  The harbour of Gorey, Jersey falls dry at low tide.

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  Brouscon's Almanach of 1546: Compass bearings of high waters in the Bay of Biscay (left) and the coast from Brittany to Dover (right).

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  Brouscon's Almanach of 1546: Tidal diagrams "according to the age of the moon".

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  Tidal prediction summing constituent parts.

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  Tidal patterns in Cook Strait. The north part (Nelson) has two spring tides per month, versus only one on the south side (Wellington and Napier).

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  US civil and maritime uses of tidal data

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  A rock, seen at low water, exhibiting typical intertidal zonation.

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  Spring tide: Sun and Moon on the same side (zero degree)

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  Neap tide: Sun and Moon at 90 degrees

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  Spring tide: Sun and Moon at opposite sides

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  Neap tide: Sun and Moon at 270 degrees

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  Spring tide: Sun and Moon at the same side (end of cycle)