Chapter 9: Tides

tides

rhythmic and periodical rise and fall of sea level.

what are the tide generating forces

gravity, motion btwn Earth, Moon, Sun.

baryceneter

common center of mass or balance point, beneath Earth’s surface bc of Earth’s greater mass.

zenith

greatest gravitational force at Z, closest to moon

weakest gravitational force

N, nadir, furthest from moon and opposite zenith

centripetal forces

a force that makes a body follow a curved path. The direction of the centripetal force is always orthogonal to the motion of the body and towards the fixed point of the instantaneous center of curvature of the path

lunar bulges

As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a “bulge” of water on the near side toward the moon

tidal period

time btwn high tides: 12 hrs, 25 mins

lunar day

time btwn two successive overhead moons, 24 hours and 50 mins

solar day

24 hours

lunar day

27 earth days, 7 hours, 43 minutes, 12 seconds to complete one orbit

lunar theory

attempts to account for motions of the moon, many irregularities in it's motion.

math to analyze general movement, generate formulas, algorithms

based on 2000 years of investigation

types of lunar months

sidereal

synodic

tropical

anomalistic

draconic

synodic month

“pertaining to a synod”, average period of moon’s orbit with respect to line joining sun and earth. period of lunar phases, bc moon’s appearance depends on position of moon with respect to sun as seen from earth

tropical month

customary to specify positions of celestial bodies w/ respect to vernal equinox. slowly moves along ecliptic. takes moon less time to return to ecliptic longitude of 0 deg. than to same point amid fixed stars. (27 d, 7 h, 43 min)

solar bulges

only 46% size of lunar bulges, less than half the size. sun’s tide generating force is 27/59 that of moon, or 46%.

moon closer to earth, greater gravitational pull

flood tide

water moves toward shore

ebb tide

water moves away from shore

tidal bulges are fixed relative to sun’s and moon’s positions

tidal cycle

29.5 days

moon changes dramatically during tidal cycle

new moon

moon btwn. earth and sun, cannot be seen from earth

full moon

moon is opposite of sun

quarter moon

moon appears half lit, right angles to sun relative to earth

tidal range

difference btwn high and low tides. tide generating forces of sun and moon combine, constructive interference btwn. lunar and solar tidal bulges.

syzygy

moon, earth, sun aligned

quadrature

moon in first or third quarter phase

spring tides

new or full moons

tidal range greatest

moon in syzygy

neap tides

nep=barely touching

quarter moons

tidal range least destructive interference

moon in quadrature

waxing crescent

moon moving from new to first quarter

waxing gibbous

moon moving from first quarter to full

waning gibbous

moon moving from full to last quarter

waning crescent

moon moving from last quarter to new moon

two most important complicating factors of tides

declination of moon and sun

elliptical shapes of earth’s and moon’s orbits

declination

angular distance of moon or sun above or below earth’s equator

23.5 degrees from perpendicular. max declination is always 23.5.

ecliptic

ekleipein: to fail to appear

perigee

tidal range greatest here, when moon is closest to earth

apogee

when tidal range is the least, moon furthest from earth.

perigee apogee cycle is 27.5 days

perigean tides

greater tidal ranges experienced here.

proxigean tides

spring tide and perigee.

exceptionally high tidal range.

every 1.5 years or so

idealized tide prediction

2 high tides, 2 low tides per lunar day

0h: high tide

6h: low tide

12h: lower high tide

18h: low tide

24h: complete lunar day cycle

how long is there between high and low tides

6h 12.5 mins in solar time

what interrupts tidal bulges

continents affect tides bc they interrupt the free movement of tidal bulges across ocean.

amphidromic point

crests and troughs of tides rotate around this point.

location in ocean where there is no tidal range

crests and troughs of tide waves rotate about htis point

cotidal lines

radiate from amphidromic points and connect all nearby locations that high tide is occurring at that time.

diurnal

one high tide/one low tide per day

tidal period: 24 h, 50 min

semidiurnal

two high tides/two low tides per day

tidal range about same

tidal period: 12 h 25 min

Atlantic coast of US

mixed tides

two high tides/two low tides per day

tidal range different

most common

pacific coast of US

standing waves

tide waves reflected by coast

amplification of tidal range

tidal bores

tide generated wall of water

moves up certain rivers

conditions needed for tidal bores

large spring tidal range of at least 6 m or 20 ft

abrupt flood tide and short ebb tide phases

low lying river with seaward current

shallowing of landward sea floor

narrowing of basin in upper reaches

bay of fundy

Nova Scotia world’s largest tidal range (17m, 56 ft)

rotary current

current that accompanies slowly turning tide crest in Northern hemisphere basin. rotates counterclockwise

reversing current

alternating current due to friction in nearshore

moves in and out of narrow coastal passages

flood current

water rushes up a bay or river with incoming tide

ebb current

water drains from bay or river as tide goes out

high slack water

HSW: peak of each high tide with no current motion

low slack water

LSW: peak of each low tide with no current motion

whirlpool or vortex

rapidly spinning seawater

restricted channel connecting two basins with diff tidal cycles

maelstrom near arctic norway.

strait of messina

bay of fundy

west coast of scotland