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Sinking City of Venice

Tidal Curiosities

Four Questions


Venice homepage

Diagram: how far moon travels in a 24-hour period

 

Why is the tide cycle 24 hours 50 minutes long instead of just 24 hours long?

You might think that, because the length of the tide cycle is determined by the Earth's rotation, the tide cycle should be 24 hours long. The reason it's longer is that, during a 24-hour period, the moon travels about 12 degrees of its 360-degree orbit. The extra 50 minutes is how long it takes any spot on the Earth to "catch up" to the moon's new position.

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Diagram: Earth/moon/sun system with moon opposite Earth and in line with sun, and Earth/moon/sun system with Earth opposite moon and in line with sun

 

Why do the biggest tides happen when the moon is new and full?

Although tides are affected mainly by the moon's gravity, gravity from the sun also pulls on the oceans. In fact, the tidal force of the sun is almost half (46%) that of the moon. When the moon, sun, and Earth line up, we see either a full moon (if we're between the moon and the sun) or a new moon (if the moon is between us and the sun). At these times, the gravitational forces of the sun and moon work together, increasing the overall pull on the Earth and its oceans.

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Diagram: Earth/sun/moon system showing how sun and moon can accelerate and impede the movement of the bulges

 

Why do high tides sometimes precede the moon's zenith and sometimes lag behind?

If the tidal bulges only followed the moon, we'd always have a high tide when the moon is at its zenith—its highest point in its journey across the sky. But sometimes a high tide precedes or lags the zenith by as much as several hours. The reason is that the sun's gravity, in addition to influencing the height of a tide, can also influence the position of the bulges. For instance, as the moon approaches its first quarter phase, the sun pulls back on the bulge. (In the image, A represents the sun's gravitational tug, C the moon's, and B the combined force of the two.) The Earth rotates into the bulge earlier than it would have if the sun weren't pulling back, causing the high tide to precede the moon's zenith. At some points in the moon's orbit the sun pulls the bulge forward, thus causing the tide to lag the moon's zenith.

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Diagram: moon orbiting Earth at an inclination, with a highlighted area on the Earth that passes through only one of the two bulges

 

Why do some places on Earth experience only one high tide (and one low tide) in a 24-hour period instead of two?

The moon doesn't orbit the Earth directly over the equator. Instead, its orbit is inclined by about five degrees from a line extending between our planet and the sun. Also, depending on our position in our orbit around the sun, the moon's orbit can be inclined by as much 23.5 degrees. This inclination causes the tidal bulges to often form above and below the equator. When this happens, it's possible for locations on the rotating Earth to pass through only one of the two bulges, thus producing only one high tide.

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Note:
The following explanations take into account only basic astronomical influences. They ignore geographical influences, such as the presence of land masses, the shape of coastlines, the varying depths and contours of the seafloor, and weather.


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