TIDES

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TIDES
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Definition

• Tides periodic rise and fall of large bodies of
  water

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• Caused by the gravitational interactions between
  the Earth and the Moon and the Sun

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Lunar Tides

• The Moon tries to pull on Earth to bring it
  closer. The Earth is able to hold onto
  everything but the water
• The oceans bulge out in the direction of the Moon

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• Another bulge occurs on the opposite side (this
  is due to the Earth being pulled towards the Moon
  but the oceans on this side staying put).
• Two tides occur each day. About 12hr and 25min
  between two high tides.

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Complicating Factors

• The Earth and Moon are not static. They orbit
  and rotate.
• The Earth is not covered in oceans.
• The oceans are of varying depths.
• Friction between the oceans and the Earth.

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Effect of the Sun

• Other objects influence the Earths tides.
• The effect of the Sun is about half that of the
  Moon.
• Particularly large tides are experienced when the
  Earth, Moon and Sun line up.

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• SPRING TIDES Highest tides. During full and new
  moon.
• 20 higher than average tides

http//home.hiwaay.net/krcool/Astro/moon/moontide
s/
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• NEAP TIDES Weaker tides. During first and third
  quarter
• 20 lower than average tides

http//home.hiwaay.net/krcool/Astro/moon/moontide
s/
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Proxigean Spring Tide

• A rare, unusually high tide.
• Occurs when the Moon is both unusually close to
  the Earth (at its closest perigee, called the
  proxigee) and in the New Moon phase (when the
  Moon is between the Sun and the Earth).
• The proxigean spring tide occurs at most once
  every 1.5 years.

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What causes these higher tides???

• SUN!!!

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Why is the Moon sometimes closer to the Earth???

• The Moon moves on an orbit which is not circular
  but elliptical, with the Earth at one focus.
• Perigee The Moon is at its closest approach to
  the Earth (356 410 km).
• Apogee The Moon is at its
• greatest separation from the
• Earth (406 697 km).

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Orbits

• Since the Sun is not at the center of an
  elliptical orbit (it is a FOCUS), the planet
  moves closer towards and further away from the
  Sun as it orbits. This is similar to how the
  Moon orbits Earth.

Perihelion The place where the planet is
closest to the Sun. (Greek peri means
near) Aphelion When the planet is furthest away
from the Sun. (Greek apo means away from)
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Why are there no ocean tides at the equator?

• Near the equator, the difference between high
  tide and low tide is actually rather small,
  compared to other latitudes.
• To see this, draw a circle inscribed in an
  ellipse, with the major axis of the ellipse
  rotated by 23 degrees with respect to the
  circle's horizontal diameter. Now measure the
  height of the elliptical contour just above the
  'equator' of the circle. You will see that it is
  quite small compared to other positions on earth,
  particularly at latitudes of 23 degrees or so.
• Even larger differences can occur depending on
  the shape of a bay or inlet or continental shelf.

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• When the Earth, Moon and Sun are aligned for
  Spring Tides, are they highest at Full or New
  Moon?

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Tides in the Bay of Fundy
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Hopewell Rocks
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The Tidal Bore

• The leading edge of the incoming tide forms a
  wave of water that travels up a river or narrow
  bay against the direction of the current.

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The Annapolis Tidal Generating Station

• Uses the massive movements of the water in the
  Bay of Fundy
• Creates enough energy to power 5000 homes.
• The only tidal power station in North America

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Earths Rotation

• Tidal interactions have slowed Earths rotational
  period.
• Eventually (billions of years) Earth and Moon
  will have the same rotational period (27.3 days)

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How do tides affect marine life?

• Evolution Plants and animals on the shoreline
  between high and low tides must be able to
  withstand stresses (waves, exposure to air,
  changes in the salinity (salt) of the water,
  drying out, freezing, overheating, starvation
  (most animals can only feed when submerged).
• Tides therefore create niches in the shoreline
  ecosystems that are available to different
  organisms, depending on their tolerance to these
  conditions. This leads to intertidal zonation,
  the tendency of plants and animals to form
  visible communities along the marine shoreline,
  between the high and low tide lines.

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How do tides affect marine life?

• Evolution and Uniqueness Tide pools are
  depressions in rocks that are flooded regularly
  with sea water, but isolated at low tide. Many
  intertidal and subtidal areas are particularly
  rich in marine life due to their exposure to
  strong tidal currents, which carry abundant
  nutrients.
• Erosion Tides also affect erosion and coastal
  sediment processes, since they control to what
  height waves can influence the land. Storm surges
  (high energy waves) that occur during spring
  tides can be particularly damaging to the
  shoreline and/or human property.

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Seasons

• The three reasons why we have Seasons
• 1) Tilt of Earth
• 2) Revolution of the Earth around the Sun.
• 3) The North Pole always POINTS in the same
  direction
• NOT DISTANCE FROM SUN!!!

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Why does the Moon not have seasons?
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Animations

• http//projects.astro.illinois.edu/data/Seasons/se
  asons.html
• http//lectureonline.cl.msu.edu/mmp/applist/seaso
  ns/cd190b.htm
• http//highered.mcgraw-hill.com/sites/007299181x/s
  tudent_view0/chapter2/seasons_interactive.html
• (also change to 0 inclination to see what could
  be!)

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Seasons

• We have seasons because the Earth is tilted at
  23.5 as it orbits of the Sun.
• The varying amounts of sunlight around the Earth
  during the year creates the seasons.
• The tilt of the Earth means the Earth will lean
  towards the Sun (Summer) or lean away from the
  Sun (Winter).

