Explain-it: How do the tides work?

 In this video Adam Hart-Davis uses a biscuit, a pickled onion and an orange to explain how the sun and moon control the tides.

How the Moon Affects the Changing of the Tides

The word “tides” is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun.

Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide. There is about 12 hours and 25 minutes between the two high tides.

Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world’s oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day.

What are the different types of Tides
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.
Spring Tides
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.
Neap Tides
During the moon’s quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.
A Few Facts About Lunar Tides
  • The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth’s centrifugal force created by its spin, you get tides.
  • The sun’s gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides.
  • The sun’s gravity also produces tides. But since the forces are smaller, as compared to the moon, the effects are greatly decreased.
  • Tides are not caused by the direct pull of the moon’s gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides.
  • Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides.
  • Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon).
  • When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides.
  • Spring tides and neap tide levels are about 20% higher or lower than average.
  • Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet
  • The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.)
  • The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada.
  • Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon’s 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours.
  • Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes.
  • The type of gravitational force that causes tides is know as “Tractive” force.
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