Tides

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!

Tides Because of their extremely long wavelengths, tides are shallow water waves that span across entire ocean basins! Caused by the combined gravitational forces of the Moon and Sun, and the motion of the Earth mrbarlow.wordpress.com/tagastronomy

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

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

A planet orbits the sun in balance between gravity and inertia Inertia ‘wants’ to keep the movement of a planet in a straight line, but gravity pulls the planet toward the sun, causing the planet to travel in a fixed path around the sun

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

The motion of the Earth around the center of mass throws up a bulge on the side of Earth opposite the Moon

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

Turning of the tides The movement into and out of the bulges results in the high and low tides, respectively

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

Tidal Bulges: The Sun’s Effect The combined effects of the Sun and Moon cause wave interference Constructive interference results when the Earth, Moon, and Sun are all in a line; additive – Results in higher high tides and lower low tides Destructive interference results when the Earth, Moon, and Sun form a right angle; subtractive – Results in diminished lunar tides

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

Effects of Elliptical Orbits The Earth revolves around the Sun in an elliptical orbit such that the Earth is presently 92.2 million miles from the Sun during the Northern Hemisphere winter and 94.5 million miles from the Sun during its summer (distance between the Earth and the Sun vary by 2.5% over the course of a year) Tidal ranges are highest when Earth is closest to the Sun, and lowest when it is farthest away

After 1 Earth day, the Moon has moved slightly forward in its orbit It becomes necessary for a spot on the Earth to regain its position relative to the Moon each day Therefore, the times of high (and low) tide from 1 day to the next are offset by 50 minutes

Tide Charts On Long Island, there are 2 high tides and 2 low tides each day, as the Earth moves into and out of the tidal bulges 2 crests and 2 troughs; 1 for each bulge Subsequent high (or low) tides occur ~12 hours and 25 minutes apart from each other

What Types of Tidal Patterns Exist? The conditions described so far have been idealized But the oceans are dynamic and there are several missing factors that have not been discussed: – Depth of ocean does not allow such large waves to move freely – Shapes of ocean basins influence tidal height – And… the CONTINENTS!!!

What Types of Tidal Patterns Exist? A semidiurnal tide describes 2 high tides and 2 low tides each day – East coast (Atlantic) of the United States A diurnal tide describes a single high tide and a single low tide each day – Gulf of Mexico and other shallow, inland seas A mixed tide describes a mixed combination with successive high (or low) tides significantly different in height – West coast of the United States

Semidiurnal, Diurnal, and Mixed Tides

What Types of Tidal Patterns Exist? As the Earth turns, landmasses obstruct the tidal crests, diverting, slowing, and complicating their movements The natural tendency of water in an enclosed basin to rock at a specific frequency modifies the pattern into a diurnal one in the Gulf of Mexico As tidal bulges become “squished” against land, water moves downslope and is influenced by the Coriolis effect!

Tidal Extremes: The Bay of Fundy The largest tidal range (vertical difference between the high and subsequent low tide) in the world is found in Nova Scotia’s Bay of Fundy Here, the tidal range during spring tide conditions is ~17 meters (56 feet)! The Bay of Fundy has a wide opening into the Atlantic Ocean which splits into 2 narrow basins at its northern end

Tidal Extremes: The Bay of Fundy ch?v=YiHP8EG_Wq4

Tidal Currents As sea level moves up and down with the tide, water currents are generated Flood currents rush water into a bay (or river) with the incoming high tide Ebb currents drain water out of a bay (or river) as a low tide is approaching No water flows during slack water (“slack tide”); occurs at peak of high and peak of low tide

Ponquogue bridge (old, left; new, right); Hampton Bays, NY

Don’t be a Sven… READ YOUR TIDE CHARTS!!!