TIDESTIDES

DefinitionDefinition

• Tides = Tides = periodic rise and fall of large periodic rise and fall of large bodies of waterbodies of water

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

Lunar TidesLunar Tides

• The Moon tries to The Moon tries to pull on Earth to pull on Earth to bring it closer. bring it closer. The Earth is able The Earth is able to hold onto to hold onto everything but the everything but the waterwater

• The oceans bulge The oceans bulge out in the direction out in the direction of the Moonof the Moon

• Another bulge occurs Another bulge occurs on the opposite side on the opposite side (this is due to the Earth (this is due to the Earth being pulled towards being pulled towards the Moon but the the Moon but the oceans on this side oceans on this side staying put). staying put).

• Two tides occur each Two tides occur each day. About 12hr and day. About 12hr and 25min between two 25min between two high tides.high tides.

Complicating FactorsComplicating Factors

• The Earth and Moon are not static. The Earth and Moon are not static. They orbit and rotate. They orbit and rotate.

• The Earth is not covered in oceans.The Earth is not covered in oceans.

• The oceans are of varying depths.The oceans are of varying depths.

• Friction between the oceans and the Friction between the oceans and the Earth.Earth.

Effect of the SunEffect of the Sun

• Other objects Other objects influence the Earth’s influence the Earth’s tides.tides.

• The effect of the Sun The effect of the Sun is about half that of is about half that of the Moon.the Moon.

• Particularly large tides Particularly large tides are experienced when are experienced when the Earth, Moon and the Earth, Moon and Sun line up.Sun line up.

• SPRING TIDES:SPRING TIDES: Highest tides. Highest tides. During full and During full and new moon.new moon.

• 20% higher than 20% higher than average tidesaverage tides

http://home.hiwaay.net/~krcool/Astro/moon/moontides/

• NEAP TIDESNEAP TIDES: : Weaker tides. Weaker tides. During first and During first and third quarterthird quarter

• 20% lower than 20% lower than average tidesaverage tides

http://home.hiwaay.net/~krcool/Astro/moon/moontides/

Proxigean Spring TideProxigean 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.

What causes these higher tides???

• SUN!!!

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).

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)

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.

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

Tides in the Bay of FundyTides in the Bay of Fundy

Hopewell RocksHopewell Rocks

The Tidal BoreThe Tidal Bore

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

The Annapolis Tidal Generating The Annapolis Tidal Generating StationStation

• Uses the massive Uses the massive movements of the movements of the water in the Bay of water in the Bay of FundyFundy

• Creates enough Creates enough energy to power energy to power 5000 homes.5000 homes.

• The only tidal power The only tidal power station in North station in North AmericaAmerica

Earth’s RotationEarth’s Rotation

• Tidal interactions Tidal interactions have slowed have slowed Earth’s rotational Earth’s rotational period.period.

• Eventually (billions Eventually (billions of years) Earth and of years) Earth and Moon will have the Moon will have the same rotational same rotational period (27.3 days)period (27.3 days)

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.

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.

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!!!

Why does the Moon not have seasons?

Animations…

• http://projects.astro.illinois.edu/data/Seasons/seasons.html

• http://lectureonline.cl.msu.edu/~mmp/applist/seasons/cd190b.htm

• http://highered.mcgraw-hill.com/sites/007299181x/student_view0/chapter2/seasons_interactive.html

• (also change to 0’ inclination to see what could be!)

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).

Northern Hemisphere Seasons

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/seasons.htm

• The height of the Sun in the sky varies with the seasons.

• The Sun is at its highest at 12:00pm (noon) on any given day.

• In the summer the Sun is higher than it is in the other three seasons.

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!

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.

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.

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!)

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.

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.

Seasons

Note: This diagram does not take into consideration an elliptical orbit of the Sun. The Earth’s perihelion is in December and its aphelion is in June.

If the Earth were not tilted…

Summer (Northern Hemisphere)

Winter (Northern Hemisphere)

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.

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)

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?

Resources• http://www.lcsd.gov.hk/CE/Museum/Space/EducationResource/Universe/framed_e/lecture/ch06/imgs/tides.jpg• http://www.enchantedlearning.com/subjects/astronomy/moon/Tides.shtml• http://www.jochemnet.de/fiu/tide2.jpg• http://home.hiwaay.net/~krcool/Astro/moon/moontides/• http://www.physicalgeography.net/fundamentals/images/earth_moon.jpg• http://photoclub.canadiangeographic.ca/photos/rosco-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/NOVA%20SCOTIA/NOVASCOTIAPHOTOS/annapolis7low.jpg• http://www.intute.ac.uk/sciences/worldguide/satellite/2559.jpg• http://www.srh.noaa.gov/jetstream/ocean/images/bayoffundy.jpg• http://www.raftingcanada.ca/• http://cord.org/cm/leot/course01_mod07/loet01-07-06new.gif• http://www.canada-maps.org/nova-scotia-map.htm• http://grant7.typepad.com/trilogy/images/tidal%20pool.bmp• http://www.zoology.ufl.edu/personal/grad/jhavird/Picture/killifish-pics-(1)-(optimal).jpg• http://www.ucmp.berkeley.edu/vertebrates/sarco/lungfish1.jpg• http://csep10.phys.utk.edu/astr161/lect/time/tides.html• http://csep10.phys.utk.edu/astr161/lect/time/tides.html• http://en.wikipedia.org/wiki/Tidal_bore• http://www.physicalgeography.net/fundamentals/images/earth_rotation.jpg• http://www.michaeltaylor.ca/Culture/hopewell-low.jpg• http://thomonthenet.com/Maritimes2002/123a.jpg