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Northern Hemisphere Seasons
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However

• The Earth's movement around the Sun causes the
  seasons, but it does not affect the temperatures
  during the seasons.
• Earth is farthest from the Sun in July (orbit,
  not tilt!)
• Earth is closest to the Sun at the beginning of
  January

http//www.woodlands-junior.kent.sch.uk/time/seaso
ns.htm
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• The height of the Sun in the sky varies with the
  seasons.
• The Sun is at its highest at 1200pm (noon) on
  any given day.
• In the summer the Sun is higher than it is in the
  other three seasons.

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BUT

• The reason the distance from the Earth to the Sun
  does not affect our seasons is because our orbit
  is more circular than elliptical (eccentricity
  0.017)
• However, if you were on a more eccentric planet,
  seasons would be affected by the distance from
  the Sun!

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Summer

• Sun's rays hit the Earth at a steep angle because
  the Sun is higher in the sky.
• The light does not spread out very much, thus
  increasing the amount of energy hitting any given
  spot.

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Summer

• The long daylight hours during the summer months
  allow the Earth plenty of time to reach warm
  temperatures (absorbs heat and light energy).
• Because the Sun is higher in the sky our shadows
  are shorter than they are in the other three
  seasons. This is due to the angle the rays hit us
  at.

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Why are days longer in the Summer?

• The daylight lasts more than 12 hours because the
  Sun is at its Northernmost position from the
  equator
• This is also why the Southern hemisphere has
  shorter days at this time!
• The farther North from the equator you are, the
  longer the daylight hours in the Summer. (North
  Pole no night time!)

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Do countries on the equator have seasons?

• YES!
• Seasons here are not temperature based like we
  have (astronomical seasons affected by the tilt
  of Earth).
• Seasons here are based on meteorological seasons
  rainy and dry seasons.
• These seasons are more affected by atmospheric
  changes.

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Winter

• Sun's rays hit the Earth at a shallow angle
  because the Sun is lower in the sky.
• These rays are more spread out, which minimizes
  the amount of energy that hits any given spot.
• The long nights and short days prevent the Earth
  from warming up.
• This is why we have cold
• winters.

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Seasons
Note This diagram does not take into
consideration an elliptical orbit of the Sun.
The Earths perihelion is in December and its
aphelion is in June.
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If the Earth were not tilted
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Summer (Northern Hemisphere)
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Winter (Northern Hemisphere)
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Day Length

• Summer is warmer and winter is colder because of
  the length of our days and nights.
• In the summer daylight lasts longer and night
  time is shorter.
• In winter the days are shorter and the nights
  longer.
• There is more time for the sun to warm us during
  long summer days. And short winter days have
  long, cold nights so there is less time for
  warming up of the Earth.
• On the equator, the days and nights are 12 hours
  long so the temperature is approximately the same.

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Day Length

• Here in Nova Scotia, our shortest days are 9
  hours. Our longest days are about 15 hours.
• At the North Pole, they have 24 hour sunlight
  during Summer Solstice and 24 darkness during
  Winter Solstice.
• This is also true for any place North of about
  66.
• (Opposite is true for South)

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Review Questions

• 1) What causes tides? 
• 2) What are spring and neap tides? How do they
  differ? How are they similar? How often do they
  occur?
• 3) What are proxigean tides? How often do they
  occur?
• 4) What effects have the tides had on life on
  Earth?
• 5) Why do we experience seasons? How do the
  seasons differ in different parts of the world?
• 6) How have the seasons affected life on Earth?
• 7) Where are tides largest (in relation to
  Moon)?
• 8) Where are the highest tides on Earth? Why?
• 9) Explain the difference between apogee/perigee
  and aphelion/perihelion.
• 10) How does the Sun affect the tides, daylength
  and the seasons on Earth?

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Resources

• http//www.lcsd.gov.hk/CE/Museum/Space/EducationRe
  source/Universe/framed_e/lecture/ch06/imgs/tides.j
  pg
• http//www.enchantedlearning.com/subjects/astronom
  y/moon/Tides.shtml
• http//www.jochemnet.de/fiu/tide2.jpg
• http//home.hiwaay.net/krcool/Astro/moon/moontide
  s/
• http//www.physicalgeography.net/fundamentals/imag
  es/earth_moon.jpg
• http//photoclub.canadiangeographic.ca/photos/rosc
  o-photo/images/43243/500x335.aspx
• http//static.howstuffworks.com/gif/ocean-current-
  6b.jpg
• http//www.huntsmanmarine.ca/images/tides_1rd.jpg
• http//upload.wikimedia.org/wikipedia/commons/5/54
  /Bay_of_Fundy.jpg
• http//www.canadacool.com/COOLFACTS/NOVA20SCOTIA/
  NOVASCOTIAPHOTOS/annapolis7low.jpg
• http//www.intute.ac.uk/sciences/worldguide/satell
  ite/2559.jpg
• http//www.srh.noaa.gov/jetstream/ocean/images/bay
  offundy.jpg
• http//www.raftingcanada.ca/
• http//cord.org/cm/leot/course01_mod07/loet01-07-0
  6new.gif
• http//www.canada-maps.org/nova-scotia-map.htm
• http//grant7.typepad.com/trilogy/images/tidal20p
  ool.bmp
• http//www.zoology.ufl.edu/personal/grad/jhavird/P
  icture/killifish-pics-(1)-(optimal).jpg
• http//www.ucmp.berkeley.edu/vertebrates/sarco/lun
  gfish1.jpg
• http//csep10.phys.utk.edu/astr161/lect/time/tides
  .html