Tides - Introduction to Oceanography - Lecture Slides, Slides of Oceanography

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Tides

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Tides

• Tides are waves with very long wavelengths
• Tides are the periodic rising and lowering of average sea level that occurs throughout the ocean
• Tides are the longest waves in the oceans with wavelengths of half the Earth’s circumference
• High tide is the crest of the wave, while low tide is the trough!

Origin of the Tides

• Tides are caused by 2 principle factors:
  • Gravitational attraction
  • Centrifugal force or inertia
• All masses are drawn to one another, and exert a gravitational pull
• The greater the mass, the greater the pull
• As distance increases, gravitational force greatly decreases

Origin of the Tides

• Although the Moon is much smaller than the Sun, it is closer in proximity to the Earth, and so exerts a greater gravitational pull on the Earth than the Sun
• The moon exerts about twice the tide-raising force on Earth as that of the sun
• The moon’s gravity attracts the ocean surface toward the moon

Origin of the Tides

• The lunar (moon) gravitational force pulling on the oceans causes water to be drawn toward the side of the Earth that faces the moon
• This creates a tidal bulge of water

http://oceanlink.island.net/oinfo/tides/tides.html

Origin of the Tides

• However, a second tidal bulge exists on the opposite side of the Earth as well
• This bulge exists results from centrifugal forces that arise as the Earth and Moon revolve around one another
• Centrifugal force describes the tendency of a given body to move in a straight line

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Origin of the Tides

• Although we have been taught that the moon revolves around the Earth, the reality is less simple
• The Earth and Moon actually rotate around a common center; the ‘balance point’ of the system
• Because the mass of Earth is 81 times that of the Moon, their common center of rotation is actually inside the Earth

Origin of the Tides

• Instead, the Earth and Moon revolve around a common center of mass located beneath the Earth’s surface

Gravitational pull

Centrifugal force

A true tidal wave…

• The resulting bulges on the Earth remain aligned as the Moon and Earth rotates on its common axis
• But the Earth spins daily around its own axis, moving into and out of the tidal bulges every day

Tidal Bulges: The Sun’s Effect

• The Sun affects the tides, too
• Like the Moon, the Sun produces tidal bulges on opposite sides of the Earth; one towards the Sun, and one away from the Sun
• These solar (sun) bulges, however, are much smaller than the lunar bulges because the sun, although larger, is ~400 times farther away from the Earth than the Moon

We are the knights that say…neap!

• The large tides caused by the lunar alignment of the Sun, Earth and Moon are called spring tides - High tides are very high; Low tides are very low - Occurs every 2 weeks during new and full moons
• The diminished tides that occur when the Sun, Earth and Moon form a right angle are called neap tides - High tides are not very high; Low tides are not very low - Occur during the quarter moon phases