Tides
description
Transcript of Tides
![Page 1: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/1.jpg)
Tides
EPS 131: Intro. to Physical Oceanography
Douglas McClure10/17/05
![Page 2: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/2.jpg)
Outline• Physical origin of tidal forces• Global effects of tidal forces on
Earth– Over one day– Over one lunar month
• Tides in Reality• Outlook for the (distant) future
![Page 3: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/3.jpg)
Origin of Tidal Forces• Gravitational force between two masses falls off
as 1/(distance)2: • Thus the Moon pulls more strongly on the Earth
at A than at B; indeed, the force difference is about 7%.
• The sun will have a similar but smaller effect; we will come back to this.
112
21 1067.6; GRmGmFG
AB
![Page 4: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/4.jpg)
Tidal Forces• Compared to its force on the center
of the Earth, the Moon’s force along on the surface looks as follows:
(moon off to one side)
• Since rock is rigid and water is not, these forces will cause oceans to rise and fall relative to the crust.
(The crust rises and falls too, but only slightly.)
![Page 5: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/5.jpg)
Effects of Tidal Forces: One Day
• The Earth completes a full rotation; the moon completes only 1/29 of its orbit.
• Hence the two bulges remain almost still on the timescale of the Earth’s rotation.
![Page 6: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/6.jpg)
Effects of Tidal Forces: One Day
• In other words, a given point on the Earth’s surface will experience both bulges and both troughs over one day.
• Small correction: after 1 day, the Moon (and thus the bulges) will have advanced by 360o/29 ≈ 12o ≈ 48 min
• Thus high tides occur every 12 hrs 24 mins instead of every 12 hrs.
![Page 7: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/7.jpg)
Variation Over a Lunar Month
• The Sun contributes a tidal force slightly less than ½ of the Moon’s.
• At new Moon and full Moon, the Sun’s contribution aligns with the Moon’s, and so tides are stronger (“spring tides”).
• At half Moons, the Sun’s contribution partially cancels the Moon’s, and so tides are weaker (“neap tides”).
(Sun is to the right)
![Page 8: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/8.jpg)
Variation due to Elliptical Orbits
• Since the Moon’s orbit is elliptical, it is sometimes closer and sometimes farther; particularly strong tides occur every several months when the Moon reaches its closest point (“perigee”) during a spring tide phase.
• Analogously, the Sun’s effect will be strongest when the Earth is at perihelion.
![Page 9: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/9.jpg)
Tides in Reality
• Tides are affected by regional topography.• Resonance effects and the Bay of Fundy
![Page 10: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/10.jpg)
Tides in Reality
• In most parts of the ocean, the effects are much more complicated.
• Both timing and amplitude may vary; in general, tides lag behind the Earth-Moon axis.
• Tidal energy as a source of mixing.
![Page 11: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/11.jpg)
Long-term Effects of Tidal Forces
• Because of the Earth’s fast rotation, the two “bulges” actually lag behind the Earth-Moon axis.
• The resulting mutual tangential acceleration causes the Earth to slow down and the Moon to speed up.
• The Earth will continue slowing until it becomes tidally locked with the Moon.
• After then, the only source of tides will be the Sun…if it is still around.
![Page 12: Tides](https://reader035.fdocuments.in/reader035/viewer/2022062816/5681563e550346895dc3e654/html5/thumbnails/12.jpg)
References• Knauss, Chapter 10• http://en.wikipedia.org/wiki/Tides• http://www.jal.cc.il.us/~mikolajsawicki/
tides_new2.pdf• http://oceanlink.island.net/oinfo/tides/
tides.html• http://homepage.eircom.net/~cronews/
geog/tides/tide.html• http://oceanworld.tamu.edu/resources/
ocng_textbook/chapter17/chapter17_04.htm